TETRAGONAL-TO-MONOCLINIC PHASE-TRANSFORMATION DURING THERMAL CYCLING AND ISOTHERMAL AGING IN YTTRIA PARTIALLY-STABILIZED ZIRCONIA

The tetragonal-to-monoclinic (T --> M) phase transformation accompanied with thermal cycling has been studied by ionic conductivity, dilatation and X-ray diffraction measurements. With repeating thermal cycling, the M(s) point shifts to a higher temperature, the amount of expansion due to T --> M transformation increases, and the transformation proceeds from the surface in the specimen interiors. These phenomena are attributed to grain boundary cracking which occurs as a result of the transformation. The time-transformation isotherms at 353 K and at room temperature can be expressed by the Johnson-Mehl equation f = 1 - exp(- bt(n)), and the same n value of 0.7 was obtained irrespective of grain size and of environment.