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Some critical issues and problems for the development of TiNi thin films and their MEMS applications were discussed in this paper. New types of TiNi thin film based microactuators, such as microgrippers, microvalves, micromirror, etc. were designed and fabricated using MEMS techniques.

Porous Ti-50.5at.%Ni shape memory alloy (SMA) samples with a range of porosities were prepared by spacer sintering. The porous structure of the alloy was examined using scanning electron microscopy (SEM). The phase constituents of the porous TiNi alloy were determined by X-ray diffraction (XRD). The shape memory behavior of the porous TiNi alloy was investigated using loading–unloading compression tests. Results indicate that the porous TiNi alloy exhibits superelasticity and the recoverable strain by the superelasticity decreases with the increase of porosity. After a prestrain of 7%, the superelastically recovered strains for the porous TiNi alloy samples with porosities of 46%, 59%, 69% and 77% are 2.0%, 1.8%, 1.5% and 1.3%, respectively. The pores in the TiNi alloy samples cause stress/strain concentration, as well as crack initiation, which adversely affect the shape memory behavior of the porous TiNi alloy.

In order to clarify the effect of annealing on the shape memory behavior of Ti-50.85at.%Ni alloy, the deformation and transformation behavior were investigated using tensile tests and differential scanning calorimeter (DSC). The martensitic transformation temperature increases with increasing annealing temperature until it reach as a maximum, and then decreases with further increasing annealing temperature. This can be rationalized by interaction between the distribution of Ti₃Ni₄ precipitates and recovery of cold-worked structure. The R-phase transformation temperature increases with increasing annealing temperature until reaching a maximum, and then decreases with a further increase of annealing temperature. This is attributed to the change of Ni content in the matrix caused by precipitation of Ti₃Ni₄. The critical stress for slip decreases rapidly with increasing annealing temperature, influenced by interaction between the distribution of Ti₃Ni₄ precipitates and recovery of cold-worked structure.

NiTi wires are successfully employed asthermal actuators: devices able to convert thermal energy into mechanical energy, generating force and displacement. So far, researchers have highlighted the influence of various parameters on shape memory characteristics of the material, such as chemical composition, cold working, heat treatment and cycling. This study is aimed to prove that drawing histories, in particular the number of drawing steps performed to achieve a certain cold working level, are fundamentally important on functional properties of wires. This process parameter involves significant changes in terms of thermal hysteresis width. These alterations are still observable on wires thermo-mechanical cycled, therefore on products ready to be employed for actuators.

To obtain TiNi foams with interconnected pores that have surface quality necessary for bone growth in addition to required mechanical performance, sintering with the space holder technique was employed in this study, which aimed to evaluate the bone healing process of TiNi graft materials. For this purpose, processed TiNi foams with three different porosities were placed into the created defects in the femur of rats. Moreover, the mechanical properties of the processed TiNi foams were conducted via monotonic compression tests in order to evaluate mechanical biocompatibility.

This paper experimentally analyzes the influence of repeated thermal cycling at zero stress on the two-way training efficiency for developing the two-way shape memory effect in TiNi shape memory alloys. Four different sets of TiNi wire are used in the study: two sets have been heat-treated to ensure a transformation path with no R-phase, whereas the other two sets have had the same heat-treatment which is then followed by repeated thermal cycling at zero stress to stabilize their phase transformation behavior. Subsequently, thermal cycling under constant stress two-way training is performed on each sample set. The study then analyzes the training efficiency and the recoverable two-way memory strain for each sample set. The results suggest that to obtain the best two-way material performance, that is, to obtain a substantial two way memory strain together with an increase in the efficiency of the training, the TiNi wire should be thermally cycled at zero stress prior to training until the phase transformation is stabilized.