The effect of Ti content on age hardening and mechanical properties of uranium-titanium alloys

•Age hardening kinetics indicate multiple mechanisms of strengthening.•Activation energy varies with Ti content and microstructure.•Ductility decreases with increasing strength and does not recover on overaging.•Alloy selection requires consideration of quench rate sensitivity and age hardening potential. The effect of Ti content on age hardening and the resulting mechanical properties are described for γ-quenched U-Ti alloys containing 0.3 to 2.0 wt.%Ti. Age hardening occurs between ∼250 and ∼450°C. Overaging occurs at higher temperatures by cellular decomposition. Age hardening kinetics suggest that different mechanisms occur depending on Ti content and initial microstructure. Strengthening in α′a acicular martensites begins by the formation of Ti clusters which evolve into thin U2Ti disc shaped precipitates and later mature into continuous U2Ti rods beginning at peak hardness. The mechanism of hardening in α′b banded martensite is more elusive, as significant hardening occurs where atomic mobility is lower than that required for precipitate formation, similar to that reported for age hardening in α′′b banded martensite in U-6%Nb. The activation energy for aging varies with Ti content and microstructure. In fully martensitic alloys containing 0.75 to 2.0%Ti it is in the vicinity of ∼44 kcal/mole (184 kJ/mole). But it is lower in alloys containing less than 0.6%Ti where quenched microstructures are less than fully martensitic. Tensile ductility is high prior to aging, decreases with age hardening, is effectively zero at peak hardness, and remains low in overaged conditions. Attractive combinations of strength and ductility are best obtained in alloys containing 0.6 to 1.0%Ti which have been partially aged to fractional hardening levels no greater than ∼0.6. This corresponds to the very early stages of aging, associated with clustering and the earliest stages of U2Ti disc formation. Alloys containing 0.45%Ti or less are not as responsive to age hardening. Alloys containing 1.5 and 2.0%Ti can be aged to higher strengths, but extreme quench rate sensitivity prevents them from being effectively heat treated in realistic section thicknesses.