Decomposition of the β-Phase in Zr-20%Nb

Abstract

Decomposition of the β-phase in the Zr-20%Nb alloy for different thermal treatments was studied. The grains of the high temperature β-phase which could be fully retained on quenching showed a distribution of plate-like features in electropolished or chemically etched samples. These plates were identified to be of the γ-hydride phase. The crystallography and the mechanism of formation of γ-hydride in the β-phase have been discussed. The formation of the β-phase during isothermal ageing at temperatures between 548 K and 698 K was studied. The appearance of ω-particles of cuboidal shape in periodic arrays along 100 _β directions and the pronounced tendency of alignment of clusters of ω-particles of the same variant have been explained in terms of a possible spinodal decomposition in the β-phase prior to ω-precipitation and the strain energy minimization tendency. Precipitation of α-plates was studied in two set of samples given the following thermal treatments: (i) quenched from 1173 K to 300 K followed by ageing at 773 K and (ii) rapidly cooled from 1173 K to 773 K and isothermally held. The former set showed a more homogeneous distribution of widmanstatten a-plates while in the latter, a majority of α-plates grew from the grain boundaries. Copious nucleation in the former set has been attributed to the presence of athermal co-particles or ω-like defects in samples quenched to 300 K. The periodic array of internal twins and the {334}_β habit plane associated with the α-plates in the former set suggested the possibility of the operation of a shear in the α-precipitation process.