Diffusionless Phase Transformations in Zirconia and Hafnia

A study, by high‐temperature diffractometry, of the monoclinic‐tetragonal phase changes in pure ZrO 2 and HfO 2 disclosed that the hysteresis loops associated with the transformations have similar shapes which are typical of many martensitic phase changes, but that the widths of the loops differ greatly for the two materials, being about 200°C for zirconia and only 20° to 30°C for hafnia. The temperature‐induced transformations are predominantly athermal. The nature of these and similar transformations is discussed. The reversible formation of a cubic form of ZrO 2 above 2200°C was confirmed.