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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.