Recrystallization and grain growth of a nano/ultrafine structured austenitic stainless steel during annealing under high hydrostatic pressure

Recrystallization and grain growth of a nano/ultrafine structured austenitic stainless steel... J Mater Sci (2018) 53:11823–11836 METALS Metals Recrystallization and grain growth of a nano/ultrafine structured austenitic stainless steel during annealing under high hydrostatic pressure 1, 2 1 3 Agnieszka Teresa Krawczynska * , Stanislaw Gierlotka , Przemyslaw Suchecki , Daria Setman , 1 1 3 Boguslawa Adamczyk-Cieslak , Malgorzata Lewandowska , and Michael Zehetbauer Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland Institute of High Pressure Physics UNIPRESS, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria Received: 20 December 2017 ABSTRACT Accepted: 14 May 2018 The aim of this study was to investigate the effect of high hydrostatic pressure Published online: applied during annealing on the processes of recrystallization and grain growth 21 May 2018 in nanostructured austenitic stainless steel 316LVM. The nanostructures were obtained by profile rolling to a total strain of 3.4 and by high-pressure torsion to The Author(s) 2018 a total strain of 79. These processes resulted in microstructures consisting of nanotwins and nanograins, respectively. The deformed samples were annealed at 900 C for 10 min under atmospheric or hydrostatic pressures of 2 and 6 GPa. The resulting microstructures were examined using transmission and scanning electron microscopy techniques. The mechanical properties were evaluated in microhardness measurements. It was established that annealing under high hydrostatic pressure retards recrystallization and grain growth, both in profile- rolled and high-pressure torsion-processed samples. The magnitude of retar- dation depends on the character of the grain boundaries. The non-equilibrium high-angle grain boundaries present in the high-pressure torsion-processed sample show higher mobility under pressure than the nanotwinned and low- angle boundaries in the profile-rolled sample. recently in nanocrystalline, materials [3–5]. During Introduction the annealing of nanocrystalline materials produced Recovery, recrystallization and grain growth are by severe plastic deformation (SPD) techniques, one some of the most important processes that affect the can observe typical processes of recovery, recrystal- properties of crystalline materials. For this reason, lization and grain growth, but they follow a different many papers have been devoted to understanding course than in materials deformed by conventional these phenomena in microcrystalline [1, 2], and techniques [6–8]. In general, nanomaterials are less Address correspondence to E-mail: akrawczynska@wp.pl; agnieszka.krawczynska@pw.edu.pl https://doi.org/10.1007/s10853-018-2459-1 J Mater Sci (2018) 53:11823–11836 thermally stable than their microcrystalline counter- V¼RTlnðmÞ=p ð1Þ parts, and recrystallization begins below their usual where R is the gas constant, m is the rate of the pro- recrystallization temperatures. However, it has also cesses investigated and p is the pressure. been shown that, by creating uniform nanostructures Furthermore, it was found during annealing of having a uniform grain size distribution and a high cold-rolled copper under a pressure of 4.2 GPa that content of high-angle grain boundaries, thermal sta- the pressure retarded recrystallization [20]. The gen- bility can be significantly improved [9–11]. The eral conclusion that can be drawn from this work enhanced thermal stability of a nanostructured aus- regarding grain growth is that the higher the hydro- tenitic stainless steel can be obtained by adding static pressure applied, the more significant the yttrium powders during mechanical milling [12]. The decrease in grain boundary mobility. Moreover, the addition of 1.5 wt% stabilizes the microstructure impact of high pressure on the \100[, \110[ and around 116 nm grain size after 3 h annealing at \111[ tilt boundaries was analyzed [18, 22]. The 1100 C. Moreover, the creation of deformation-in- results showed that the movement of \110[ tilt duced nanotwins makes the microstructure stable up boundaries depends on the activation energy, as this to 800 C[13]. The processes of recrystallization and movement is effected by the cooperative motion of grain growth in nanostructured, and especially sin- several atoms. The movement of the \100[ and gle-phased, materials have been quite well described. \111[ tilt boundaries is not related to the activation Nevertheless, one issue which remains almost com- energy and is effected by a single atom mechanism. It pletely neglected is how such materials behave dur- was also noticed during experiments on normal grain ing annealing under high hydrostatic pressure. This growth in aluminum under high hydrostatic pressure subject has been raised only in the author’s previous that the migration of low-angle grain boundaries was work [14, 15]. In those studies, annealing under high slowed down even more, nearly frozen in compar- hydrostatic pressure was applied to a hydrostatically ison with the case of general grain boundaries [26]. extruded (HE) austenitic stainless steel to optimize its This is because low-angle grain boundaries can mechanical properties, particularly its strength to migrate by vacancy grain boundary migration, which ductility balance. It was possible to achieve a con- is highly limited under high pressure. trolled slowing of recrystallization processes to pro- In the view of published work within the subject of duce a nanostructured austenitic stainless steel annealing under the high hydrostatic pressure, it possessing a good combination of strength and duc- seems that annealing under high hydrostatic pressure tility (an ultimate tensile strength of 1247 MPa and a can be an effective way to control the grain growth in total elongation of 24.4%). The only experiments that nanostructured materials and produce fine grain have been performed under high pressure recently materials which could not be obtained under atmo- refer to the Ge precipitation rate in Ge? ion-im- spheric pressure due to fast and uncontrollable grain planted SiO films [16], a study on the effect of growth. However, this requires full understanding of annealing under pressure on the material properties processes taking place during annealing under high of Cu2ZnSn(S,Se)4 thin films [17] and an enhance- hydrostatic pressure. Therefore, the aim of the cur- ment of magnetic refrigeration performance in rent study was to investigate the influence of high metamagnetic MnCoSi alloy by high-pressure pressure on the recrystallization and grain growth of annealing [18]. a nanostructured stainless steel 316LVM. To this end, However, several authors have studied the effect of specimens of an austenitic stainless steel were hydrostatic pressure on diffusion, dislocation climb deformed by profile rolling (PR)—a conventional and glide, recrystallization and grain growth mobility deformation technique, and by high-pressure torsion in microcrystalline materials [19–27]. It is well known (HPT)—an SPD technique; the samples were then that high hydrostatic pressure has an impact on dif- annealed under a pressure of 2 and 6 GPa and, for fusion processes that are correlated with the motion comparison, under atmospheric pressure. The HPT of vacancies, as it affects the activation volume V*of process was chosen because it is one of the most the crystal related to atomic rearrangements during efficient SPD processes for grain refinement [28–33]. thermally activated processes. The influence of pres- sure on V* can be expressed by the equation: J Mater Sci (2018) 53:11823–11836 11825 of the sample for the annealing experiments. The Materials and methods dimensions of the samples were limited by the Sandvik Bioline 316LVM austenitic stainless steel was dimensions of the toroidal high-pressure cell. The used; this is a low-carbon, vacuum-melted 316L samples were annealed at 900 C for 10 min, either at grade stainless steel, UNS S31673 certified to ASTM atmospheric pressure (0.1 MPa) or at a hydrostatic F138, supplied as annealed in the form of 50-mm- pressure of 2 or 6 GPa