Analysis of Phase Transformations in Fe-Ni-Al Alloys Using Diffusion Couples of Fe/Fe-33at.%Ni-33at.%Al Alloy/Ni

The present work focused on analyzing the phase transformation in Fe-Ni-Al alloys employing a diffusion couple of Ni/Fe-Ni-Al/Fe, and Calphad-based diffusion and thermodynamic software. Diffusion couples were prepared by annealing at 1100 degrees C for 200 h and then air-cooled. These couples were also aged at 750 degrees C for 100 and 275 h. Both numerical and experimental results indicated that the diffusion path between Ni or Fe pure metal and the Fe-33at.%Ni-33at.%Al alloy is not linear. The phases formed during the diffusion anneal at 1100 degrees C correspond to those shown in the Calphad-calculated Fe-Ni-Al equilibrium diagram. The aging treatment at 750 degrees C promoted the inverse precipitation beta'-> beta' + alpha, which caused the softening of the alloy. Moreover, the normal precipitation reactions, alpha ->alpha +beta' and gamma -> gamma +gamma', were also observed to occur during the aging of diffusion couple at 750 degrees C, originating precipitation hardening.