THERMALLY ACTIVATED MARTENSITIC TRANSFORMATIONS IN MG-PSZ

The thermally activated, stress-assisted martensitic tetragonal --> mo noclinic (t --> m) and tetragonal --> orthorhombic (t --> o) transform ations in a high-toughness Mg-PSZ were investigated by monitoring the phase assemblage with Raman spectroscopy after a variety of heat treat ments and loading conditions, After a short anneal at 1000 degrees C, which transforms m- and o-ZrO2 to the t polymorph, isothermal t --> m and t --> o transformations occur at room temperature during the month s following the anneal. The transformation rates in the annealed sampl es are greatly enhanced under external stress, Alternatively, samples containing regions of significant residual stress, introduced by inden tation for example, and then annealed at relatively low temperatures, underwent additional thermally activated transformation in the stresse d regions. The thermodynamics and kinetics of this complex transformat ion ''plasticity,'' and its effect on mechanical properties, are discu ssed.