and. To verify whether diameter rods, possessing the chemical composition annealing under high hydrostatic pressure leads to shown in Table 1. the creation of similar microstructures as conven- The samples were conventionally deformed using tional annealing for a shorter time, additional profile rolling (PR) with a reduction in cross section experiments of annealing under 0.1 MPa for 4 min of 23.8, which corresponds to a strain value of were performed. A toroidal high-pressure cell [34, 35] approximately 3.4. In this case, the strain was calcu- was used for the high-pressure annealing, and a lated according to the equation e = 2ln (d /d ), where conventional furnace for annealing at atmospheric 1 2 d (/ = 12 mm) is the initial diameter and d (/ = pressure. Further on in this text, these samples will be 1 2 2.2 mm) the final diameter. It must be pointed out referred to as PR_10_0.1 MPa, PR_10_2GPa, that this calculation is only an approximation, since PR_10_6GPa, PR_4_0.1 MPa, HPT_10_0.1 MPa, the cross section is not actually a circle. Additionally, HPT_10_2GPa, HPT_10_6GPa and HPT_4_0.1 MPa. the material was cut into disks with a diameter of The microstructure of the samples was investigated 10 mm and a thickness of 0.8 mm. The disks were using a Hitachi SU 8000 scanning electron micro- processed at room temperature using an HPT device scope working at 5 kV in the BSE mode and a JEOL at a constant pressure of 6.0 GPa. The disks were JEM 1200 EX transmission electron microscope torsionally strained to 5 revolutions. The strain was working at 120 kV. For the scanning electron micro- well defined as simple shear, c, and was calculated scopy examination, the samples were prepared by according to the equation c =2p 9 r 9 n/t, where r, electropolishing using Struers electrolyte A3. The n and t are the distance from the torsion axes, the parameters for electropolishing were as follows: number of applied revolutions and the mean thick- voltage - 15 V, time - 15 s. For the transmission ness of the sample, respectively. The equivalent electron microscopy examination, the samples were strains eeq = c/H3 calculated 3.5 mm from the cen- prepared by mechanical polishing to a disk thickness tral point of the sample after 5 revolutions were equal of about 100 lm. Further thinning to reach a thick- to 79. A phase analysis of the as-received, HPT- and ness appropriate for electron transparency was car- PR-processed samples was performed on a Bruker D8 ried out by electropolishing using Struers electrolyte Advance diffractometer with nickel-filtered copper A2. In the case of the PR samples, the microstructure radiation (k = 0.154056 nm). The data were collected was analyzed in the center of the samples. In the case in a range between 10 and 120 2H, with a step of the HPT samples, for microstructure analysis of the width of D2H = 0.02 and a counting time 5 s. The cross sections, disks with a diameter of 3 mm were energy of the emitter beam was 40 kV, and the cur- cut from the edge regions of each disk so that the rent was 40 mA. areas of observation were 3.5 mm from the central After PR, samples of 3 mm in height were cut from point. Further thinning was carried out by means of a a 500 mm rod. After the HPT experiments, disks of conventional procedure. Qualitative and quantitative 5 mm in diameter were cut in such a way that the studies as well as the percentage share of the radius of the sample after HPT became the diameter recrystallized areas of the microstructures were con- ducted using stereological and image analysis meth- ods [36, 37]. The grain size was determined as the Table 1 Chemical composition (wt%) of austenitic stainless steel equivalent diameter, d , defined as the diameter of a 316LVM 2 circle having an area equal to the surface area of a C Si Mn PSCr Ni Mo Cu N given grain. The grain shape was described by grain elongation factor, defined as the ratio of the maxi- 0.025 0.6 1.7 0.025 0.003 17.5 13.5 2.8 0.1 \0.1 mum to the equivalent diameter d /d . To establish max 2 the variation of the size of individual grains, a vari- ation coefficient, CV(d ), defined as the ratio of the 2 J Mater Sci (2018) 53:11823–11836 standard deviation SD (d ) to the mean value, was determined. Microhardness measurements were conducted on polished cross sections of the PR and HPT samples. These measurements were made using a Zwick microhardness tester under a load of 200 g. The val- ues of the Vickers microhardness, Hv, were recorded along a diameter of the PR and HPT samples with a separation of 0.1 mm. Results X-ray diffraction analysis Figure 1 shows X-ray diffraction profiles of the as- received, PR- and HPT-processed samples. The X-ray diffraction profiles reveal a c-austenite phase (fcc) without any evidence of e-martensite (hcp) or a’- martensite (bcc). The greatest broadening of peaks is observed for the HPT sample and is a result of microstructure refinement and microstrains. Microhardness measurements Microhardness measurements on cross sections of the Figure 2 Microhardness Hv0.2 of PR- a and HPT-processed PR- and HPT-processed samples after annealing at b austenitic stainless steel annealed under 0.1 MPa and 2 and 900 C under various pressures for 4 and 10 min are 6 GPa at 900 C for 10 min and additionally for 4 min under presented in Fig. 2. After HPT, the microhardness 0.1 MPa. reaches an average value of 471 Hv0.2, which is deviation equals 14 Hv0.2, whereas after PR it is higher by 46 units than after PR. Moreover, the HPT twice as high (it must be pointed out that the diam- causes a more homogeneous distribution of micro- eter of the HPT sample is two times greater than that hardness on the diameter since the standard after PR). Annealing under high hydrostatic pressure has a considerable impact on the microhardness values of HPT- and PR-processed samples. There is a tendency for both samples that the higher the pres- sure during annealing, the higher the microhardness value retained. However, the PR samples showed greater microhardness values for annealing under a pressure of 2 and 6 GPa (301 and 390 Hv0.2, respec- tively) than the HPT samples (254 and 282 Hv0.2, respectively). Additionally, annealing was performed for 4 min at 0.1 MPa. In the case of the PR samples, the microhardness value after annealing for 4 min can be compared with that obtained for annealing under 2 GPa for 10 min (287 and 301 Hv0.2, respec- tively). In the case of the HPT samples, annealing for 4 min at 0.1 MPa causes changes in the microhard- ness similar to annealing under 6 GPa (282 and 284 Figure 1 X-ray diffraction profiles of as-received, PR-, and HPT- Hv0.2, respectively). processed samples. J Mater Sci (2018) 53:11823–11836 11827 Microstructure observation an e-martensite phase was not revealed in the X-ray analysis, probably due to the small volume of this Microstructure observation after plastic deformation phase beyond the detection threshold. Microscopy observations at greater magnifications The microstructure of the cross sections is severely reveal that the microstructures of the PR and HPT refined in both the PR- and HPT-processed samples, samples differ considerably. The microstructure of as presented in Fig. 3. The microstructure observa- the PR sample in cross section is non-homogeneous. tions are supported by the selected area diffraction It consists of elongated deformation bands divided (SAED) patterns from an area having a diameter of into subgrains of thickness in the range of approximately 4 lm. The presence of diffraction 50–100 nm and length of 100–300 nm, and defor- rings in the SAED confirms that the microstructure mation nanotwins of thickness of 5–10 nm, as has been refined. However, the non-uniform intensity shown in detail in Fig. 4. In the case of the HPT of the rings in the SAED of the PR sample indicates sample, in cross section the microstructure has been that the microstructure is more textured after PR than transformed into small fragments forming after HPT. Moreover, the SAED patterns indicate that nanocrystallites. The average grain size is below the PR- and HPT-processed samples consist of an c- 100 nm. Inside some grains, one can notice a high austenite phase. In the case of the HPT-processed