The microstructures of 4 alloys (5, 9, 13 or 18at%Nb) were investigated, and self-accommodating twin structures were considered in terms of the lattice parameters of the martensite phases. Active deformation twinning systems which operated during deformation of the 13at%Nb shape-memory alloy were identified. The as-transformed twin structure consisted of finely twinned laths or bands. The fine twins were either (021) or (¯130), with other approximate twin orientations between the bands. The specific twin relationships which were present in the microstructures could be understood in terms of the distortions of the relevant crystal structures, and changes in lattice parameter with composition. The monoclinic distortion which was associated with the α" structure greatly decreased the shear of specific twinning systems; thus facilitating twinning deformation. The initial deformation of α" alloys was dominated by the motion of {¯172} twins which moved easily through the as-transformed microstructure due to their low twinning shear, and ability to cross existing twins. In the later stages of deformation within the shape memory regime, fine twins from the as-transformed structure were eliminated; presumably via the cooperative motion of {¯130} and crossing {¯172} twin boundaries. Large single-orientation regions were created, and were assumed to be a result of the migration of the remaining twin boundaries in the structure.

Martensitic Structures and Deformation Twinning in the U-Nb Shape-Memory Alloys. R.D.Field, D.J.Thoma, P.S.Dunn, D.W.Brown, C.M.Cady: Philosophical Magazine A, 2001, 81[7], 1691-724