Direct Evidence for Three Transformation Mechanisms for the β→α Phase Change in Uranium - Chromium Alloys

Abstract

PURE uranium exists in three allotropic forms: α, which is orthorhombic and stable up to 667° C; β, which is tetragonal and stable between 667° and 775° C; and γ which is body-centred cubic and is stable between 775° C and the melting point 1,132° C. In the pure metal the phase changes occur with great speed, and investigation of the mechanisms of the changes is extremely difficult. The addition of small concentrations (∼ 0.5 atomic per cent) of certain solutes, of which chromium is one, retards the β → α change so effectively that β can be retained at moderate cooling rates to sub-critical temperatures and then allowed to proceed isothermally. White¹ and others^2,3, using either dilatometric or metallographic techniques, showed that the time–temperature–transformation (TTT) diagram associated with isothermal β→α transformation in various dilute uranium alloys consisted of two separate C-curves. Metallographic and crystallographic investigations^4–7 have shown conclusively that the low-temperature C-curve (hereafter referred to as C₃), which in U–0.5 per cent Cr alloy extends from room temperature to about 400° C, is associated with a martensitic mechanism. It was supposed that the upper C-curve, covering the range 400°–620° C in this alloy, was associated with transformation involving thermally activated atomic movement. However, metallographic observation showed that the micro-structure of the α formed at temperatures between about 525° C and 620° C was quite different from that formed between 525° C and 400° C, suggesting that two formation mechanisms were possible, one in each temperature range. In order to explore this possibility further Dixon and Burke⁸ redetermined the TTT diagram for a U–0.5 atomic per cent Cr alloy using an electrical resistivity method which was regarded as a more sensitive technique than those used in previous investigations. Evidence was found to suggest that the upper C-curve was composed of two overlapping C-curves, intersecting at about 520° C as required by the two-mechanism hypothesis.