NiTi shape memory alloy (SMA), as a typical functional material, has been widely used in many fields, such as biomedical devices, solid-state refrigeration and actuators. Recently, with the fast development of micro-electromechanical systems, the thermomechanical properties of the NiTi SMA at microscale have attracted wide attention but are very little explored so far. In this thesis, the microscale processing method was developed to manipulate the microstructure of NiTi for enhancing the thermomechanical performance, such as functional degradation, thermal expansion, and temperature dependent Young’s modulus. Firstly, the cuboidal micropillar with a dual-pillar method was exploited to measure the accurate stress-strain response of materials at microscale using the focused ion beam and...[ Read more ]

NiTi shape memory alloy (SMA), as a typical functional material, has been widely used in many fields, such as biomedical devices, solid-state refrigeration and actuators. Recently, with the fast development of micro-electromechanical systems, the thermomechanical properties of the NiTi SMA at microscale have attracted wide attention but are very little explored so far. In this thesis, the microscale processing method was developed to manipulate the microstructure of NiTi for enhancing the thermomechanical performance, such as functional degradation, thermal expansion, and temperature dependent Young’s modulus. Firstly, the cuboidal micropillar with a dual-pillar method was exploited to measure the accurate stress-strain response of materials at microscale using the focused ion beam and nanoindentation. Then, a compression-based plastic deformation was conducted on cuboidal NiTi micropillars and it is found that, at the moderate plastic deformation with residual strain of 3.5%, the dense and saturated dislocation structures and residual martensite were created in the micropillars, so the functional fatigue resistance was significantly improved together with the reduced transformation stress and hysteresis loop area, which is attractive to elastocaloric refrigeration applications; at severe plastic deformation (residual strain of 50%), the polycrystalline NiTi was extremely grain-refined into 10-nm nanocrystals and the volume fractions of B2 and B19′ phases can be well-manipulated by deformation temperature, so that a temperature independent Young’s modulus and a giant coefficient of stress-induced thermal expansion were achieved in dual-phase and single-phase nanocrystalline NiTi micropillars, respectively. Such results provide a new insight via micro-fabrication to develop high performance materials.

Keywords: NiTi shape memory alloy, microstructure manipulation, micropillar compression, functional fatigue, temperature dependent Young’s modulus, thermal expansion.