Research topic for PhD study Supervisors: Dr Wenyi Yan (Monash University) and Prof. Qingping Sun (Hong Kong University of Science and Technology) Indentation method to measure the mechanical properties of shape memory alloy Shape memory alloy (SMA) is a well known functional material for possessing shape memory effect and superelasticity behaviour, which are due to intrinsic microstructure transition of thermoelastic martensitic transformation. The representing NiTi polycrystalline SMA has been used in human implants and surgical devices for many years with a famous example of NiTi vascular stents to reinforce blood vessels by applying either shape memory behavior or superelastic mechanisms. Recently, SMA thin film has been recognized as a promising and high performance material in the field of Micro-Electro-Mechanical Systems (MEMS). To accurately acquire the mechanical properties of a material applied in micro- machines, such as micro-sensors and micro-actuators in MEMS, is prerequisite for the purpose of structure design and safety evaluation. Conventional testing method such as tensile test at macro-scale can not be applied any more to obtain reliable property data for micro-structure materials. On the other hand, nano-indenters provide accurate measurements of the continuous variation of indentation load down to µN, as a function of the indentation depth down to nm. To extract hardness and other mechanical properties become possible from the indentation load versus the indentation depth measured from nano-indentation test at micro-scale on the material surface. The proposed project will apply the method of dimensional analysis, transformation constitutive models and the finite element approach to investigate the nano- indentation tests. From this investigation, the relationships between the indentation responses, such as hardness, contact area, initial unloading slope, and mechanical properties of superelastic SMA will be established. To solve the reverse problem, a procedure to extract the mechanical properties of NiTi SMA from nano-indentation will be developed. This developed technique will be applied to determine the mechanical properties of NiTi thin films, which will be directly used to assist the design of NiTi-based micro-machines in MEMS and the development of wear- resistant NiTi coating in surface engineering.
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