The formation of reverted austenite in 18 percent Ni 350 grade maraging steel

The nucleation mechanism of reverted austenite, especially of the coupled twin morphology, is studied. At a heating rate of 250 C /min, Ni3Ti precipitates as well as interlath and intralath reverted austenites are formed. The reverted austenite has Nishyama-Wassermann and Kurjumov-Sachs orientation relationships with the martensite matrix. The faster heating rate of 1000 C/min eliminates the formation of Ni3Ti precipitates, and hence the coupled twin morphology of Widmanstaetten austenite is not observed, confirming that these plates nucleate on Ni3Ti precipitates. It is suggested that the nucleation of gamma-twins is a diffusionless transformation mechanism and that they further grow by a diffusion-controlled shear transformation mechanism. (AIAA)