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 alpha" monoclinic (U-6 wt.% Nb), or gamma" 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 gamma(0)-phase and stress-induced phase transformation from the gamma" to the alpha" structure. The volume fraction of the gamma'-phase decreases from 100% initially to similar to 26% after 4% total strain and some reversion is observed on release. The initial stress state of the stress-induced alpha" grains will be discussed as well as the load sharing between the two phases. (c) 2005 Elsevier B.V. All rights reserved.