Critical grain size in the γ→α martensite transformation. Thermodynamic analysis with regard to spatial scales characteristic of martensite nucleation

Abstract

In the work, it is shown that taking into account the ratio of spatial scales characteristic of martensite crystal nucleation in the elastic field of an individual dislocation localized in a grain of diameter D makes it possible (i) to estimate the critical grain size D c (∼1 μm) characteristic of the γ→α transformation at a martensite starting temperature M s ∼ 100 K from the requirements that threshold strain arises in the elastic region, (ii) to consistently describe the D dependence of M s with M s(D c) = 0 from analysis of the phase free energy difference, (iii) to express D c through macroparameters and interpret the limiting case M s∞→0, D c→∞ (D c ∼ (M s∞) −1 with M s∞ ≡ M s (∞)), and (iv) to more exactly specify the qualitative dynamic model of the effect of dislocation grain boundaries on the onset of initial excitation.