Limit conditions for the onset of abnormal grain growth in a homogeneous microstructure: Theory and Experiments

Abstract

Abstract In previous papers it has been demonstrated that abnormal grain growth in a polycrystalline microstructure is a natural phenomenon even in the presence of an homogeneous microstructure. Namely, in a statistical sense, each portion of the material has the same grain size distribution and no local heterogeneities are involved. The only necessary condition is the presence of a grain growth inhibitor which can be represented by second phase particles (Zener Drag), homogeneously distributed as well, or by segregating atoms on the grain boundary (Atoms drag). Considering specifically the Zener Drag effect, it can be shown that in the presence of an evolution of the restraining force ( even if extremely slow), due to Ostwald ripening or to dissolution of the particles, the corresponding grain growth process allows the grain size distribution to continuously evolve reaching a critical shape. Such peculiar shape corresponds to a “structural” instability which irreversibly determines the onset of an abnormal growth phase with a drastic change of grain growth kinetics (kinetics exponents for the mean radius largely above the canonical 0.5). In this context the mechanism of Abnormal Grain Growth onset is discussed as a general physical phenomenon and a set of experimental data on several steels will be presented for supporting such conclusion.