TY - JOUR
AU - Awane,, Tohru
AB - Abstract Epoxy resin attached to a fatigue fracture surface of Ti–Al–Nb alloy was removed using a removal method for hardly soluble organic material attached to metallic material, which has been developed by the author. In the removal method process, the epoxy resin attached to the fracture surface was treated with an organic solvent, `tetrahydrofuran', and cold concentrated sulfuric acid of nearly 100% purity. After the epoxy resin was removed from the fracture surface with the removal method, damage of the microscopic feature of the fracture surface was investigated using a scanning electron microscope (SEM). For the first time, the degree of the removal of the epoxy resin with the method was investigated by energy dispersive X-ray spectroscopy (EDS) in this research. After the removal, no damage of the fracture surface was found with SEM observation. In addition, C Kα derived from the epoxy resin was not detected with the EDS after removal. The result of the EDS analysis clarified that the epoxy resin was completely removed with the removal method. SEM sample preparation, fracture surface, epoxy resin, metallic material, Titanium alloy, embedding resin Introduction Microscopic features of fracture surfaces of metallic materials are dependent on many factors, including the type of stress loading, temperature and environmental factors. Therefore, it is vital to observe and analyze the microscopic features for research into the mechanical properties of metallic structural materials or investigation of failure accident of metallic structures. Such microscopic features are usually observed using a scanning electron microscope (SEM). Observations of microstructures around the fracture surfaces are also vital for the research or the investigation. When microstructures of fractured metallic material samples are observed, the samples are embedded into resin such as epoxy resin. Then the samples are polished and chemically etched to reveal the microstructures. Thus, countless samples are embedded into epoxy resins in research institutes for materials, machine manufacturers or manufacturers of metallic materials every day. After the observation of the microstructure of the polished surface, it is necessary to take the sample out from the epoxy resin in the following cases: In case the fracture surface is needed to be reobserved (Fig. 1a). In case of an observation of a region in a crack that occurs when the sample is fractured (Embedding resin before hardening enters the crack) (Fig. 1b). In case of an observation to investigate a relationship between the microscopic feature of the fracture surface and microstructure of the polished surface (Fig. 1c). Fig. 1. Open in new tabDownload slide Observational cases of fractured metallic material samples taken out of embedding resin. (a) In case the fracture surface needs to be reobserved. (b) In case of an observation of a region in a crack that occurs when the sample is fractured. (c) In case of an observation to investigate a relationship between the microscopic feature of the fracture surface and the microstructure of the polished surface. Fig. 1. Open in new tabDownload slide Observational cases of fractured metallic material samples taken out of embedding resin. (a) In case the fracture surface needs to be reobserved. (b) In case of an observation of a region in a crack that occurs when the sample is fractured. (c) In case of an observation to investigate a relationship between the microscopic feature of the fracture surface and the microstructure of the polished surface. However, the fracture surfaces are covered with epoxy resin in any case as shown in Fig. 1. The fracture surface covered with the epoxy resin is impossible to observe. Therefore, the residual resin needs to be removed from the metallic fracture surface without damaging the microscopic feature of the surface. However, any effective method to remove the resin without damaging the microscopic feature of the metallic fracture surface had not been previously known. In the field of biology, Steffens developed a method for the removal of epoxy resins from tissue in preparation for scanning electron microscopy [1]. In this method, a concentrated solution of sodium methoxide, which is strongly alkaline, is used to dissolve the epoxy resin. However, such a method in which a strongly alkaline solution is used cannot be applied to metallic material samples, because the solution corrodes many metallic materials. Particularly, the solution reacts violently with amphoteric substances such as Al, Zn, Sn and Pb. The author has developed an effective method to remove hardly soluble organic materials, such as epoxy resin, from a metallic fracture surface without damaging its microscopic feature [2]. This removal method is unique in terms of using cold concentrated sulfuric acid of nearly 100% purity. In the reference [2], the author showed the effect of the removal method by SEM observations. However, such a SEM observation cannot clarify whether epoxy resin is completely removed or not. Therefore, the degree of epoxy resin removal by this