COMPOSITION DEPENDENCE OF MARTENSITIC TRANSFORMATIONS IN FE-NI INVAR-ALLOYS UNDER HYDROSTATIC PRESSURES

The composition dependence of martensitic transformations in Fe-Ni invar alloys (Fe-31.7 and -32.3 at%Ni) under hydrostatic pressures has been examined by carrying out electrical resistivity and magnetization measurements under pressures up to 1.5 GPa and optical microscope observations, the results were compared with those for an Fe-29.9 at%Ni invar alloy previously examined. As a result, M(s) temperatures for the present two alloys decreased parabolically with increasing hydrostatic pressure, as in the previous alloy. The hydrostatic pressure dependences of M(s) temperatures for the two alloys were calculated by putting the atomic volumes of austenite and martensite phases and the spontaneous volume magnetostrictions of austenite phase, omega(s), which were also measured in the present study, into a previously derived equation including a term of the omega(s) related to the invar effect. Consequently, the calculated dependences for the two alloys were all in good agreement with the measured ones over the range of applied hydrostatic pressures. Thus, the previously proposed effect of spontaneous volume magnetostriction and the derived equation were further confirmed to be appropriate. Optical microscope observation showed that the morphology of martensites formed under hydrostatic pressure was the same as that of thermally-induced martensites in both the alloys irrespective of hydrostatic pressures.