Dynamic Theory of the Effect of a Strong Magnetic Field on the Martensitic Transformation in Steels with Austenite Grain Sizes Close to a Critical Value

Abstract

In the dynamic theory of martensitic transformations, temperature M_s of the start of transformation corresponds to conditions that are optimal for generating waves by nonequilibrium d electrons that control the growth of a martensitic crystal. The recognition of relative damping ${{\Gamma}}_{{\text{e}}}^{'}$ of s electrons plays a substantial role in this case. A general analysis that has made it possible to propose, for the first time, an analytical formula for critical size D_c$\left( {{{\Gamma}}_{{\text{e}}}^{'}} \right)$ of austenite grains is used to interpret the results for chromium–nickel steels, in which the transformation is initiated by strong magnetic fields. Under conditions of positive bulk magnetostriction, a magnetic field with intensity H lowers the chemical potential of electrons. As a result, the D_c(${{\Gamma}}_{{\text{e}}}^{'},$ H) value decreases and austenite with a grain diameter of D, which was stabilized by condition D < D_c(${{\Gamma}}_{{\text{e}}}^{'},$ 0) in the absence of a field, becomes destabilized, since inequality D > D_c(${{\Gamma}}_{{\text{e}}}^{'},$H) is fulfilled. The dynamic theory predicts a sharp increase in the D_c(${{\Gamma}}_{{\text{e}}}^{'},$ 0) value at ${{\Gamma}}_{{\text{e}}}^{'}$ → 1. This is evidenced by value D_c$\left( {{{\Gamma}}_{{\text{e}}}^{'}} \right)$ ≥ 1 mm for steel 67Kh2N22, which is three orders of magnitude larger than D_c(${{\Gamma}}_{{\text{e}}}^{'},$ 0) ≈ 1 μm for the Fe–31Ni alloy. Other peculiarities of the effect of magnetic field on the martensitic transformation are also discussed.