Thermomechanical deformation of shape memory alloys

NiTi is a shape memory alloy and can undergo crystallographically reversible martensitic transformation under applied loads resulting in recoverable of strains of the order of 5 %. The single crystal properties of shape memory alloys have been studied extensively in the past and a good understanding of the mechanical properties of the material in this form has been acquired. However, when used in practical applications shape memory alloys are used in their polycrystalline form. In a polycrystalline form the deformation behaviour may be quite different to that of a single crystal due to the constraints of surrounding grains and anisotropy of material properties. In the case of shape memory alloys these are anisotropic elastic and transformation properties. The main focus of the work in this thesis is the deformation behaviour of commercial rod samples of NiTi while under thermomechanical loads. The grain-orientation-specific internal strain development and phase faction evolution within particular grain orientations is evaluated during deformation by the in-situ neutron diffraction technique. The experimental results presented include stress-induced martensitic transformation, cooling through the martensitic transformation under a fixed stress, the generation of recovery stresses while heated under constraint, and studies of the detwinning of the B 19' martensite phase under compressive and tensile loading. In addition, the effect of ageing on mechanical properties of NiTi is investigated via the method. Changes in the load partitioning behaviour is noted for NiTi cooled under a fixed tensile stress of 200 MPa which compare well with modelling predictions in the literature. Large changes in the mechanical properties of NiTi as a results of ageing are ascribed to the presence of the R-phase due to the formation of precipitates during ageing. Evidence of detwinning of B 19' martensite in both tension and compression is found, in contrast to other work in the literature.