The plastic deformation of alpha-uranium

Thermomechanical Response of Polycrystalline alpha-Uranium

“…Grain Boundary S liding (di f f usion or slip accommodated)-Grain boundary sliding is consistent with the creep experiments that were run between 300-550 o C and at similar stress levels µ ⇠ 10 4 [38], which saw a stress exponent of ⇠2.4. A similar stress exponent was reported elsewhere in the literature at similar temperatures [17,69].…”

“…In other words, the sample went through primary creep again upon returning to temperature, as opposed to going straight to steady state creep as would be expected in most other metals, especially pure metals. Creep experiments have reported a stress exponent of ⇠2.5 [17,38], which is consistent with a grain boundary sliding mechanism. Grain boundary sliding is often reported to have n=2-3, and solute drag dislocation creep which is reported as n=3.…”

“…It is noted that the creep experiments reported a temperature dependence that was much more subtle (meaning the transitions is much smoother) for the steady state response [38]. It is noted that the magnitude of strains are such that deformation would be in the primary creep regime, so parameters coming from steady-state creep are not directly applicable.…”

“…Unlike most metals, a cooling and heating cycle in the middle of a creep experiment causes a shift in the deformation rate in deplete ↵-uranium. Young showed that cooling and returning to temperature while under stress induced a plastic strain during the thermal cycle and essentially reset the creep state of the material [38]. In other words, the sample went through primary creep again upon returning to temperature, as opposed to going straight to steady state creep as would be expected in most other metals, especially pure metals.…”

Transformation superplasticity of zirconium

“…[1] The former type of superplasticity relies on grain-boundary sliding and is operative in metals with grains smaller than 10 m, which must be stable at the temperature of deformation. These mismatch stresses and the resulting strain increments can be repeatedly produced by thermal cycling of pure metals exhibiting coefficients of thermal expansion anisotropy [1,2] (e.g., Zn, [3,4,5] and U [3,4,6] ) and/or an allotropic phase transformation [1,7] (e.g., Fe, [8,9,10] Co, [8,11] Ti, [8,12] Zr, [8,13] and U [8] ). [1] Since pure metals display neither duplex structures nor grain-boundary pinning, they exhibit rapid grain growth at elevated temperatures and are, thus, typically incapable of fine-structure superplasticity.…”