Diffusionless Phase Transformations in Zirconia and Hafnia

Abstract

A study, by high-temperature diffractometry, of the monoclinic-tetragonal phase changes in pure ZrOz and HfOz 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 ZrOz above 22OOOC was confirmed.

Direct high-temperature single-crystal observation of orientation relationship in zirconia phase transformation

“…The observational evidence does not furnish a clear decision as to whether the transformation is of the brittle martensitic or the true martensitic type (Wolten 1963), but favors the latter point of view.…”

Peridynamic simulations of the tetragonal to monoclinic phase transformation in zirconium dioxide

“…Assuming transformation of the grain begins in the centre and expands outwards, the upper limit for the time taken for the entire grain to transform can be calculated by the radius of the grain (~0.6 x 10 -3 mm) divided by the speed of sound in zirconia (3.82 x 10 6 mm s -1 ), giving a total transformation time of 1.6 x 10 -10 s. Although researchers often define the martensitic transformation as being close to the speed of sound (Deville et al, 2004), this is very difficult to measure experimentally. It has even been suggested that martensitic transformations might occur at closer to around one third the speed of sound (Wolten, 1963).…”

X-ray diffraction study of the yttria-hafnia system

“…When monoclinic hafnia is heated to about 1700®C, it undergoes a nonquenchable transformation to the tetragonal phase* Wolten (7) has found that the transformation occurs over a temperature interval of about 150 C©* On cooling, the reverse transformation occurs at lower temperatures with hysteresis of 20 to 30 C°. At a constant temperature within the transformation interval, the relative proportions of the two phases do not change with time,…”