Abstract
A systematic study of the microstructure and mechanical properties of zirconium-hydrogen alloys formed from the a-phase region under a variety of cooling conditions has been made. The tensile properties of these alloys, at room temperature and at a variety of strain rates, has been related to their microstructure in terms of volume fractions and nature of the hydride phases present. Nomarski Differential Interference Microscopy has been used to make detailed optical examinations of hydride morphology in unetched specimens and the hydride phases Q and Y have been identified unambiguously, even when present in low concentrations, by means of a Nonius Guinier quadruple focussing X-ray camera technique. The microstructure and constitution of the alloys has been found to vary with increases in cooling rate from the solid solution phase field and it has been shown that while slow cooling favours precipitation mainly at grain boundary sites, increases in cooling rate favour the formation of Widmanstatten structures. It has also been shown that slow cooling favours the precipitation reaction alpha → alpha+delta while increases in cooling rate progressively favour the reaction alpha → alpha + gamma. The magnitude of the effect of hydride on the mechanical properties of zirconium-hydrogen alloys has been shown to depend more on the volume fraction of the second phase than on its nature. It has been shown that low strain failures may be associated with the presence of continuous networks of hydride throughout the specimens and in these cases it has been demonstrated that there is no significant difference between the embrittling effects of the hydride phases Q and Y. It has been found that the presence of a triaxial stress system markedly reduces the strains to failure in these alloys and that specimens which normally fail at plastic strains of about 10% may be made to do so after about 1% strain in the presence of a notch.