Experimental investigation of the U–Zr–Al ternary phase diagram: Isothermal sections at 673 K and 1073 K

Abstract

Isothermal sections at 673K and 1073K of the ternary U–Zr–Al system were established in the whole concentration range, by means of powder X-ray diffraction, scanning electron microscopy–energy dispersive X-ray spectroscopy and differential thermal analysis. All measured compositions and unit-cell refinements were performed at room temperature from quenched samples annealed at 1073K and 673K for four and eight weeks respectively. For both temperatures, the Al-rich corner of the phase diagram is characterized by extended homogeneity ranges due to mutual exchange between U and Zr in UAl3 (cubic, AuCu3-type) and in the Laves phase UAl2 (cubic, MgCu2-type). Minute U solubility in ZrAl2 (hexagonal, MgZn2-type) and in Zr2Al (hexagonal, Ni2In-type) was evaluated to be of the order of 1at.%U. For the other binary compounds, the solubility of the third component was found negligible. At 1073K, the solid solution based on γU (cubic, W-type) which covers the U–Zr binary axis up to 95.5at.% Zr, allows also some limited solubility of Al [maximum of 5at.%]. For Al-content below 66at.%, most of the phase relations comprise equilibria between the Zr–Al binaries and the γ(U,Zr,Al) solid solution. At 673K, the U–Zr axis is found in agreement with the literature data and no Al solubility could be detected in αU, αZr and UZr2 (δ phase). The phase relations are mainly established between Zr–Al binaries and αU. For monolithic UMo fuel with a Zr diffusion barrier foil cladded with Al, the main interaction product is expected to involve the U-based alloy with the Zr–Al binary compounds and the pseudo-binary U1−xZrxAl2 and U1−xZrxAl3 phases.