The transformation behavior of a U-16.4 at % Nb-5.6 at % Zr alloy

The isothermal transformation kinetics of a γ-phase U-16.4 at % Nb-5.6 at % Zr alloy have been studied by resistometric techniques between 130 and 590° C. Three distinct regions of transformation were observed. Above 420° C the transformation proceeded by nucleation and growth and progressed to the equilibrium $\text{α + γ}{}_2$ phases. Aging at temperatures between 270 and 400° C resulted in the appearance of a transition $\text{α}{}^{\mathrm{″}}$ monoclinic phase that remained stable after even 7 days of aging at 380° C. Aging below 250° C produced a tetragonal $\text{γ}{}^0$ phase. A negative temperature coefficient of resistance is observed at temperatures below 400° C when the material is quenched from a 900° C anneal. It is suggested that 400° C represents an athermal γ to $\text{γ}{}^{\mathrm{<mtext>S</mtext>}}$ transformation and that the negative resistance coefficient results from a continual change of lattice parameters of $\text{γ}{}^{\mathrm{<mtext>S</mtext>}}$ with decreasing temperature.