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

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 Dc (∼1 μm) characteristic of the γ→α transformation at a martensite starting temperature Ms ∼ 100 K from the requirements that threshold strain arises in the elastic region, (ii) to consistently describe the D dependence of Ms with Ms(Dc) = 0 from analysis of the phase free energy difference, (iii) to express Dc through macroparameters and interpret the limiting case Ms∞→0, Dc→∞ (Dc ∼ (Ms∞)−1 with Ms∞ ≡ Ms (∞)), and (iv) to more exactly specify the qualitative dynamic model of the effect of dislocation grain boundaries on the onset of initial excitation.