density of dislocations (Fig. 5). It was also observed sample, one can notice, apart from the c-austenite that the longest grain axis is oriented parallel to the phase (fcc), a weak ring from e-martensite phase HPT shear plane. However, as this has been the (hcp). Some authors suggest that e-martensite is not a subject of previous research, this issue will not be perfect hcp structure, but consider it as a heavily further explored here [39, 40]. faulted fcc c-austenite structure with a special arrangement of stacking faults [38]. The presence of Figure 3 Microstructures of the a PR- and b HPT-processed samples—cross sections, SAED patterns of c PR- and d HPT-processed samples. J Mater Sci (2018) 53:11823–11836 Figure 4 Microstructures of the PR-processed sample—cross area of 0.6 lm in diameter, e deformation nanotwins in the dark section: a global view, b deformation bands, c deformation field, f matrix in the dark field. nanotwins in the bright field, d SAED pattern from nanotwinned Microstructure observations after annealing microstructures of HPT_10_0.1 MPa and PR_10_0.1 MPa leads to the conclusion that the The microstructures of the PR samples and HPT samples are fully recrystallized. The average equiv- samples after annealing at 900 C under 0.1 MPa, 2 alent diameter is greater for the PR_10_0.1 MPa than and 6 GPa for 10 min are shown in Fig. 6 (in BSE for the HPT_10_0.1 MPa (4.3 and 2.0 lm, respec- mode SEM) and Fig. 7 (TEM). The average equivalent tively). Furthermore, the microstructure elongation diameter d , standard deviation SD (d ) and coeffi- 2 2 factor values are comparable (1.35 and 1.28, respec- cient of variation CV(d ) of grains are presented in the tively, for PR_10_0.1 MPa and HPT_10_0.1 MPa), form of charts in Fig. 7. An analysis of the which indicates fully equiaxial grain structure. J Mater Sci (2018) 53:11823–11836 11829 Figure 5 Microstructures of the HPT-processed sample—a in the bright field, b in the dark field, obtained by selecting a part of a diffraction ring from the (111) planes. Figure 6 a Microstructures of PR-processed samples after anneal- 900 C for 10 min under 0.1 MPa, 2 and 6 GPa; c microstructures ing at 900 C for 10 min under 0.1 MPa, 2 GPa and 6 GPa; of PR- and HPT-processed samples after annealing at 900 C for b microstructures of HPT-processed samples after annealing at 4 min under 0.1 MPa; SEM in BSE mode. J Mater Sci (2018) 53:11823–11836 Figure 7 a Microstructures of PR-processed samples after anneal- 900 C for 10 min under 0.1 MPa, 2 and 6 GPa; c microstructures ing at 900 C for 10 min under 0.1 MPa, 2 and 6 GPa; of PR- and HPT-processed samples after annealing at 900 C for b microstructures of HPT-processed samples after annealing at 4 min under 0.1 MPa; TEM. Nevertheless, they differ in the value of the coeffi- and only 0.087 lm under 6 GPa. Between the cient of variation, which is significantly higher for the recrystallized areas, one can notice areas where there PR_10_0.1 MPa (0.79) than for the HPT_10_0.1 MPa has been no recrystallization. In the case of the HPT (0.47). This implies that the distribution of the samples, the equivalent diameter reaches 1.35 lm equivalent diameter in the PR_10_0.1 MPa is wider under 2 GPa and 0.58 lm under 6 GPa, which is than in the HPT_10_0.1 MPa. The smaller grains in greater than in the PR samples. Moreover, in contrast the PR_10_0.1 MPa contain some dislocations, to the PR samples, there are no non-recrystallized whereas the HPT_10_0.1 MPa contains grains free areas between the recrystallized grains. As well as from dislocations. investigating the effect of high hydrostatic pressure Applied annealing pressure at 900 C hinders on the equivalent diameter of the PR and HPT sam- recrystallization and grain growth in the PR-pro- ples, attention was paid to the impact of high cessed samples and grain growth in the HPT-pro- hydrostatic pressure on the elongation parameter and cessed samples. In the case of the PR samples, the coefficient of variation. It seems that the high equivalent diameter reaches 0.42 lm under 2 GPa hydrostatic pressure applied had no impact on the J Mater Sci (2018) 53:11823–11836 11831 elongation parameter, which was approximately 1.3 differences are visible after annealing for 4 and for all annealing conditions for both the PR and HPT 10 min. After 4 min of annealing, it can be noticed samples. Nevertheless, the pressure applied during that in the PR-processed samples discontinuous annealing had a significant impact on the coefficient recrystallization occurred, whereas in the HPT-pro- of variation. There are two tendencies visible. In the cessed samples the recrystallization was continuous. case of the PR samples, the coefficient of variation This discontinuous recrystallization (frequently decreases with an increase of pressure from 0.79 called primary recrystallization) is the result of an under 0.1 MPa to 0.46 under 6 GPa, whereas in the inhomogeneous microstructure produced during case of the HPT samples it increases from 0.47 under deformation. If the microstructure after deformation 0.1 MPa to 0.64 under 6 GPa. It means that the is inhomogeneous, which is true for low-to-moderate abnormal grain growth, represented by a high coef- strains (in the case of the PR-processed samples ficient of variation, is favorized in the PR sample - e = 3.4) during deformation, it means that there are under the atmospheric pressure and in the HPT preferred sites for the formation of nuclei having high sample under the increased pressure. local misorientations, e.g., highly misoriented regions To verify whether annealing under high hydro- within deformation bands [41]. The process of static pressure leads to the creation of similar heterogeneous nucleation leads, over a longer microstructures as does conventional annealing, but annealing time, to a microstructure that is fully over a shorter time, additional experiments of recrystallized but of diversified grain size. In the case annealing under 0.1 MPa were performed. The of the austenitic stainless steel, continuous recrystal- microstructures obtained are presented in Figs. 6 and lization was reported for a total strain of 6.4 reached 7. The average equivalent diameter d , standard during multi-axial compression [42]. During anneal- deviation SD (d ) and coefficient of variation CV(d ) ing of the HPT-processed sample that was deformed 2 2 of the grains are presented in Fig. 8. This experiment to a high strain of 79, the strain-induced high-angle makes it possible to observe that the retardation of grain boundaries change to conventional ones. As a result, homogeneous nucleation and, for longer annealing induced by annealing at 2 GPa for 10 min in the case of the PR samples can be compared with times, normal grain growth were perceived. This annealing for 4 min under 0.1 MPa (d = 0.42, d / kind of behavior has been previously observed in 2 max d = 1.31, CV(d ) = 0.78—for PR_10_2 GPa, d = 0.44, austenitic stainless steels deformed by HPT and 2 2 2 d /d = 1.39, CV(d ) = 0.74—for PR_4_0.1 MPa). In subsequently annealed [43]. max 2 2 the case of the HPT samples, 4-min annealing at Moreover, after 10 min of annealing, the HPT- 0.1 MPa causes similar changes in the microstructure processed samples showed smaller grain size than as annealing at 6 GPa for 10 min (d = 0.58, d / the PR-processed samples. The visibly enhanced 2 max d = 1.31, CV(d ) = 0.64—for HPT_10_6 GPa, thermal stability here may be explained by the fact 2 2 d = 0.55, d /d = 1.33, CV(d ) = 0.54—for that, during heating, the recovery of the non-equi- 2 max 2 2 HPT_4_0.1 MPa). One difference in the microstruc- librium grain boundary structure (definitely present ture between HPT_10_6GPa and HPT_4_0.1 MPa lies in a higher volume in the HPT-processed samples in the value of the coefficient of variation, which is than in the PR-processed samples) proceeds quite significantly higher for annealing at 6 GPa than at rapidly due to their high diffusivity [44]. Non-equi- 0.1 MPa. librium grain boundaries are specific grain bound- aries that possess an increased free energy density, increased width, a high density of dislocations asso- Discussion ciated with the near-boundary region, and corre- spondingly large residual