Transformation Kinetics, Microplasticity and Aging of Martensite in FE-31 Ni.

Abstract : The effects of testing temperature and volume fraction of martensite on the microplastic response of unaged Fe-31 Ni martensite-austenite aggregates have been determined. These results, supplemented by other experimental studies, show that two unusual mechanisms of plastic deformation give rise to the apparent softness of the quenched structures. The transformation of retained austenite to martensite during the application of stress gives rise in specified conditions to large microplastic strains. The other strain component, which dominates the microdeformation above the temperature range where transformation occurs, is attributed to stress-assisted relaxation of internal stresses. The measured changes in elastic modulus and internal friction (at 80 kHz) during aging of Fe-31 Ni after quenching cannot be explained by the diffusion of point defects to dislocations. It is suggested that this aging effect is instead due to the motion of dislocations which occurs in order to decrease the internal stress. Experimental results pertaining to the kinetics of the austenite to martensite phase transformation show that no distinguishable athermal component of transformation exists in the Fe-31 Ni alloy. A model for martensitic kinetics is developed which treats the propagation of the interface as analogous to plastic deformation. This model and reconsideration of the implications of dislocation motion in the kinetics of embryo formation both lead to the conclusion that martensitic kinetics should be of a 'fast isothermal' character. (Author)