ABSTRACT
The monoclinic (B) to cubic (C) transformation in terbia (Tb2O3) was examined for possible use as a high temperature transformation toughener. It involves an ~8% volume expansion on cooling and a unit cell shape change of ~10°, and it occurs at ~1650°C Composites of 20 vol% B-Tb2O3–80 vol% MgO were prepared and the mechanical properties were evaluated at temperatures up to 1400°C. The high temperature B phase of Tb2O3 could only be retained under ambient conditions by rapid quenching or cooling under pressure in a HIP. The B to C transformation could not be stress induced at room temperature, and elevated temperatures were required to overcome the nucleation barrier for transformation.
Single edge notched beam (SENB) specimens were fabricated and broken at ambient and elevated temperatures and the fracture surfaces were examined by X-ray diffraction (XRD). From room temperature to 1000°C, no transformation was observed. From 1075° to 1150°C, the B to C transformation was observed only on fracture surfaces. At 1300°C transformation was also observed on free surfaces and at 1400°C, spontaneous transformation was observed throughout the material. The toughness of specimens increased with increasing temperature above 600°C, with a significant increase in toughness at 1400°C.