microstrains [8]. This leads Comparison of the thermal stability of HPT- to a rapid decrease in the driving force of the grain and PR-processed samples under 0.1 MPa growth. Another important factor explaining the enhanced thermal stability of a HPT sample is the Even though the TEM observations confirmed the more uniform microstructure after deformation in the refinement of the microstructures of the HPT- and case of HPT than in a PR-processed sample. The PR-processed samples, their behavior during impact of the uniform microstructure on the thermal annealing under 0.1 MPa was different. The stability was proven in the experiment on J Mater Sci (2018) 53:11823–11836 Figure 8 a Average equivalent diameter d , b elongation factor d /d , and c coefficient of variation CV(d ) of grains in PR- and HPT- 2 max 2 2 processed samples after annealing at 900 C for 10 min under 0.1 MPa, 2 and 6 GPa and for 4 min under 0.1 MPa. molybdenum [9]. The thermal stability of molybde- found that a pressure of 4.2 GPa applied while num processed by HPT exceeded significantly the annealing polycrystalline copper cold-rolled to 98% respective one for multi-step forging despite the retarded both the initiation and the rate of recrystal- lower deformation degree in the latter case, which lization [20]. In another work, it was proved that that was related to the more homogeneous grain size grain growth decreased by a factor of 1.3 under a distribution in the former case. As a result, the high pressure of 1.2 GPa in aluminum rolled to a 90% mobility of grain boundaries decreases, which results reduction [26]. A similar effect of recrystallization in their enhanced thermostability. retardation was observed during annealing at 300 C of an Al-2%Mg alloy under an applied stress of Impact of high hydrostatic pressure 10 MPa [45]. on recrystallization and grain growth In the present study, one fact that requires some in nanostructured austenitic steel 316LVM explanation is the considerable difference in the retardation of the rate of recrystallization and grain The above results indicate that hydrostatic pressures growth between the PR- and HPT-processed samples. of 2 and 6 GPa have a considerable impact on the Such an explanation must take into consideration the process of recrystallization and grain growth in the different nature of the grain boundaries in the PR- PR samples and mainly on grain growth in the HPT and HPT-processed samples, despite the fact that samples. The fact that increasing the annealing both samples possess highly refined microstructures. pressure retards recrystallization and grain growth 1. Firstly, in the PR-processed samples, most of the has been investigated in the past in numerous studies microstructure is occupied by nanotwins and [20–27]. For example, in one such early study, it was J Mater Sci (2018) 53:11823–11836 11833 elongated deformation bands consisting of sub- samples. This means that even though the grains, whereas in the HPT-processed samples vacancy concentration decreases with increasing there is a preponderance of nanograins. In pre- pressure during annealing, in the HPT-processed vious studies on micrograined materials, it was samples it must be much higher than in the PR- explained that low-angle grain boundaries, pre- processed samples during annealing under high sent in the PR samples, move by the vacancy hydrostatic pressure, enabling dislocation climb- diffusion mechanism [26]. However, the vacancy ing and the migration of grain boundaries even concentration in the material decreases with under a pressure of 6 GPa. increasing pressure during annealing [26]. For 3. Thirdly, in the HPT-processed samples one can this reason, the movement of low-angle grain expect to find a high density of non-equilibrium boundaries is strongly slowed down during grain boundaries. The specific structure of grain annealing under high hydrostatic pressure. This boundaries affects diffusivity, which is much explains the fact that there are non-recrystallized higher than in the case of general high-angle areas in the PR-processed samples after annealing grain boundaries and highly defected twin under high hydrostatic pressure. Moreover, in the boundaries. It is also possible that there is a PR samples there is a high density of nanotwin certain fraction of non-equilibrium grain bound- boundaries viewed as 60\111[twist boundaries aries present in the PR-processed samples, as it is or 70.5 \110[ tilt boundaries. It is known that highly deformed. Nevertheless, that value is \110[tilt boundaries move by some cooperative much lower than in the HPT-processed samples. motion of several atoms, while\100[ and\111[ During annealing under 0.1 MPa, the rapid tilt boundaries move by a single atom mecha- recovery of such boundaries may decrease their nism, which can be achieved more easily under mobility. In contrast, under high pressure, such a high hydrostatic pressure [21, 22]. This fact also rapid recovery may be suppressed, and the explains the retardation of recrystallization under enhanced diffusivity and excess energy of the high hydrostatic pressure in the PR-processed non-equilibrium boundaries may act as a driving samples. A strong retardation of grain growth force for grain growth. was also observed in a nanotwinned austenitic stainless steel refined by HE after annealing for Does annealing under high hydrostatic 10 min under a pressure of 6 GPa (d = 133 nm). pressure result in the same microstructure The behavior of HE-processed and PR-processed as under atmospheric pressure? samples during annealing under high hydrostatic pressure confirms that the migration of twins In order to answer this question, experiments in boundaries is hampered under high pressure. which PR- and HPT-processed samples were 2. Secondly, these samples differ in the equivalent annealed for 4 min under 0.1 MPa were performed. strain applied during deformation, which was 3.4 The results were compared with the microstructures and 79 for the PR- and HPT-processed samples, obtained for annealing under 2 and 6 GPa for 10 min. respectively. According to previous studies, the It was discovered that, in the case of the PR-pro- higher the plastic deformation and hydrostatic cessed samples, the microhardness and microstruc- pressure applied during deformation, the higher ture after 4 min of annealing under 0.1 MPa are the density of vacancies in the material [46, 47]. comparable to the values obtained after 10 min under The excess vacancy concentration in pure Cu and 2GPa. However, they differ slightly in the percentage Ni samples processed by HPT can achieve values of the recrystallized area, which is greater for the of (0.9–20)*10^(-4) [46, 47], whereas in a’-marten- PR_10_2GPa—approximately 80% than the site processed by HPT the equivalent value are PR_4_0.1 MPa—approximately 70%. The HPT-pro- (5.2 ± 3.6)*10^(-4) [48]. Since austenitic stainless cessed samples annealed for 4 min under 0.1 MPa steel is a material having a low stacking fault were fully recrystallized and can be compared with energy and a high melting temperature, one can the samples annealed for 10 min under 6 GPa. predict that a high density of agglomerated However, they differ slightly in the value of the vacancies will appear in the HPT-processed coefficient of variation, which is greater for the samples, as opposed to the PR-processed J Mater Sci (2018) 53:11823–11836 International License (http://creativecommons.org/ samples annealed under high hydrostatic pressure. licenses/by/4.0/), which permits unrestricted use, This might result from the fact that under high distribution, and reproduction in any medium, pro- pressure applied during annealing the migration of vided you give appropriate credit to the original vacancies is highly reduced and various grain author(s) and the source, provide a link to the Crea- boundaries tend to migrate by various mechanisms tive Commons license, and indicate if changes were and consequently at various rates [49]. made. 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Recrystallization and grain growth of a nano/ultrafine structured austenitic stainless steel during annealing under high hydrostatic pressure

