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

摘要

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 γ~0 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 γ~0-phase and stress-induced phase transformation from the γ~0 to the α" structure. The volume fraction of the γ~0-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.