Journal of Materials Science

Passerone, Alberto; Zieba, Pawel; Asthana, Rajiv

doi: 10.1007/s10853-010-4453-0pmid: N/A

Dezellus, O.; Eustathopoulos, N.

doi: 10.1007/s10853-009-4128-xpmid: N/A

After a brief presentation of thermodynamics and kinetics of non-reactive wetting, the recent results and theoretical developments concerning the reactive wetting of solids by liquid metals are reviewed. A section is devoted to illustrate and discuss the effect of interfacial reactions in removing the wetting barriers existing on many ceramic and metallic solids.

Passerone, D.; Pignedoli, C.; Valenza, F.; Muolo, M.; Passerone, A.

doi: 10.1007/s10853-010-4427-2pmid: N/A

The relative stability of different realizations of the Ag(111)/Alumina interfaces with varying oxygen partial pressures is investigated by means of ab initio density functional theory (DFT) simulations. Previous theoretical studies of similar systems always involve oversimplified geometries like stoichiometric Al-terminated, Al-rich, or O-terminated alumina interfaces. Such framework cannot explain the experimental behavior observed at intermediate oxygen partial pressure. Our approach, instead, suggests that the oxygen at the interface can play an important role at intermediate concentrations, leading to a more realistic interpretation of the experimental data.

Straumal, B.; Kogtenkova, O.; Straumal, A.; Kuchyeyev, Yu.; Baretzky, B.

doi: 10.1007/s10853-010-4377-8pmid: N/A

The microstructure of binary Co–13.6 wt% Cu and Cu–4.9 wt% Co alloys after long anneals (930–2,100 h) was studied between 880 and 1,085 °C. The contact angles between (Co) particles and (Cu)/(Cu) grain boundaries (GBs) in the Cu–4.9 wt% Co alloy are between 50° and 70°. In the Co–13.6 wt% Cu alloy, the transition from incomplete to complete wetting (coverage) of (Co)/(Co) GBs by the second solid phase (Cu) has been observed. The portion of completely wetted (Co)/(Co) GBs increases with increasing temperature beginning from T wss = 970 ± 10 °C and reaches a maximum of 15% at 1,040 °C. This temperature is very close to the Curie point in the Co–Cu alloys (1,050 °C). Above 1,040 °C, the amount of completely wetted (Co)/(Co) GBs decreases with increasing temperature and reaches zero at T wsf = 1,075 ± 5 °C. Such reversible transition from incomplete to complete wetting (coverage) of a GB by a second solid phase is observed for the first time.

Asthana, Rajiv; Singh, Mrityunjay; Sobczak, Natalia

doi: 10.1007/s10853-010-4647-5pmid: N/A

High-temperature sessile-drop wettability tests were conducted on unpolished C–C and SiC–SiC composite substrates using commercial braze alloys Palco (Pd-35Co), Palni (Pd-40Ni), Cusil-ABA (63Ag–35.3Cu–1.75Ti), and Ticusil (68.8Ag–26.7Cu–4.5Ti). Observations revealed non-uniform, anisotropic spreading, copious braze infiltration of the composite substrates, particularly C–C composite, and Ti enrichment at the composite/braze interface together with dissolution of Si (from SiC–SiC composite) in braze and diffusion of Co (from Palco) in the composite. The droplet/composite contact region near the droplet center revealed intimate and microstructurally sound bonding. However, inter-laminar shear cracking within the SiC–SiC composite in contact with Ticusil, Palco, and Palni, and partial substrate/droplet de-cohesion near the edge of the droplet were also observed. In Palco and Palni droplets, fiber tows in the contact region de-laminated from the main body of the composite via inter-laminar shear cracking resulting in fiber flotation, segregation, and surface degradation. The study is one of the first empirical enquiries into the complex wetting and spreading behavior of brazes on commercial C–C and SiC–SiC composites.

Wojewoda-Budka, J.; Sobczak, N.; Morgiel, J.; Nowak, R.

doi: 10.1007/s10853-010-4379-6pmid: N/A

The microstructure studies of the reaction product region (RPR) obtained due to the interaction between the liquid aluminium and polycrystalline zinc oxide substrate at 1273 K has been studied. The RPR extended over the oxide substrate and showed a typical C4 (co-continuous-ceramic-composites) structure composed of two interpenetrating phases. The scanning electron microscopy studies revealed that the large crystals of alumina were surrounded by an Al(Zn) metallic phase. Moreover, the transmission electron microscopy investigation showed the presence of a thin (~250 nm) layer next to the ZnO. The chemical analysis accompanied by the selected area electron diffraction patterns indicated in both cases the same stoichiometric aluminium oxide but of different crystallographic structure, i.e., large crystals had α-Al2O3 structure while the layer was identified as metastable δ-Al2O3. The results were compared to those reported for interaction between liquid aluminium and monocrystalline ZnO.

