Maraging behavior of an Fe-19.5Ni-5Mn alloy II: Evolution of reverse-transformed austenite during overaging

The evolution of reverse-transformed austenite in a maraging Fe-19.5Ni-5Mn alloy was examined using transmission electron microscopy. The types of reversed austenite observed were determined by the aging temperature and time. It was found that matrix austenite appeared first, followed by lath-like austenite and then by recrystallized austenite, the latter after prolonged aging at higher temperatures. Each evolved at different preferential sites. The first two types of reversed austenite contained dense dislocations that probably contributed to the subsequent recrystallization. The lath austenite was twin-related, and its orientation relationship with the residual martensite films obeyed either Nishiyama (N) or Kurdjumov-Sachs (K-S) relations, depending on the aging temperature. Each individual austenite lath nucleated independently with a habit plane close to {111}_f, and constituted a lamellar structure with the residual martensite. Its formation possibly involved a shear mechanism accompanied by element redistribution. The matrix austenite and the lath-like austenite eventually recrystallized to form fine grains of austenite. Part of the recrystallized austenite transformed to martensite after cooling so that the resultant microstructure was a mixture of a few newly formed martensite laths and oval/spherical ferrite particles distributed in the recrystallized austenite matrix. The recrystallized austenite and ferrite particles, rather than the lamellar structure of lath-like austenite, are thought to be the equilibrium phases at 550°C.