The influence of dislocations on B19' and R-phase transformations in a NiTi shape memory alloy

The ability to control the stress-induced phase transformation of the shape memory alloy, NiTi, is an important technological challenge that must be understood for their wide application in devices that can exploit their reversible strain properties. This study elucidates the direct relationship between dislocation density and the martensitic, B19' & R-phase transformations, including its formation temperature from interrupted annealing of rolled NiTi samples. Deformation is shown to determine the enthalpy change required for the B2 -> R -> B19' transformation, with associated transformation temperatures being modifiable via dislocation density and recovery processes. Recovery is shown to be rapid, highly heterogeneous and sensitive to crystal orientation. Grains with a < 100 > direction close to the macroscopic rolling direction recover more rapidly than < 110 > and < 111 > orientated grains. Considered to be governed by processing induced residual stresses and resultant crystallographic dependent annihilation/slip pathways, there are opportunities to tune B2 -> R -> B19' transformation on either a grain-averaged or an orientation dependant per-grain basis.