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INTERDIFFUSION BETWEEN A Ni-Cr ALLOY AND W, AND THE USE OF Ni FOR LIQUID PHASE BONDING OF ZrC/W COMPOSITES
The Ni-Cr-W system is of significant technological and scientific interest due to the oxidation resistance provided by Cr and the excellent high-temperature mechanical properties of Ni-Cr-based alloys and W. Several active phenomena during the processing of such materials (such as solidification, precipitation, sintering) or during the service of such materials (such as oxidation) are governed by diffusion of elements in this system. In this study, interdiffusion in the Ni-Cr-W ternary system was investigated at 1000-1200oC. Diffusion couples, consisting of a Ni-20wt%Cr alloy and pure W, were used to evaluate interdiffusion coefficients in the Ni-Cr-W system using a method developed by Dayananda and coworkers. The effective average interdiffusion coefficients obtained by this method were used to reproduce a concentration profile with a good fit to the profiles as measured from experiments. The activation energies for interdiffusion of Ni, Cr, and W were within the range of 318±27 kJ/mol for diffusion in the Ni-rich phase , and 271±32 kJ/mol for diffusion in the W-rich phase. These values were consistent with values reported in other studies of similar systems. The role of W grain boundaries on the interdiffusion behavior of Ni, Cr, and W was also investigated. This study was conducted at 1000oC using a diffusion couple between fine-grained, hot-pressed W (dave= 2.3 μm) and a Ni-20wt%Cr alloy. Although a slight preference for Ni and Cr diffusion along W grain boundaries was detected though STEM-EDX analysis, the temperature was sufficiently high that the volume diffusion rates of Ni and Cr were appreciable compared to the grain boundary diffusion of Ni and Cr, so that the overall concentration profiles were similar to those obtained from diffusion couples with coarse-grained W (dave=29.3 μm).
A liquid phase bonding technique to join ZrC/W-based composites was also evaluated. ZrC/W composites exhibit superior thermal conductivity and failure strength at high temperatures relative to nickel-based alloys. ZrC/W composites produced by the Displacive Compensation of Porosity (DCP) method were joined together by a liquid-phase bonding technique using Ni as an interlayer material. The effects of the bonding temperatures, ranging from 1300-1500oC, on the microstructure and microchemistry of the bonded area, and on the bonding strength at room temperature, were investigated. The cross-section from a specimen bonded at 1300 oC exhibited several 25 – 90 μm voids along the bonded interface while the cross-sections from specimens bonded at 1400 oC and 1500 oC exhibited no such voids. The Zr, and C contents at the bonded interface increased as the bonding temperature increased from 1400 oC to 1500 oC. On the other hand, the Ni content at the bonded interface area decreased as the bonding temperature increased. The shear strength values obtained after bonding at 1300-1500oC for the same time of 2 hours were similar and in the range of 127.0 ±63.0 MPa.