Martensitic nucleation in ZrO 2

Abstract

A small particle experiment to investigate the tetragonal to monoclinic ZrO 2 (martensitic) transformation was performed using a CuZrO 2 model material prepared by internal oxidation. Incoherent spherical particles formed above 1173 K remained essentially untransformed at 4.2 K and after complete dissolution of the Cu matrix. Transformation was induced by deformation, and the transformed monoclinic particles were stable during reheat up to the highest original oxidation temperature. A twinning substructure was associated with all the monoclinic particles after transformation but no retained tetragonal phase was found. The results demonstrated that the transformation is nucleation controlled, that the strong constraint effect which suppresses the martensitic nucleation is due to the parent tetragonal matrix, and that the cause is the deviatoric transformation strain energy. Such high strain energy can be overcome only if the nucleation is assisted by the interaction energy with a defect. A closer examination of the application of the defect model containing [001]dislocations further suggests that the monoclinic nucleus has a coherent interface. The results of model calculations are in good agreement with the observation of the martensitic transformation at the M s temperature around 1223 K in bulk ZrO 2 and the stability of small ZrO 2 particles at much lower temperatures; in the latter case, it was shown that the mere generation of one dislocation, e.g. by cold work, suffices to trigger the nucleation. Lastly, the particle size effect in CuZrO 2 is attributed to the size dependence of the interfacial stresses arising from the plastic deformation of the Cu matrix.