Shape memory alloy - wenyi by few12005


									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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