Theory and experiment of martensitic nucleation in ZrO2 containing ceramics and ferrous alloys

Abstract

An in situ TEM experiment on martensitic nucleation was performed using sub-micron ZrO2 particles which were initially defect free. Extrinsic, spontaneous and heterogeneous nucleation was induced by introducing dislocation loops with a strong shear component and Hertzian contact stresses into selected particles. The critical characteristics of these defects which were required to trigger barrierless nucleation, in terms of the loop radius, Burgers vector, contact stress, and contact area were in good agreement with an analytic elasticity model. Along with dislocation walls, these defects which have a short range stress field can also account for intrinsic nucleation in ZrO2 containing ceramics and ferrous alloys. The essential features of nucleating defects, which may be classified into intrinsic or extrinsic ones depending on their origins, and short range or long range ones depending on their stress fields, are analyzed and contrasted in view of their experimental evidence.