Texture development and deformation mechanisms during uniaxial straining of U–Nb shape-memory alloys

Abstract

The shape-memory effect is well documented in uranium–niobium alloys near the α″–γ^o metastable phase boundary. In situ neutron diffraction measurements during uniaxial loading indicate that U–14 at.% Nb (in the α″ monoclinic phase field) deforms by stress-induced twin reorientation. Alternatively, U–16 at.% Nb (initially γ^o tetragonal) undergoes a stress-induced phase transformation to the α″ monoclinic phase. The crystallographic texture of the monoclinic phase of both compositions has been measured and qualitatively interpreted by considering the orientation relationship between the most favoured α′′ variant and the parent phase. In addition, previously published observations of deformation structures within the shape-memory regime of a U–13 at.% Nb alloy are discussed within the context of the same model.

Acknowledgements

The authors wish to thank Dr Loren Jacobson for his assistance in creating the plots for figure 5. Stimulating discussions with Drs J.P. Hirth and A.G. Crocker are also gratefully acknowledged. Lastly, we would like to thank Alan Patterson and Paul Dunn for their continued support of research in uranium metallurgy. This work was supported under DOE contract No. W-7405-ENG-36.

Notes

Previously published in Vol. 85, No. 13, pp. 1441–1458

Although the actual composition of the alloy investigated by the current authors was U–13 at.% Nb, U–14 at.% Nb was chosen as the nominal composition for the SME alloy. Note that the strains calculated in the ensuing discussion will be slightly different for other compositions (including U–13 at.% and U–16 at.% Nb), but the overall results regarding preferred variants and driving strains for variant switching will be similar.

The proper unit cell for the γ^o structure is rotated 45° from that of the cubic γ phase. Thus, the ‘a’ lattice parameter of the tetragonal γ^o phase should be divided by

for comparison to the ‘a’ lattice parameter of the cubic γ phase. See [17] or [23] for more details regarding these structures.