Critical Grain Size Estimation in the γ-α Martensitic Transformation with Athermal Macrokinetics by the Example of Fe-Ni-Cr System

Resumo

The existence of the critical grain size D_c for reconstructive martensitic transformations implies that at austenite grain diameters D smaller than D_c the transformation is suppressed during cooling down to absolute zero temperature. In the case of athermal macrokinetics, martensite crystals divide connected (free from boundaries) volumes of austenite, which allows the use of fractal type models for the processing of results. This study shows that the symmetric model of orthogonal coupling of martensite crystals, developed to estimate the amount of formed martensite in single-crystal samples, can also be applied for an initial polycrystalline sample with a known austenite grain size distribution. A step-by-step algorithm for the theoretical estimation of D_c is proposed under the assumption that the formation of each succeeding generation of martensite crystals begins in the largest continuous volumes of retained austenite. If the estimated cumulative fraction of martensite coincides with the observed resultant value, the count is stopped, and the size of the largest of untransformed continuous volumes of retained austenite is taken as D_c. A detailed analysis of results was carried out for a sample of the alloy Fe - 29.96%Ni - 1.83% Cr in which the volume fraction of the largest grains (with the size D = 310–315 µm) was approximately 58% and four autocatalytic bursts were detected during cooling; the bursts corresponded mainly to the generations of martensite crystals associated with the austenite regions related to the transformation of the initial coarse grains. The amount of martensite crystals was determined by the increase in magnetization and by X-ray diffraction. With the cumulative fraction of martensite ≈70%, D_c was estimated to be ≈ 25.44 µm.