Martensitic Variant Selection During Compression/Tension Cycling of a Uranium–Niobium Shape Memory Alloy

Abstract The shape memory alloy depleted uranium 6-wt pct (~ 14 at. pct) niobium was tested in cyclic compression/tension with a nominal amplitude of ± 2 pct strain to 200 cycles where the test was halted without failure. Throughout the test an accumulated total strain of roughly 700 pct or a total plastic strain of 430 pct was achieved without fracture. Rapid cyclic hardening was observed throughout the extent of the cyclic test. At the same time a reduction of the area of the hysteresis loop with increased cycle was observed. While the elastic and elastic-plastic transition regions of the stress/strain curve were unchanged by cycling, the hardening rate beyond 1 pct strain increased significantly with cycle. In situ neutron diffraction tests collected during 13 interrupted cycles reveal that the deformation mechanism that is responsible for the shape memory effect, reorientation of martensitic twin-related variants (de-twinning) does not change with cycling, but the driving force to move the twin boundaries increases, resulting in the increased flow strength.