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Materials Science; Materials Science, general; Characterization and Evaluation of Materials; Polymer Sciences; Continuum Mechanics and Mechanics of Materials; Crystallography and Scattering Methods; Classical Mechanics
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J Mater Sci (2018) 53:11823–11836 METALS Metals Recrystallization and grain growth of a nano/ultrafine structured austenitic stainless steel during annealing under high hydrostatic pressure 1, 2 1 3 Agnieszka Teresa Krawczynska * , Stanislaw Gierlotka , Przemyslaw Suchecki , Daria Setman , 1 1 3 Boguslawa Adamczyk-Cieslak , Malgorzata Lewandowska , and Michael Zehetbauer Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland Institute of High Pressure Physics UNIPRESS, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria Received: 20 December 2017 ABSTRACT Accepted: 14 May 2018 The aim of this study was to investigate the effect of high hydrostatic pressure Published online: applied during annealing on the processes of recrystallization and grain growth 21 May 2018 in nanostructured austenitic stainless steel 316LVM. The nanostructures were obtained by profile rolling to a total strain of 3.4 and by high-pressure torsion to The Author(s) 2018 a total strain of 79. These processes resulted in microstructures consisting of nanotwins and nanograins, respectively. The deformed samples were annealed at 900 C for 10 min under atmospheric or hydrostatic pressures of 2 and 6 GPa. The resulting microstructures were examined using transmission and scanning electron microscopy techniques. The mechanical properties were evaluated in microhardness measurements. It was established that annealing under high hydrostatic pressure retards recrystallization and grain growth, both in profile- rolled and high-pressure torsion-processed samples. The magnitude of retar- dation depends on the character of the grain boundaries. The non-equilibrium high-angle grain boundaries present in the high-pressure torsion-processed sample show higher mobility under pressure than the nanotwinned and low- angle boundaries in the profile-rolled sample. recently in nanocrystalline, materials [3–5]. During Introduction the annealing of nanocrystalline materials produced Recovery, recrystallization and grain growth are by severe plastic deformation (SPD) techniques, one some of the most important processes that affect the can observe typical processes of recovery, recrystal- properties of crystalline materials. For this reason, lization and grain growth, but they follow a different many papers have been devoted to understanding course than in materials deformed by conventional these phenomena in microcrystalline [1, 2], and techniques [6–8]. In general, nanomaterials are less Address correspondence to E-mail: akrawczynska@wp.pl; agnieszka.krawczynska@pw.edu.pl https://doi.org/10.1007/s10853-018-2459-1 J Mater Sci (2018) 53:11823–11836 thermally stable than their microcrystalline counter- V¼RTlnðmÞ=p ð1Þ parts, and recrystallization begins below their usual where R is the gas constant, m is the rate of the pro- recrystallization temperatures. However, it has also cesses investigated and p is the pressure. been shown that, by creating uniform nanostructures Furthermore, it was found during annealing of having a uniform grain size distribution and a high cold-rolled copper under a pressure of 4.2 GPa that content of high-angle grain boundaries, thermal sta- the pressure retarded recrystallization [20]. The gen- bility can be significantly improved [9–11]. The eral conclusion that can be drawn from this work enhanced thermal stability of a nanostructured aus- regarding grain growth is that the higher the hydro- tenitic stainless steel can be obtained by adding static pressure applied, the more significant the yttrium powders during mechanical milling [12]. The decrease in grain boundary mobility. Moreover, the addition of 1.5 wt% stabilizes the microstructure impact of high pressure on the \100[, \110[ and around 116 nm grain size after 3 h annealing at \111[ tilt boundaries was analyzed [18, 22]. The 1100 C. Moreover, the creation of deformation-in- results showed that the movement of \110[ tilt duced nanotwins makes the microstructure stable up boundaries depends on the activation energy, as this to 800 C[13]. The processes of recrystallization and movement is effected by the cooperative motion of grain growth in nanostructured, and especially sin- several atoms. The movement of the \100[ and gle-phased, materials have been quite well described. \111[ tilt boundaries is not related to the activation Nevertheless, one issue which remains almost com- energy and is effected by a single atom mechanism. It pletely neglected is how such materials behave dur- was also noticed during experiments on normal grain ing annealing under high hydrostatic pressure. This growth in aluminum under high hydrostatic pressure subject has been raised only in the author’s previous that the migration of low-angle grain boundaries was work [14, 15]. In those studies, annealing under high slowed down even more, nearly frozen in compar- hydrostatic pressure was applied to a hydrostatically ison with the case of general grain boundaries [26]. extruded (HE) austenitic stainless steel to optimize its This is because low-angle grain boundaries can mechanical properties, particularly its strength to migrate by vacancy grain boundary migration, which ductility balance. It was possible to achieve a con- is highly limited under high pressure. trolled slowing of recrystallization processes to pro- In the view of published work within the subject of duce a nanostructured austenitic stainless steel annealing under the high hydrostatic pressure, it possessing a good combination of strength and duc- seems that annealing under high hydrostatic pressure tility (an ultimate tensile strength of 1247 MPa and a can be an effective way to control the grain growth in total elongation of 24.4%). The only experiments that nanostructured materials and produce fine grain have been performed under high pressure recently materials which could not be obtained under atmo- refer to the Ge precipitation rate in Ge? ion-im- spheric pressure due to fast and uncontrollable grain planted SiO films [16], a study on the effect of growth. However, this requires full understanding of annealing under pressure on the material properties processes taking place during annealing under high of Cu2ZnSn(S,Se)4 thin films [17] and an enhance- hydrostatic pressure. Therefore, the aim of the cur- ment of magnetic refrigeration performance in rent study was to investigate the influence of high metamagnetic MnCoSi alloy by high-pressure pressure on the recrystallization and grain growth of annealing [18]. a nanostructured stainless steel 316LVM. To this end, However, several authors have studied the effect of specimens of an austenitic stainless steel were hydrostatic pressure on diffusion, dislocation climb deformed by profile rolling (PR)—a conventional and glide, recrystallization and grain growth mobility deformation technique, and by high-pressure torsion in microcrystalline materials [19–27]. It is well known (HPT)—an SPD technique; the samples were then that high hydrostatic pressure has an impact on dif- annealed under a pressure of 2 and 6 GPa and, for fusion processes that are correlated with the motion comparison, under atmospheric pressure. The HPT of vacancies, as it affects the activation volume V*of process was chosen because it is one of the most the crystal related to atomic rearrangements during efficient SPD processes for grain refinement [28–33]. thermally activated processes. The influence of pres- sure on V* can be expressed by the equation: J Mater Sci (2018) 53:11823–11836 11825 of the sample for the annealing experiments. The Materials and methods dimensions of the samples were limited by the Sandvik Bioline 316LVM austenitic stainless steel was dimensions of the toroidal high-pressure cell. The used; this is a low-carbon, vacuum-melted 316L samples were annealed at 900 C for 10 min, either at grade stainless steel, UNS S31673 certified to ASTM atmospheric pressure (0.1 MPa) or at a hydrostatic F138, supplied as