Influence of processing methods on microstructure and multiscale functional properties of nitinol
Keywords: EPSC modelling
texture refinement
neutron diffraction
laue micro-diffraction
phase transformation
superelasticity
shape memory alloys
shape memory ceramics
grain structure
shape memory polymers
NiTi shape memory alloys
ageing
lattice rotation
EBSD
temperature effect
nickel-based superalloys
DSC
powder diffraction
synchrotron X-ray diffraction
multiscale
heat treatment

This thesis focuses on investigating the processing-microstructure-property relationships for the Nitinol material. Processing routes include metal forming process and heat treatment process. Metal forming processes include casting, hot rotary swaging, hot rolling, and cold drawing. The following aspects of the microstructure were taken into consideration: grain morphology (shape), grain size, grain boundary type, crystallinity, preferred grain orientation, precipitates, lattice defects, and phase composition. The functional properties of Nitinol considered in this thesis include Superelasticity and Shape Memory Effect. The mutual link between the functional properties and microstructure evolution is explored.
State-of-the-art experimental techniques were employed to studying the microstructure evolution during functional use of the material. Several key techniques applied form part of synchrotron science that in particular enables bulk material characterisation under operando conditions. An X-ray powder diffraction technique was used to provide mesoscale information regarding the lattice strain, phase volume fraction, texture, and microstrain within the material volume. Synchrotron X-ray Laue micro-diffraction provided microscale information regarding crystal lattice orientation, deviatoric elastic strain, and phase distribution. Time-Of-Flight neutron diffraction provided mesoscale information on the lattice strain, phase volume fraction, texture, microstrain, lattice parameters, and precipitate population. In addition, electron microscopy (EM) was extensively used to provide complementary observations from microscale down to the nanoscale. EM techniques included Focused Ion Beam (FIB) milling, Scanning Electron Microscopy, Transmission Electron Microscopy, Electron Back Scatter Diffraction, Energy-dispersive X-ray Spectroscopy. This thesis presents comprehensive employment of the advanced techniques mentioned above, and demonstrates how the combination of the techniques can bring about new findings to the field.
Computer simulations were conducted to mimic the microstructure evolution in silico. The modelling approaches included are Elastic-Plastic Self Consistent (EPSC) modelling and Crystal Plasticity Finite Element Modelling (CPFEM). Once the models were calibrated and verified by experimental observations, they could be used to predict the material microstructure evolution under much wider range of conditions than those used for model validation and refinement.
Through integrating both the experiment and simulation results, this thesis can be used as a systematic / practical guidance to customise the functional properties of Nitinol by means of microstructure engineering.