The constitution of thorium-zirconium alloys containing more than 15% zirconium and the effect of some third elements on the stability of the body-centred-cubic phase in these alloys

Abstract

The constitution of thorium-zirconium alloys containing more than 15 at. % zirconium has been investigated, chiefly by metallographic methods. The decomposition of the high-temperature body-centred-cubic solid solution (β) has been found to be markedly affected by the purity of the starting materials. The β-phase in alloys prepared from commercial purity thorium and from “hafnium-free” zirconium or from zirconium containing 1 at. % hafnium has been found to break down to a β 1 + β 2 solid state miscibility gap which has a maximum at 52.5 at.% zirconium and a temperature of 950 ± 10° C. Such a miscibility gap has not been observed in alloys made from “iodide” thorium. Some observations on the metastable microstructures obtained by quenching from the β-field. are also given, together with the crystal structures and lattice parameters of these alloys. The effect of additions of 2 at. % each of cerium, hafnium, indium, titanium and uranium on the decomposition of the β-field in alloys prepared from zirconium containing 1 at. % hafnium has also been studied. Indium, titanium and uranium increase the size of the miscibility gap, indium having the least effect and titanium the greatest. Additions of cerium and hafnium cause the gap to disappear, a smooth β-decomposition curve with a point of inflection in the composition range 45–55 at. % zirconium being obtained.