Effect of Spontaneous Volume Magnetostriction on Martensitic Transformation in an Fe-29.9 at%Ni Invar Alloy under Hydrostatic Pressures

It was shown in the previous studies that measured hydrostatic pressure dependence of Ms temperature in Fe–Ni and Fe–Ni–C invar alloys was not in agreement with the one calculated by using a modified Patel-Cohen’s equation, although it was in good agreement in an Fe–Ni–C non-invar alloy. It was then speculated that the disagreement for invar alloys might be caused by the existence of spontaneous volume magnetostriction, which was one of physical properties related to the invar nature. In order to quantitatively confirm the speculation, the spontaneous volume magnetostriction of an Fe-29.9 at%Ni invar alloy has been obtained by measuring the spontaneous magnetization under hydrostatic pressures. Then, a new equation including a term due to the spontaneous volume magnetostriction to estimate the pressure dependence of Ms temperature has been derived by referring to the Patel-Cohen’s equation. The pressure dependence of Ms temperature for the Fe–Ni invar alloy has been then calculated by putting the obtained spontaneous volume magnetostriction and other physical quantities, such as bulk modulus, into the newly derived equation. As a result, the calculated pressure dependence was in almost good agreement with the previously measured one in the pressure range examined. It was thus concluded that the previous speculation and the newly derived equation were quantitatively confirmed.