Martensite microstructure transformed from ultra-fine-grained Fe–32%Ni alloy austenite

The martensite microstructure transformed from the ultra-fine-grained Fe–32%Ni alloy austenite produced by severe plastic deformation was examined by the SEM and TEM observations. The Fe–32%Ni alloy was multi-axially forged with cumulative strain up to 9.0 at the temperature of 773 K and a strain rate of 10−2 s−1. Then the large strain deformed specimen was quenched in liquid nitrogen to cause martensitic transformation. The results indicate that the severe plastic deformed microstructure was characterized by equilaxed grains with non-equilibrium high-angle boundaries, high-level internal stresses and elastic distortions in crystal lattice. The lenticular martensite plates transformed from the severe plastic deformed austenite are not integral but are broken and fragmental. Twins and high-density dislocations act as martensite substructure. The formation of broken and fragmental martensite is resulted from the strengthening of parent phase austenite, the high-level distortions in crystal lattice and the large fraction non-equilibrium curved grain boundaries formed during SPD.