Interdiffusion in Co-Based Co-Al-W Ternary Alloys at Elevated Temperatures

L1(2)-type Co-3(AI,W) compound is a very important phase in Co-based high temperature materials. This study investigated the interdiffusions of Co-based Co-Al-W ternary alloys at 1 423, 1 473 and 1 523 K. Two types of diffusion couples were employed to estimate four diffusivity matrix coefficients ((D) over tilde (Co)(AlAl), (D) over tilde (Co)(AlW), (D) over tilde (Co)(WW) and (D) over tilde (Co)(WAl)). A modified ternary Boltzmann-Matano method was employed for the evaluation of interdiffusion coefficients. The estimated cross-interdiffusion coefficients ((D) over tilde (Co)(AlW) and (D) over tilde (Co)(WAl)) had opposite signs at 1 523 K which is not contradictory to but rather consistent with the Onsager's reciprocity theorem. The second derivative of the Gibbs free energy with respect to the Al concentration, (partial derivative(2)G(c)/partial derivative C-Al(2)), may have a sign opposite to that of W concentration, (partial derivative(2)G(c)/partial derivative C-W(2)). Experimental results revealed that the effect of W on the Al diffusion flux is greater than that of Al on the diffusion flux of W by about one order of magnitude.