Effect of Hydrostatic Pressures on Martensitic Transformations in Fe-24.0 at%Pt Invar Alloys with Different Degrees of Order

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The effect of hydrostatic pressures on martensitic transformations in Fe-24.0 at%Pt invar alloys with different degrees of order, S, (S≈0, S>0, S≈0.6, S≈0.8) has been examined by measuring electrical resistivity and magnetization under pressures up to 1.5 GPa. As a result, it was found that the transformation temperatures for the alloys with S≈0, S>0 and S≈0.6 were only decreased with increasing hydrostatic pressure, while those for the alloy with S≈0.8 were inversely increased under pressures lower than 0.25 GPa although they decreased under pressures higher than 0.25 GPa. The hydrostatic pressure dependences of equilibrium temperatures, which were obtained from the measured transformation temperatures, for the alloys with S≈0.6 and S≈0.8 have been calculated by putting the atomic volumes of austenite and martensite phases and the spontaneous volume magnetostrictions of austenite phase, ωs, measured in the present study into a previously derived equation including a term of the ωs related to the invar effect. Consequently, the calculated dependences for those alloys were all in good agreement with the measured ones in the applied hydrostatic pressure range, as in an Fe-29.9 at%Ni invar alloy previously examined. Thus, the previously proposed effect of spontaneous volume magnetostriction and the derived equation were further confirmed to be appropriate.
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