A top-down strategy to determine interdiffusivities for fcc Ni-Co-Al-Ta quaternary system adopting diffusion triple and quadruple techniques

Abstract

To determine interdiffusivities in multi-component systems, the CALPHAD method traditionally follows a bottom-up sequence (i.e. from unary, binary up to ternary) in both experimental and optimization procedures before extrapolating to higher-order systems. However, this process faces challenges like low efficiency and considerable errors, often leading to reassessing sub-systems when studying higher-order systems. To address this problem, the present study on the fcc Ni-Co-Al-Ta system proposes a top-down strategy that designs samples simultaneously for binary and ternary sub-systems from a quaternary top-design perspective, significantly reducing experimental samples yet enabling mutual verification of sub-systems across samples. It is achieved by assembling diffusion triples and quadruples connecting six alloys end-to-end, and analyzing them using our newly developed two-dimensional inverse scheme. Atomic mobilities are inversely deduced from composition-distribution data sufficiently covering four surfaces of the quaternary single-phase composition quasi-pentahedron. This top-down strategy demonstrates its significant enhancement in efficiency and quality.