method has been required to be investigated by another analytical method. In this work, the removal method was applied to remove epoxy resin attached to a fatigue fracture surface of Ti–Al–Nb alloy and the effectiveness of the removal method was investigated with an SEM and an EDS. Since the Ti–Al–Nb alloy is greatly expected as a next-generation lightweight and heat-resistant alloy, it is energetically researched all over the world [3–5]. Both the fracture surfaces and microstructures of Ti–Al–Nb alloy samples embedded into epoxy resin are often observed. If the removal method is effective in removing epoxy resin attached to the Ti–Al–Nb alloy, the method will be helpful for the continued research into the Ti–Al–Nb alloy. Materials and methods The apparatus used for the observation of the Ti–Al–Nb sample comprised an SEM: JSM-5400 (JEOL) and an EDS system: JED-2140 (JEOL). At first, secondary electron (SE) images of microscopic features of the fatigue fracture surface sample of the Ti-10.7Al-45.3Nb (mass %) alloy were taken with the SEM. In this research, SE images and X-ray maps of an identical area were taken to compare states of the fracture surface at each phase of the removal method. Then the sample was embedded into epoxy resin. The epoxy resin used for this experiment was specifix-20 (Struers). The resin to embed the sample was made by mixing 7.29 g of base resin and 1.1 g of curing agent. After the sample was embedded into the resin, the sample was treated in the following stages: (1)–(5) as shown in Fig. 2. Cooling of bulk resin with liquid nitrogen and mechanical crushing of the bulk resin (The 1st stage) The fracture surface sample was taken out of the embedding resin by crushing the bulk resin that was made brittle with cooling with liquid nitrogen. The sample taken out of the resin was washed in acetone using an ultrasonic washing machine for 5 min and it was coated with gold using a sputtering device: JFC-1100 (JEOL) to prevent electron charge-up of residual resin in the SEM observation. After that, SE images of the area were taken with the SEM. Moreover, an X-ray map of C Kα emitted from the residual resin and X-ray maps of Nb Lα and Ti Kα emitted from the fracture surface were collected with the EDS system. Swelling of the residual resin with tetrahydrofuran (The 2nd stage) After the 1st stage, the sample was soaked in organic solvent: tetrahydrofuran for 46 h. After that, the sample was washed in tetrahydrofuran using an ultrasonic washing machine for 10 min. After that, the sample was again coated with gold using a sputtering device: JFC-1100 (JEOL), because most of the previous gold coating would have been stripped away after the treatment of the 2nd stage. Then SE images of the area were taken with the SEM. Moreover, X-ray maps of C Kα, Nb Lα and Ti Kα were collected with the EDS system. Decomposition and removal of the residual resin in cold concentrated sulfuric acid (The 3rd stage) After the 2nd stage, the sample was soaked in stock solution of sulfuric acid (made by KANTO CHEMICAL CO., INC, Japan.) for 30 min in a sealed glass bottle with cover. The glass bottle was placed in a room where the temperature was 292 K. Liquid temperature of the sulfuric acid was 292 K both before and after soaking. Since concentrated sulfuric acid of nearly 100% purity hardly ionizes at room temperature, it does not corrode many kinds of metallic materials. On the other hand, since the concentrated sulfuric acid has dehydrating action on organic materials including hydrogen and oxygen in their molecules, the sulfuric acid is capable of decomposing the organic materials. Therefore, the residual resin on the metallic fracture surface can be removed with the dehydrating action without damaging the microscopic feature of the fracture surface. Washing the sample with saturated aqueous sodium hydrogen carbonate solution (The 4th stage) After the sample was soaked in the sulfuric acid, it was washed with saturated aqueous sodium carbonate solution to remove the sulfuric acid from the sample. Since the aqueous sodium hydrogen carbonate solution is weakly basic, it can neutralize the ionized sulfuric acid in the solution. Therefore, damage of the sample due to ionized sulfuric acid can be avoided by this washing method. Washing the sample in pure water (The 5th stage) After the 4th stage, the sample was washed in pure water to remove the solution. After the water washing, the sample was dried in hot air. Additional treatments for SEM observations of the sample in this research After the 5th stage, the sample was washed in acetone using an ultrasonic washing machine for 3 min. Then, the sample was treated with ion etching using JFC-1100 to remove the residual gold film. Since the sample surface was covered with sodium hydrogen carbonate residue, it was washed in pure water using an ultrasonic washing machine for 10 min. After the ultrasonic washing using the water, the sample was treated with ion etching using JFC-1100 to remove the residual gold film again. After that, SE images of the area were taken with the SEM. Moreover, X-ray maps of C Kα, Nb Lα and Ti Kα were