Interdiffusion and Atomic Mobility for Face-Centered Cubic (FCC) Co-W Alloys
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ase alloys based on the Co-Al-W system have been considered as potential next-generation hightemperature structural materials.[1] The same as the Nibase superalloys, the new type of Co-base alloys sustain superior temperature capabilities mostly because the microstructure following heat treatment consists of the face-centered cubic (fcc) c (A1) matrix and the ordered fcc c¢ (L12)precipitate. Knowledge of both thermodynamics and diffusion characteristics is of critical importance in understanding and developing the new class of hightemperature alloys. The phase relation of the Co-Al-W base system, especially the thermodynamic (meta)stability of c¢[2,3] and the alloying effect,[4–6] have been greatly clarified. The morphology and composition of the c¢ precipitate are also strongly dependent on the diffusion interaction between the c¢ and c phases. The interdiffusion and atomic mobility of the Co-Al binary of the Co-Al-W base system have been recently studied.[7] However, the bulk interdiffusion data of another key binary, i.e., Co-W, remain tentative[8,9] and inconsistent,[10] the latest work reported an increase in the interdiffusion with the W content and an activation energy as small as for a grain Y.-W. CUI, Staff Researcher, is with the Computational Alloy Design Group, IMDEA Materials Institute, Getafe, Madrid 28906, Spain. Contact e-mail: [email protected] GUANGLONG XU, Doctoral Candidate, is with the Computational Alloy Design Group, IMDEA Materials Institute, and also with the Facultad de Informa´tica de la Universidad Polite´cnica de Madrid, Madrid 28660, Spain. R. KATO, Graduate Student, R. KAINUMA and K. ISHIDA, Professors, are with the Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. XIAO-GANG LU, Professor, is with the School of Materials Science and Engineering, Shanghai University, Shanghai 200072, P.R. China. Manuscript submitted August 29, 2012. Article published online January 15, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
boundary diffusion in the concentrated Co-W alloys.[9,11] In this article, interdiffusion studies were carried out over a temperature range from 1273 K to 1573 K (1000 C to 1300 C) by using the solid-state incremental diffusion couples. The interdiffusion coefficients were evaluated and then employed to assess the atomic mobility for the fcc Co-W alloys as a key element to our long-term diffusion research of the Co-base alloys. Co-base binary alloys with nominal compositions of 8, 12, and 14 at. pct W were prepared from electrolytic cobalt (99.9 pct) and tungsten (99.9 pct) by arc melting under an argon atmosphere. The arc melting was repeated ten times to attain a homogeneous composition. The final melting run was completed by gradually reducing the arc current to ensure the alloy ingots with larger average grain size. Small pieces of specimens cut from the ingots were solid-solution treated under vacuum in quartz capsules at 1573 K (1300 C) for 72 hours followed by water quenching. The solution treatment resulted in the alloys
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