EFFECTS OF HYDROSTATIC-PRESSURE ON MARTENSITIC TRANSFORMATIONS

The hydrostatic pressure is one of the important state variables in ph ase equilibrium, and its generation techniques have been so much devel oped that it is now possible to study in detail the effect of pressure on phase transformations. In this overview, the effects of pressure o n phase transformations, especially on martensitic transformations, ar e described in detail, centering around our recent systematic work, si nce there are still a small number of work in this research field. The effect of hydrostatic pressure on martensitic transformation is funda mentally brought about through an interaction of the hydrostatic press ure with the volume change, expansion or contraction, accompanying the transformations. The volume change is affected by the invar effect in ferrous invar alloys. Therefore, the hydrostatic pressure effect on m artensitic transformations may be varied with the type of the martensi tic transformations, thermoelastic or non-thermoelastic, athermal or i sothermal, and it also depends on the kind of alloys, invar or non-inv ar. From this point of view, the hydrostatic pressure effects have bee n examined for those various types of martensitic transformations in v arious kinds of alloys, which include Fe-Ni, Fe-Ni-C, Cu-Al-Ni, disord ered and ordered Fe-Pr, aged Ti-Ni, ausaged Fe-Ni-Co-Ti and Fe-Ni-Mn a lloys. It is clearly shown that the martensitic transformation start t emperature, M-s, is decreased or increased with increasing hydrostatic pressure, depending on whether the volume change is expansion or cont raction. These changes in M are in good agreement with theoretical cal culations in which hydrostatic pressure dependences of volume change a nd invar effect are both taken into consideration, using an equation d erived by modifying that of Patel and Cohen. Despite those changes in M-s, martensite morphology is not varied with the hydrostatic pressure , irrespective of the type of martensitic transformations and the kind of alloys.