Neutron diffraction study of the deformation mechanisms of the uranium–7 wt.% niobium shape memory alloy

The shape memory effect (SME) has been reported in the uranium–niobium alloy system in the region of the phase diagram surrounding U–6.5 wt.% Nb. In this regime, the material may have either an α″ monoclinic (U–6 wt.% Nb), or γ0 tetragonal structure (U–7 wt.% Nb) and is two phase near 6.5 wt.% niobium. In situ neutron diffraction studies during uniaxial compressive loading of U–7 wt.% Nb indicate that strain in the recoverable region is accommodated by both motion of existing twin boundaries within γ0-phase and stress-induced phase transformation from the γ0 to the α″ structure. The volume fraction of the γ0-phase decreases from 100% initially to ∼26% after 4% total strain and some reversion is observed on release. The initial stress state of the stress-induced α″ grains will be discussed as well as the load sharing between the two phases.