annealed in the form of 50-mm- pressure of 2 or 6 GPa and. To verify whether diameter rods, possessing the chemical composition annealing under high hydrostatic pressure leads to shown in Table 1. the creation of similar microstructures as conven- The samples were conventionally deformed using tional annealing for a shorter time, additional profile rolling (PR) with a reduction in cross section experiments of annealing under 0.1 MPa for 4 min of 23.8, which corresponds to a strain value of were performed. A toroidal high-pressure cell [34, 35] approximately 3.4. In this case, the strain was calcu- was used for the high-pressure annealing, and a lated according to the equation e = 2ln (d /d ), where conventional furnace for annealing at atmospheric 1 2 d (/ = 12 mm) is the initial diameter and d (/ = pressure. Further on in this text, these samples will be 1 2 2.2 mm) the final diameter. It must be pointed out referred to as PR_10_0.1 MPa, PR_10_2GPa, that this calculation is only an approximation, since PR_10_6GPa, PR_4_0.1 MPa, HPT_10_0.1 MPa, the cross section is not actually a circle. Additionally, HPT_10_2GPa, HPT_10_6GPa and HPT_4_0.1 MPa. the material was cut into disks with a diameter of The microstructure of the samples was investigated 10 mm and a thickness of 0.8 mm. The disks were using a Hitachi SU 8000 scanning electron micro- processed at room temperature using an HPT device scope working at 5 kV in the BSE mode and a JEOL at a constant pressure of 6.0 GPa. The disks were JEM 1200 EX transmission electron microscope torsionally strained to 5 revolutions. The strain was working at 120 kV. For the scanning electron micro- well defined as simple shear, c, and was calculated scopy examination, the samples were prepared by according to the equation c =2p 9 r 9 n/t, where r, electropolishing using Struers electrolyte A3. The n and t are the distance from the torsion axes, the parameters for electropolishing were as follows: number of applied revolutions and the mean thick- voltage - 15 V, time - 15 s. For the transmission ness of the sample, respectively. The equivalent electron microscopy examination, the samples were strains eeq = c/H3 calculated 3.5 mm from the cen- prepared by mechanical polishing to a disk thickness tral point of the sample after 5 revolutions were equal of about 100 lm. Further thinning to reach a thick- to 79. A phase analysis of the as-received, HPT- and ness appropriate for electron transparency was car- PR-processed samples was performed on a Bruker D8 ried out by electropolishing using Struers electrolyte Advance diffractometer with nickel-filtered copper A2. In the case of the PR samples, the microstructure radiation (k = 0.154056 nm). The data were collected was analyzed in the center of the samples. In the case in a range between 10 and 120 2H, with a step of the HPT samples, for microstructure analysis of the width of D2H = 0.02 and a counting time 5 s. The cross sections, disks with a diameter of 3 mm were energy of the emitter beam was 40 kV, and the cur- cut from the edge regions of each disk so that the rent was 40 mA. areas of observation were 3.5 mm from the central After PR, samples of 3 mm in height were cut from point. Further thinning was carried out by means of a a 500 mm rod. After the HPT experiments, disks of conventional procedure. Qualitative and quantitative 5 mm in diameter were cut in such a way that the studies as well as the percentage share of the radius of the sample after HPT became the diameter recrystallized areas of the microstructures were con- ducted using stereological and image analysis meth- ods [36, 37]. The grain size was determined as the Table 1 Chemical composition (wt%) of austenitic stainless steel equivalent diameter, d , defined as the diameter of a 316LVM 2 circle having an area equal to the surface area of a C Si Mn PSCr Ni Mo Cu N given grain. The grain shape was described by grain elongation factor, defined as the ratio of the maxi- 0.025 0.6 1.7 0.025 0.003 17.5 13.5 2.8 0.1 \0.1 mum to the equivalent diameter d /d . To establish max 2 the variation of the size of individual grains, a vari- ation coefficient, CV(d ), defined as the ratio of the 2 J Mater Sci (2018) 53:11823–11836 standard deviation SD (d ) to the mean value, was determined. Microhardness measurements were conducted on polished cross sections of the PR and HPT samples. These measurements were made using a Zwick microhardness tester under a load of 200 g. The val- ues of the Vickers microhardness, Hv, were recorded along a diameter of the PR and HPT samples with a separation of 0.1 mm. Results X-ray diffraction analysis Figure 1 shows X-ray diffraction profiles of the as- received, PR- and HPT-processed samples. The X-ray diffraction profiles reveal a c-austenite phase (fcc) without any evidence of e-martensite (hcp) or a’- martensite (bcc). The greatest broadening of peaks is observed for the HPT sample and is a result of microstructure refinement and microstrains. Microhardness measurements Microhardness measurements on cross sections of the Figure 2 Microhardness Hv0.2 of PR- a and HPT-processed PR- and HPT-processed samples after annealing at b austenitic stainless steel annealed under 0.1 MPa and 2 and 900 C under various pressures for 4 and 10 min are 6 GPa at 900 C for 10 min and additionally for 4 min under presented in Fig. 2. After HPT, the microhardness 0.1 MPa. reaches an average value of 471 Hv0.2, which is deviation equals 14 Hv0.2, whereas after PR it is higher by 46 units than after PR. Moreover, the HPT twice as high (it must be pointed out that the diam- causes a more homogeneous distribution of micro- eter of the HPT sample is two times greater than that hardness on the diameter since the standard after PR). Annealing under high hydrostatic pressure has a considerable impact on the microhardness values of HPT- and PR-processed samples. There is a tendency for both samples that the higher the pres- sure during annealing, the higher the microhardness value retained. However, the PR samples showed greater microhardness values for annealing under a pressure of 2 and 6 GPa (301 and 390 Hv0.2, respec- tively) than the HPT samples (254 and 282 Hv0.2, respectively). Additionally, annealing was performed for 4 min at 0.1 MPa. In the case of the PR samples, the microhardness value after annealing for 4 min can be compared with that obtained for annealing under 2 GPa for 10 min (287 and 301 Hv0.2, respec- tively). In the case of the HPT samples, annealing for 4 min at 0.1 MPa causes changes in the microhard- ness similar to annealing under 6 GPa (282 and 284 Figure 1 X-ray diffraction profiles of as-received, PR-, and HPT- Hv0.2, respectively). processed samples. J Mater Sci (2018) 53:11823–11836 11827 Microstructure observation an e-martensite phase was not revealed in the X-ray analysis, probably due to the small volume of this Microstructure observation after plastic deformation phase beyond the detection threshold. Microscopy observations at greater magnifications The microstructure of the cross sections is severely reveal that the microstructures of the PR and HPT refined in both the PR- and HPT-processed samples, samples differ considerably. The microstructure of as presented in Fig. 3. The microstructure observa- the PR sample in cross section is non-homogeneous. tions are supported by the selected area diffraction It consists of elongated deformation bands divided (SAED) patterns from an area having a diameter of into subgrains of thickness in the range of approximately 4 lm. The presence of diffraction 50–100 nm and length of 100–300 nm, and defor- rings in the SAED confirms that the microstructure mation nanotwins of thickness of 5–10 nm, as has been refined. However, the non-uniform intensity shown in detail in Fig. 4. In the case of the HPT of the rings in the SAED of the PR sample indicates sample, in cross section the microstructure has been that the microstructure is more textured after PR than transformed into small fragments forming after HPT. Moreover, the SAED patterns indicate that nanocrystallites. The average grain size is below the PR- and HPT-processed samples consist of an c- 100 nm. Inside some grains, one can notice a high