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| PHASE FIELD SIMULATION OF EFFECTS OF PORES ON B2-R PHASE TRANSFORMATION IN NiTi SHAPE MEMORY ALLOY |
| KE Changbo, MA Xiao, ZHANG Xinping |
| School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640 |
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Cite this article:
KE Changbo MA Xiao ZHANG Xinping. PHASE FIELD SIMULATION OF EFFECTS OF PORES ON B2-R PHASE TRANSFORMATION IN NiTi SHAPE MEMORY ALLOY. Acta Metall Sin, 2011, 47(2): 129-139.
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Abstract Recently, porous NiTi shape memory alloys (SMAs) have drawn great interest in various engineering fields, in particular for biomedical applications as one of the promising biomaterials for hard-tissue replacements and orthopedic implants. It is well known that the porous NiTi SMAs exhibit three transformations, B2-B19', B2-R and R-B19'. Among these phase transformations B2-B19' and R-B19' involve high lattice distortion and large transformation hysteresis. Consequently, these distortion and transformations usually introduce structural defects which may result in degradation of mechanical stability for the functional application. On the contrary, B2-R transformation is governed by small lattice distortion which indicates less damage to the microstructure and lower sensitivity to the defects, and thus generates higher reversibility and mechanical stability. Due to these unique virtues, it is of great importance to study the R-phase transformation behavior in porous NiTi SMAs, since the pore has a significant influence on B19' martensitic transformation as well as the R-phase transformation. In this paper, a three-dimensional phase field model aiming at accounting for the pore effect on phase transformation in NiTi SMAs was developed to study the B2-R phase transformation behavior in porous NiTi SMAs. The model was applied to characterize the microstructure evolution of B2-R phase transformation as well as the influence of porosity ratio and pore size on growth kinetics of R-phase variants. The simulation results show that the R-phase variants can form three-dimensional banded structure and two-dimensional herring-bone microstructure through self-accommodation between R-phase variants. The R-phase variants nucleate preferentially around the pores, and there are more R-phase variants to nucleate around the large pores than the smaller ones. Two types of twinning planes are found, which are {101}B2 and {001}B2 respectively, and four variants meet at <010>B2. It has been shown that the average size of R-phase variants decreases with increasing porosity ratio, but increases with increasing pore size. Moreover, the average size of R-phase variants decreases in NiTi system containing irregular pores compared to that the system containing regular circular pores. The size uniformity of R-phase variants increases with increasing porosity ratio but shows no sensitivity to pore size and pore shape. It is also shown that the B2-R transformation can generate uniform and fine R--phase microstructure only when the NiTi matrix contains a large number of small size pores.
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Received: 25 August 2010
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| Fund: Supported by National Natural Science Foundation of China (Nos.50871039 and 50801029) and Chinese Government Graduate Student Overseas Study Program (No.2008615024) |
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