Transmission electron microscopy observations of the f.c.c.-to-h.c.p. martensite transformation in Co-Ni alloys

Abstract

Nucleation and growth mechanisms for the f.c.c. austenite-h.p.c. martensite reaction in fine-grain-size Co-Ni alloys have been investigated by a combination of in situ heating and cooling in a high-voltage transmission electron microscope, and lattice imaging in a high-resolution transmission electron microscope. The small austenite grain size leads to a depression of the martensite start temperature and a slow rate of martensite reaction. Martensite plates grow discontinuously, with glissile motion of Shockley partial transformation dislocations at the martensite-austenite interface interrupted periodically by pinning. Martensite stacking faults can result from austenite stacking faults, austenite dislocations and martensite plate impingement. Secondary martensite plates do not nucleate autocatalytically adjacent to primary plates. The martensite reaction is microscopically reversible.