austenite phase. In the case of the HPT-processed density of dislocations (Fig. 5). It was also observed sample, one can notice, apart from the c-austenite that the longest grain axis is oriented parallel to the phase (fcc), a weak ring from e-martensite phase HPT shear plane. However, as this has been the (hcp). Some authors suggest that e-martensite is not a subject of previous research, this issue will not be perfect hcp structure, but consider it as a heavily further explored here [39, 40]. faulted fcc c-austenite structure with a special arrangement of stacking faults [38]. The presence of Figure 3 Microstructures of the a PR- and b HPT-processed samples—cross sections, SAED patterns of c PR- and d HPT-processed samples. J Mater Sci (2018) 53:11823–11836 Figure 4 Microstructures of the PR-processed sample—cross area of 0.6 lm in diameter, e deformation nanotwins in the dark section: a global view, b deformation bands, c deformation field, f matrix in the dark field. nanotwins in the bright field, d SAED pattern from nanotwinned Microstructure observations after annealing microstructures of HPT_10_0.1 MPa and PR_10_0.1 MPa leads to the conclusion that the The microstructures of the PR samples and HPT samples are fully recrystallized. The average equiv- samples after annealing at 900 C under 0.1 MPa, 2 alent diameter is greater for the PR_10_0.1 MPa than and 6 GPa for 10 min are shown in Fig. 6 (in BSE for the HPT_10_0.1 MPa (4.3 and 2.0 lm, respec- mode SEM) and Fig. 7 (TEM). The average equivalent tively). Furthermore, the microstructure elongation diameter d , standard deviation SD (d ) and coeffi- 2 2 factor values are comparable (1.35 and 1.28, respec- cient of variation CV(d ) of grains are presented in the tively, for PR_10_0.1 MPa and HPT_10_0.1 MPa), form of charts in Fig. 7. An analysis of the which indicates fully equiaxial grain structure. J Mater Sci (2018) 53:11823–11836 11829 Figure 5 Microstructures of the HPT-processed sample—a in the bright field, b in the dark field, obtained by selecting a part of a diffraction ring from the (111) planes. Figure 6 a Microstructures of PR-processed samples after anneal- 900 C for 10 min under 0.1 MPa, 2 and 6 GPa; c microstructures ing at 900 C for 10 min under 0.1 MPa, 2 GPa and 6 GPa; of PR- and HPT-processed samples after annealing at 900 C for b microstructures of HPT-processed samples after annealing at 4 min under 0.1 MPa; SEM in BSE mode. J Mater Sci (2018) 53:11823–11836 Figure 7 a Microstructures of PR-processed samples after anneal- 900 C for 10 min under 0.1 MPa, 2 and 6 GPa; c microstructures ing at 900 C for 10 min under 0.1 MPa, 2 and 6 GPa; of PR- and HPT-processed samples after annealing at 900 C for b microstructures of HPT-processed samples after annealing at 4 min under 0.1 MPa; TEM. Nevertheless, they differ in the value of the coeffi- and only 0.087 lm under 6 GPa. Between the cient of variation, which is significantly higher for the recrystallized areas, one can notice areas where there PR_10_0.1 MPa (0.79) than for the HPT_10_0.1 MPa has been no recrystallization. In the case of the HPT (0.47). This implies that the distribution of the samples, the equivalent diameter reaches 1.35 lm equivalent diameter in the PR_10_0.1 MPa is wider under 2 GPa and 0.58 lm under 6 GPa, which is than in the HPT_10_0.1 MPa. The smaller grains in greater than in the PR samples. Moreover, in contrast the PR_10_0.1 MPa contain some dislocations, to the PR samples, there are no non-recrystallized whereas the HPT_10_0.1 MPa contains grains free areas between the recrystallized grains. As well as from dislocations. investigating the effect of high hydrostatic pressure Applied annealing pressure at 900 C hinders on the equivalent diameter of the PR and HPT sam- recrystallization and grain growth in the PR-pro- ples, attention was paid to the impact of high cessed samples and grain growth in the HPT-pro- hydrostatic pressure on the elongation parameter and cessed samples. In the case of the PR samples, the coefficient of variation. It seems that the high equivalent diameter reaches 0.42 lm under 2 GPa hydrostatic pressure applied had no impact on the J Mater Sci (2018) 53:11823–11836 11831 elongation parameter, which was approximately 1.3 differences are visible after annealing for 4 and for all annealing conditions for both the PR and HPT 10 min. After 4 min of annealing, it can be noticed samples. Nevertheless, the pressure applied during that in the PR-processed samples discontinuous annealing had a significant impact on the coefficient recrystallization occurred, whereas in the HPT-pro- of variation. There are two tendencies visible. In the cessed samples the recrystallization was continuous. case of the PR samples, the coefficient of variation This discontinuous recrystallization (frequently decreases with an increase of pressure from 0.79 called primary recrystallization) is the result of an under 0.1 MPa to 0.46 under 6 GPa, whereas in the inhomogeneous microstructure produced during case of the HPT samples it increases from 0.47 under deformation. If the microstructure after deformation 0.1 MPa to 0.64 under 6 GPa. It means that the is inhomogeneous, which is true for low-to-moderate abnormal grain growth, represented by a high coef- strains (in the case of the PR-processed samples ficient of variation, is favorized in the PR sample - e = 3.4) during deformation, it means that there are under the atmospheric pressure and in the HPT preferred sites for the formation of nuclei having high sample under the increased pressure. local misorientations, e.g., highly misoriented regions To verify whether annealing under high hydro- within deformation bands [41]. The process of static pressure leads to the creation of similar heterogeneous nucleation leads, over a longer microstructures as does conventional annealing, but annealing time, to a microstructure that is fully over a shorter time, additional experiments of recrystallized but of diversified grain size. In the case annealing under 0.1 MPa were performed. The of the austenitic stainless steel, continuous recrystal- microstructures obtained are presented in Figs. 6 and lization was reported for a total strain of 6.4 reached 7. The average equivalent diameter d , standard during multi-axial compression [42]. During anneal- deviation SD (d ) and coefficient of variation CV(d ) ing of the HPT-processed sample that was deformed 2 2 of the grains are presented in Fig. 8. This experiment to a high strain of 79, the strain-induced high-angle makes it possible to observe that the retardation of grain boundaries change to conventional ones. As a result, homogeneous nucleation and, for longer annealing induced by annealing at 2 GPa for 10 min in the case of the PR samples can be compared with times, normal grain growth were perceived. This annealing for 4 min under 0.1 MPa (d = 0.42, d / kind of behavior has been previously observed in 2 max d = 1.31, CV(d ) = 0.78—for PR_10_2 GPa, d = 0.44, austenitic stainless steels deformed by HPT and 2 2 2 d /d = 1.39, CV(d ) = 0.74—for PR_4_0.1 MPa). In subsequently annealed [43]. max 2 2 the case of the HPT samples, 4-min annealing at Moreover, after 10 min of annealing, the HPT- 0.1 MPa causes similar changes in the microstructure processed samples showed smaller grain size than as annealing at 6 GPa for 10 min (d = 0.58, d / the PR-processed samples. The visibly enhanced 2 max d = 1.31, CV(d ) = 0.64—for HPT_10_6 GPa, thermal stability here may be explained by the fact 2 2 d = 0.55, d /d = 1.33, CV(d ) = 0.54—for that, during heating, the recovery of the non-equi- 2 max 2 2 HPT_4_0.1 MPa). One difference in the microstruc- librium grain boundary structure (definitely present ture between HPT_10_6GPa and HPT_4_0.1 MPa lies in a higher volume in the HPT-processed samples in the value of the coefficient of variation, which is than in the PR-processed samples) proceeds quite significantly higher for annealing at 6 GPa than at rapidly due to their high diffusivity [44]. Non-equi- 0.1 MPa. librium grain boundaries are specific grain bound- aries that possess an increased free energy density, increased width, a high density of dislocations asso- Discussion ciated with the