MECHANISM OF ISOTHERMAL MARTENSITIC-TRANSFORMATION

The mechanism of isothermal martensitic transformation has been studied, using the most typical alloy, Fe-Ni-Mn, of this type of martensitic transformation. The martensitic nucleation rate was investigated by measuring effect of applied stress on the transformation rate. The volume fraction of the martensite transformed was measured in situ by a magnetic detector with the coil covering the specimen. The transformation at liquid nitrogen temperature was studied in detail. The most important feature of observed results is that there is a critical stress level above which the initial transformation rate is drastically increased. This stress level coincides with the yield stress of the austenitic specimen in the case of alloys where no isothermal martensitic transformation occurs unless an external stress is applied. This fact, along with other characteristic observations, is qualitatively explained by the basic concept proposed by the present author that the nucleation rate of isothermal martensite is controlled by thermally activated motion of dislocations which accommodate the shape strain of a nucleating martensite. Furthermore, in the quantitative analysis of this thermally activated process, it is found that the activation volumes determined by three different methods coincide with one another and the calculated activation energy for thermally activated motion of dislocations in austenite is in very good agreement with the measured activation energy of isothermal transformation. These results confirm that the above mentioned proposition by the present author is correct.