The effects of Ti content and quenching on phase transformations, microstructures, and mechanical properties in uranium-titanium alloys

Abstract

• Increases in Ti content increase critical cooling rate. • Quenched microstructures progress from massive to acicular and banded martensites. • Acicular martensite forms directly from γ-phase: γ → α′ a . • Transition to banded martensite may occur when γ 0 intervenes: γ → γ 0 → α′ b . • Quenched alloys are ductile, supersaturated, and amenable to age hardening. 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 γ-phase and facilitate diffusionless transformation to supersaturated variants of α-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 γ-phase to supersaturated variants of α-phase. Very dilute alloys transform via a γ → β → α m sequence of massive transformations. Martensitic γ → α′ a transformation begins at ∼0.4%Ti, and 100% α′ a microstructures are obtained from ∼0.65% to ∼1.4% Ti. A transition to banded α′ b martensite occurs at ∼1.5%Ti. Evidence suggests that the α′ a to α′ b transition may occur when the cubic γ-phase first transforms to tetragonal γ°, which in turn transforms to orthorhombic α′ b via the sequence γ → γ° → α′ 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.