near-boundary region, and corre- spondingly large residual microstrains [8]. This leads Comparison of the thermal stability of HPT- to a rapid decrease in the driving force of the grain and PR-processed samples under 0.1 MPa growth. Another important factor explaining the enhanced thermal stability of a HPT sample is the Even though the TEM observations confirmed the more uniform microstructure after deformation in the refinement of the microstructures of the HPT- and case of HPT than in a PR-processed sample. The PR-processed samples, their behavior during impact of the uniform microstructure on the thermal annealing under 0.1 MPa was different. The stability was proven in the experiment on J Mater Sci (2018) 53:11823–11836 Figure 8 a Average equivalent diameter d , b elongation factor d /d , and c coefficient of variation CV(d ) of grains in PR- and HPT- 2 max 2 2 processed samples after annealing at 900 C for 10 min under 0.1 MPa, 2 and 6 GPa and for 4 min under 0.1 MPa. molybdenum [9]. The thermal stability of molybde- found that a pressure of 4.2 GPa applied while num processed by HPT exceeded significantly the annealing polycrystalline copper cold-rolled to 98% respective one for multi-step forging despite the retarded both the initiation and the rate of recrystal- lower deformation degree in the latter case, which lization [20]. In another work, it was proved that that was related to the more homogeneous grain size grain growth decreased by a factor of 1.3 under a distribution in the former case. As a result, the high pressure of 1.2 GPa in aluminum rolled to a 90% mobility of grain boundaries decreases, which results reduction [26]. A similar effect of recrystallization in their enhanced thermostability. retardation was observed during annealing at 300 C of an Al-2%Mg alloy under an applied stress of Impact of high hydrostatic pressure 10 MPa [45]. on recrystallization and grain growth In the present study, one fact that requires some in nanostructured austenitic steel 316LVM explanation is the considerable difference in the retardation of the rate of recrystallization and grain The above results indicate that hydrostatic pressures growth between the PR- and HPT-processed samples. of 2 and 6 GPa have a considerable impact on the Such an explanation must take into consideration the process of recrystallization and grain growth in the different nature of the grain boundaries in the PR- PR samples and mainly on grain growth in the HPT and HPT-processed samples, despite the fact that samples. The fact that increasing the annealing both samples possess highly refined microstructures. pressure retards recrystallization and grain growth 1. Firstly, in the PR-processed samples, most of the has been investigated in the past in numerous studies microstructure is occupied by nanotwins and [20–27]. For example, in one such early study, it was J Mater Sci (2018) 53:11823–11836 11833 elongated deformation bands consisting of sub- samples. This means that even though the grains, whereas in the HPT-processed samples vacancy concentration decreases with increasing there is a preponderance of nanograins. In pre- pressure during annealing, in the HPT-processed vious studies on micrograined materials, it was samples it must be much higher than in the PR- explained that low-angle grain boundaries, pre- processed samples during annealing under high sent in the PR samples, move by the vacancy hydrostatic pressure, enabling dislocation climb- diffusion mechanism [26]. However, the vacancy ing and the migration of grain boundaries even concentration in the material decreases with under a pressure of 6 GPa. increasing pressure during annealing [26]. For 3. Thirdly, in the HPT-processed samples one can this reason, the movement of low-angle grain expect to find a high density of non-equilibrium boundaries is strongly slowed down during grain boundaries. The specific structure of grain annealing under high hydrostatic pressure. This boundaries affects diffusivity, which is much explains the fact that there are non-recrystallized higher than in the case of general high-angle areas in the PR-processed samples after annealing grain boundaries and highly defected twin under high hydrostatic pressure. Moreover, in the boundaries. It is also possible that there is a PR samples there is a high density of nanotwin certain fraction of non-equilibrium grain bound- boundaries viewed as 60\111[twist boundaries aries present in the PR-processed samples, as it is or 70.5 \110[ tilt boundaries. It is known that highly deformed. Nevertheless, that value is \110[tilt boundaries move by some cooperative much lower than in the HPT-processed samples. motion of several atoms, while\100[ and\111[ During annealing under 0.1 MPa, the rapid tilt boundaries move by a single atom mecha- recovery of such boundaries may decrease their nism, which can be achieved more easily under mobility. In contrast, under high pressure, such a high hydrostatic pressure [21, 22]. This fact also rapid recovery may be suppressed, and the explains the retardation of recrystallization under enhanced diffusivity and excess energy of the high hydrostatic pressure in the PR-processed non-equilibrium boundaries may act as a driving samples. A strong retardation of grain growth force for grain growth. was also observed in a nanotwinned austenitic stainless steel refined by HE after annealing for Does annealing under high hydrostatic 10 min under a pressure of 6 GPa (d = 133 nm). pressure result in the same microstructure The behavior of HE-processed and PR-processed as under atmospheric pressure? samples during annealing under high hydrostatic pressure confirms that the migration of twins In order to answer this question, experiments in boundaries is hampered under high pressure. which PR- and HPT-processed samples were 2. Secondly, these samples differ in the equivalent annealed for 4 min under 0.1 MPa were performed. strain applied during deformation, which was 3.4 The results were compared with the microstructures and 79 for the PR- and HPT-processed samples, obtained for annealing under 2 and 6 GPa for 10 min. respectively. According to previous studies, the It was discovered that, in the case of the PR-pro- higher the plastic deformation and hydrostatic cessed samples, the microhardness and microstruc- pressure applied during deformation, the higher ture after 4 min of annealing under 0.1 MPa are the density of vacancies in the material [46, 47]. comparable to the values obtained after 10 min under The excess vacancy concentration in pure Cu and 2GPa. However, they differ slightly in the percentage Ni samples processed by HPT can achieve values of the recrystallized area, which is greater for the of (0.9–20)*10^(-4) [46, 47], whereas in a’-marten- PR_10_2GPa—approximately 80% than the site processed by HPT the equivalent value are PR_4_0.1 MPa—approximately 70%. The HPT-pro- (5.2 ± 3.6)*10^(-4) [48]. Since austenitic stainless cessed samples annealed for 4 min under 0.1 MPa steel is a material having a low stacking fault were fully recrystallized and can be compared with energy and a high melting temperature, one can the samples annealed for 10 min under 6 GPa. predict that a high density of agglomerated However, they differ slightly in the value of the vacancies will appear in the HPT-processed coefficient of variation, which is greater for the samples, as opposed to the PR-processed J Mater Sci (2018) 53:11823–11836 International License (http://creativecommons.org/ samples annealed under high hydrostatic pressure. licenses/by/4.0/), which permits unrestricted use, This might result from the fact that under high distribution, and reproduction in any medium, pro- pressure applied during annealing the migration of vided you give appropriate credit to the original vacancies is highly reduced and various grain author(s) and the source, provide a link to the Crea- boundaries tend to migrate by various mechanisms tive Commons license, and indicate if changes were and consequently at various rates [49]. made. 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Journal of Materials ScienceSpringer Journals

Published: May 21, 2018

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