collected. Fig. 2. Open in new tabDownload slide Treatments to remove epoxy resin from a metallic sample embedded in epoxy resin. Fig. 2. Open in new tabDownload slide Treatments to remove epoxy resin from a metallic sample embedded in epoxy resin. Results and discussion An SE image of the fracture surface and the X-ray maps of C Kα, Nb Lα, and Ti Kα after the 1st stage are shown in Fig. 3. Since much of the fracture surface was covered with residual resin, microscopic features of the covered areas were impossible to observe. Especially the remaining resin was thicker in the area surrounded by the red dotted line in the range of the SE image shown in Fig. 3b. C Kα emitted from the residual resin was detected, and the intensities of Nb Lα and Ti Kα detected from the area covered with the thicker residual resin were comparatively low as shown in Fig. 3c–e. Before the development of the removal method, the metallic fracture surfaces taken out of the embedding resins had been observed in such a state shown in Fig. 3. Fig. 3. Open in new tabDownload slide Secondary electron (SE) images of a fatigue fracture surface of Ti–Al–Nb alloy and X-ray maps of C Kα emitted from epoxy resin, Ti Kα and Nb Lα emitted from the fracture surface with the EDS analysis method after the embedding resin was cooled with liquid nitrogen and crushed (Stage 1). (a) An SE image of the fracture surface at a low magnification, (b) An SE image of an area of the X-ray maps. (In this research, SE images and X-ray maps of an identical area were taken to compare states of the fracture surface at each phase of the removal method.) (c) An X-ray map of C Kα emitted from the epoxy resin. (d) An X-ray map of Ti Kα emitted from the fracture surface. (e) Nb Lα emitted from the fracture surface. Fig. 3. Open in new tabDownload slide Secondary electron (SE) images of a fatigue fracture surface of Ti–Al–Nb alloy and X-ray maps of C Kα emitted from epoxy resin, Ti Kα and Nb Lα emitted from the fracture surface with the EDS analysis method after the embedding resin was cooled with liquid nitrogen and crushed (Stage 1). (a) An SE image of the fracture surface at a low magnification, (b) An SE image of an area of the X-ray maps. (In this research, SE images and X-ray maps of an identical area were taken to compare states of the fracture surface at each phase of the removal method.) (c) An X-ray map of C Kα emitted from the epoxy resin. (d) An X-ray map of Ti Kα emitted from the fracture surface. (e) Nb Lα emitted from the fracture surface. An SE image of the fracture surface and the X-ray maps of C Kα, Nb Lα and Ti Kα after the 2nd stage are shown in Fig. 4. The residual resin was drastically reduced as shown in Fig. 4a, compared with the state shown in Fig. 3a. The resin in contact with the fracture surface became thinner, but it still remained as shown in the SE image in Fig. 4b. The microscopic feature of the fracture surface could not be observed due to the residual resin. The intensity of C Kα emitted from the residual resin in Fig. 4c was lower compared with that of C Kα in Fig. 3c, because the residual resin was thinner due to the treatment of the 2nd stage. Along with the change in the thickness of the residual resin, the intensity of Nb Lα in Fig. 4e became higher compared with that of Nb Lα in Fig. 3e. Ti Kα in Fig. 4d was hardly detected even after the 2nd stage. Thus, the residual resin could be reduced by soaking the sample in tetrahydrofuran, but the residual resin could not be completely removed from the fracture surface. However, the 2nd stage to reduce the residual resin was useful because the time for soaking the sample in the concentrated sulfuric acid can be shortened in the 3rd stage. Fig. 4. Open in new tabDownload slide SE images of the fracture surface and X-ray maps of C Kα emitted from the epoxy resin, Ti Kα and Nb Lα emitted from the fracture surface with the EDS analysis method after swelling the residual resin in tetrahydrofuran (Stage 2). (a) An SE image of the fracture surface at a low magnification. (b) An SE image of the area of the X-ray maps. (c) An X-ray map of C Kα emitted from the epoxy resin. (d) An X-ray map of Ti Kα emitted from the fracture surface. (e) Nb Lα emitted from the fracture surface. Fig. 4. Open in new tabDownload slide SE images of the fracture surface and X-ray maps of C Kα emitted from the epoxy resin, Ti Kα and Nb Lα emitted from the fracture surface with the EDS analysis method after swelling the residual resin in tetrahydrofuran (Stage 2). (a) An SE image of the fracture surface at a low magnification. (b) An SE image of the area of the X-ray maps. (c) An X-ray map of C Kα emitted from the epoxy resin. (d) An X-ray map of Ti Kα emitted from the fracture surface. (e) Nb Lα emitted from the fracture surface. The SE images of the fracture surface and the X-ray maps of C Kα, Nb Lα and Ti Kα after all the removal processes are shown in Fig. 5. Moreover, the SE images of the fracture surface before being embedded into the resin are shown in Fig. 6. Any difference in the microscopic features of the fracture surface could not be found between the SE images in Fig. 