Liu, G.; Valenza, F.; Muolo, M.; Passerone, A.

doi: 10.1007/s10853-010-4337-3pmid: N/A

A composite joining technique, using a Ni–56Si filler alloy and Mo as interlayers, was used to join SiC to SiC and to Kovar. The wetting of the Ni–Si alloy on SiC ceramic was studied in a vacuum at 1,350 °C by the sessile drop technique as a function of time; the non-reactive wetting characteristics in the Ni–Si/SiC system were confirmed, with an equilibrium contact angle of about 23°. SiC/SiC joints were fabricated by two processes using a Ni–Si/Mo/Ni–Si structure as the interlayer. SiC/Kovar joints were produced by means of a multilayer structure: molybdenum, which is used as the interlayer, was joined to Kovar on one side by means of transient liquid phase bonding and to SiC on the other side, using a Ni–Si coating as a filler alloy. The resulting joints were analyzed and discussed in terms of joint morphology and microstructure, joint strength, and fracture behavior. Two interfacial layers form at the Kovar/Mo and the Mo/Ni–Si interfaces due to dissolution and interdiffusion phenomena between the metallic elements, without there being any observable reactions with the SiC component. The type of joining process and the experimental conditions used play a key role in determining the joint microstructure and composition, the joint strength and its fracture behavior.

Singh, M.; Asthana, R.

doi: 10.1007/s10853-010-4510-8pmid: N/A

Zirconium diboride–SiC (ZS) particulate ceramic-matrix composites containing either carbon powder (termed ZSC composite) or SCS-9a silicon carbide fibers (termed ZSS composite) were joined to titanium and Inconel 625 using Pd-base brazes, Palco and Palni (T L ~ 1492–1513 K). The joints exhibited intimate contact and evidence of interdiffusion of Zr, Si, Pd, and Co, with the Palni joints exhibiting most extensive chemical interaction, greater propensity toward cracking, and partial melting of the Inconel substrate. The joint region comprised of braze-plus-interaction zone exhibited comparable Knoop hardness in Palni and Palco joints. The fully dense ZS had the highest (2000–2600 HK200) and ZSC the lowest (300–750 HK200) Knoop hardness. The ZSS composites displayed a large dispersion in hardness because of incomplete densification (~30% porosity) and transversal cracking from the CTE mismatch between SCS-9a fibers and the ZS matrix. Steady-state thermal calculations reveal that for joined assemblies (~0.51 cm total thickness in the study), joining Ti or Inconel to ZS shall decrease the thermal resistance by nearly 33–43% relative to the metal substrate, thus enhancing the heat dissipation capability in advanced components made using such joints.

López, V.; Kennedy, A.; García, R.

doi: 10.1007/s10853-010-4418-3pmid: N/A

The effect of a K–Al–F-based flux was investigated on the wettability of TiC by an Al–7 wt%Si alloy in the interval of temperatures between 660 and 900 °C in Ar and in atmospheric air. Null spreading was observed without flux whereas perfect wetting was enabled by the flux in both atmospheres. The liquid flux, which provides a locally protective atmosphere by spreading on the surfaces of the substrate and eventually on the Al alloy, dissolves the aluminium oxide covering the molten alloy enabling thus direct contact between the liquid alloy and the TiC substrate. The low tensions for the solid/flux and liquid metal/flux interfaces facilitate spontaneous spreading and instantaneous wetting. Meanwhile, the flux is displaced to the lateral periphery of the substrate and to the surface of the liquid. Under the resolution of the scanning electron microscope, microstructural examination of the interfaces did not reveal reaction products. Rapid infiltration of the alloy into TiC/flux compacts, at low temperatures, correlated well with the flux-assisted spreading kinetics observed.

Elrefaey, Ahmed; Tillmann, Wolfgang

doi: 10.1007/s10853-010-4357-zpmid: N/A

Evaluations of vacuum brazed commercially pure titanium and low-carbon steel joints using one copper-based alloy (Cu–12Mn–2Ni) and two silver-based braze alloys (Ag–34Cu–2Ti, Ag–27.25Cu–12.5In–1.25Ti) have been studied. Both the interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate the joint quality. The optical and scanning electron microscopic results showed that all the filler metals interact metallurgically with steel and titanium, forming different kinds of intermetallic compounds (IMC) such as CuTi, Cu2Ti, Cu4Ti3, and FeTi. The presence of IMC (interfacial reaction layers) at the interfacial regions strongly affects the shear strength of the joints. Furthermore, it was found that the shear strength of brazed joints and the fracture path strongly depend on the thickness of the IMC. The maximum shear strength of the joints was 113 MPa for the specimen brazed at 750 °C using an Ag–27.25Cu–12.5In–1.25Ti filler alloy.