首页|The effects of Ti content and quenching on phase transformations, microstructures, and mechanical properties in uranium-titanium alloys
The effects of Ti content and quenching on phase transformations, microstructures, and mechanical properties in uranium-titanium alloys
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NSTL
Elsevier
The effect of Ti content on phase transformations, microstructures, and mechanical properties of U-Ti alloys are described for alloys containing 0.3 wt.% to 2.0 wt.%Ti. Rapid cooling is required to overcome diffusional decomposition of gamma-phase and facilitate diffusionless transformation to supersaturated variants of alpha-phase. Critical cooling rate increases with increasing Ti content, opposite to the trend observed in U-Mo and U-Nb alloys. This difference occurs because the martensite transformation temperatures in these relatively dilute U-Ti alloys are above the knee of the C-curve for diffusional decomposition, unlike those in the more concentrated U-Mo and U-Nb alloys. In these U-Ti alloys critical cooling rate depends on the amount of undercooling required to reach M-s, which increases with increasing Ti content, and the time for diffusional decomposition to occur just above M-s, which decreases with increasing Ti content. The net result is that higher cooling rates are required as Ti content increases. Full quenching results in diffusionless transformation of gamma-phase to supersaturated variants of alpha-phase. Very dilute alloys transform via gamma -> beta -> alpha(m) sequence of massive transformations. Martensitic gamma -> alpha'(a) transformation begins at similar to 0.4%Ti, and 100% alpha'(a) microstructures are obtained from similar to 0.65% to similar to 1.4% Ti. A transition to banded alpha'(b) martensite occurs at similar to 1.5%Ti. Evidence suggests that the alpha'(a) to alpha'(b) transition may occur when the cubic gamma-phase first transforms to tetragonal gamma degrees, which in turn transforms to orthorhombic alpha'(b) via the sequence gamma -> gamma degrees -> alpha'(b). Fully quenched alloys exhibit moderate strengths and ductilities, and their supersaturation with Ti makes them amenable to subsequent age hardening. Subcritical quenching typically results in two-phase microstructures with lower ductilities and near-zero Ti-supersaturation, eliminating the possibility of subsequent age hardening. (C) 2021 Published by Elsevier B.V.
NSTL
Elsevier