5b, b-1, b-2 and those in Fig. 6b, b-1, b-2. Therefore, the microscopic features of the fracture surface show no damage after all the removal treatments. No residual resin was observed in the SE image of Fig. 5. Though the intensity of C Kα in Fig. 4c was partially higher due to the residual resin, the intensity C Kα in Fig. 5c is extremely low and uniformly distributed because C Kα emitted from the residual resin was not detected and only background X-rays were detected. Since the residual resin was completely removed from the fracture surface, both the intensities of Ti Kα and Nb Lα in Fig. 5d and e became higher and uniform excluding areas that are too deep where the X-rays could not escape. Fig. 5. Open in new tabDownload slide SE images of the fracture surface and X-ray maps of C Kα emitted from the epoxy resin, Ti Kα and Nb Lα emitted from the fracture surface with the EDS analysis method after all the removal processes (Stage 3–6). (a) An SE image of the fracture surface at a low magnification. (b) An SE image of the area of the X-ray maps. (b-1) and (b-2) Magnified images of (b). (c) An X-ray map of C Kα emitted from the epoxy resin. (d) An X-ray map of Ti Kα emitted from the fracture surface. (e) Nb Lα emitted from the fracture surface. Fig. 5. Open in new tabDownload slide SE images of the fracture surface and X-ray maps of C Kα emitted from the epoxy resin, Ti Kα and Nb Lα emitted from the fracture surface with the EDS analysis method after all the removal processes (Stage 3–6). (a) An SE image of the fracture surface at a low magnification. (b) An SE image of the area of the X-ray maps. (b-1) and (b-2) Magnified images of (b). (c) An X-ray map of C Kα emitted from the epoxy resin. (d) An X-ray map of Ti Kα emitted from the fracture surface. (e) Nb Lα emitted from the fracture surface. Fig. 6. Open in new tabDownload slide SE images of the fracture surface before the sample was embedded into the epoxy resin. (a) An SE image of the fracture surface including the area of the X-ray maps. (b) The areas of the X-ray maps. (b-1) and (b-2) Magnified images of (b). Fig. 6. Open in new tabDownload slide SE images of the fracture surface before the sample was embedded into the epoxy resin. (a) An SE image of the fracture surface including the area of the X-ray maps. (b) The areas of the X-ray maps. (b-1) and (b-2) Magnified images of (b). Concluding remarks The fatigue fracture surface sample of Ti-10.7Al-45.3Nb (mass %) alloy was embedded into the epoxy resin. The sample was taken out of the resin by cooling the embedding resin with liquid nitrogen and crushing the resin. Since much of the fracture surface was covered with residual resin, microscopic features of the covered areas were impossible to observe. In order to remove the residual resin, the sample was mainly treated with tetrahydrofuran and cold concentrated sulfuric acid. The SEM observations and the EDS analyses revealed that the residual resin was completely removed after all the removal processes. Any difference in the microscopic features of the fracture surface could not be found between the SE image of the fracture surface before embedding the sample into the resin and the SE image of the fracture surface after all the removal treatments. Therefore, the microscopic features of the fracture surface showed no damage after the removal of the resin. Acknowledgements The author thanks Mr Chris Jeavons (English Event Company, Itoshima-City, Japan) for the proofreading. References 1 Steffens W L . A method for the removal of epoxy resins from tissue in preparation for scanning electron microscopy , J. Microsc. , 1978 , vol. 113 (pg. 95 - 99 ) Google Scholar Crossref Search ADS PubMed WorldCat 2 Awane T . Removal of organic compounds from metallographic surfaces using cold concentrated sulfuric acid , J. Japan Inst. Metals , 1999 , vol. 63 (pg. 551 - 552 ) OpenURL Placeholder Text WorldCat 3 Tang F , Awane T , Hagiwara M . Effect of compositional modification on Young's modulus of Ti2AlNb-based alloy , Script. Mater. , 2002 , vol. 46 (pg. 143 - 147 ) Google Scholar Crossref Search ADS WorldCat 4 Matlakhova L , Matlakhov A , Monteiro S . Temperature effect on the elastic modulus, internal friction and related phase transformations in Ti–Nb-2%Al quenched alloys , Mater. Character. , 2008 , vol. 59 (pg. 1234 - 1240 ) Google Scholar Crossref Search ADS WorldCat 5 Huang L , Liaw P K , Liu Y , Huang J S . Effect of hot-deformation on the microstructure of the Ti–Al–Nb–W–B alloy , Intermetallics , 2012 , vol. 28 (pg. 11 - 15 ) Google Scholar Crossref Search ADS WorldCat © The Author 2013. Published by Oxford University Press [on behalf of The Japanese Society of Microscopy]. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com
TI - The method of removing hardly soluble organic material from metallic specimen used in fracture surface analysis by scanning electron microscope
JO - Microscopy
DO - 10.1093/jmicro/dft025
DA - 2013-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/the-method-of-removing-hardly-soluble-organic-material-from-metallic-QnbYumzC4c
SP - 615
EP - 621
VL - 62
IS - 6
DP - DeepDyve
ER -