Effect of Plastic Deformation and Stress on Martensite Transformation in Fe-Ni Alloy

Abstract

The form, shape and hardness of martensite structure transformed from strained austenite were investigated for an Fe-31.7%Ni alloy in order to make clear the strengthening mechanism of steel by ausforming treatment. From the results of hardening characteristics of martensite plates and of the change in X-ray diffraction lines from martensite and austenite due to deformation before transformation to martensite and also microscopic observation of the structure, it was confirmed that a strengthening. by ausforming could be attributed to both the refinement of martensite structure and the change in the internal strain of martensite induced by austenite deformation prior to transformation.
The study was also extended to the determination of the influence of applied stress on martensite transformation. Ms was raised by tensile stress at the rate of 1.6.. /kg/mm² in the temperature range between -45.. and -30.. and at 0.16.. /kg/mm² in the temperature range between -51.. and -45... It was found that the rate of 1.6.. /kg/mm² showed good agreement with the result obtained from the calculation of stress dependence on Ms by Cohen's treatment. A discussion was also made to estimate the relation between the energy to initiate martensite transformation and the energy to deform the material under an applied stress.