Phase stability, thermal properties and microstructure characteristics of U-15 wt.% Cr alloy

The thermal stability, thermal properties, phase transformation and microstructure characteristics of U-15 wt.%Cr alloy were investigated through a combination of calorimetry, dilatometry, x-ray diffraction and electron microscopy techniques. It is found that at room temperature, both as-cast and homogenized alloys contained metastable βm-U phase, in addition to equilibrium α-U and Cr. The electron microscopy of homogenized sample revealed fine scale nanosized dispersions of βm-U+Cr phases. The presence of metastable βm-U phase influenced the on-heating thermal stability significantly. It is found that upon heating, βm-U→α-U+Cr transformation preceded the occurrence of equilibrium α-U+Cr→β-U+Cr→γ-U+Cr transformations. The nonequilibrium βm-U→α-U conversion took place in a continuous manner. On the other hand, the transformations, α-U+Cr→β-U+Cr→γ-U+Cr occurred at distinct transformation temperatures with measurable changes in enthalpy. The calorimetry results are found to be in accord with dilatometry measurements, which showed a positive jump in the dilatational strain (Δl/lo) across α-U+Cr→β-U+Cr→γ-U+Cr transformations.