中文摘要：

建立了适用于孔隙NiTi合金中B2-R相变的相场模型,并用该相场模型研究了多孔NiTi合金中B2-R转变的微观组织演化过程以及孔隙率和孔尺寸对R相变体生长动力学行为的影响.多孔NiTi合金中R相变体以相互协调的方式形成＂带状＂的三维结构和＂鱼骨＂状的二维组织,变体之间形成的孪晶面包括{101}_（B2）和{001}_（B2） 2种,4组变体相交于〈010〉_（B2）;这些结果与致密NiTi合金B2-R相变过程相同.多孔NiTi合金中R相优先在孔周围形核,且较大的孔周围有较多的变体形核;R相变体的平均尺寸随孔隙率增大而逐渐减小,随孔径增大而增加;相对于规则圆孔,不规则孔隙可导致R相变体尺寸略微减小;变体的尺寸均匀性则随孔隙率增大而提高,但对孔径大小和孔形状不敏感;孔隙数量越多且孔径越小,则B2-R转变越倾向于产生均匀而细密的R相组织.

英文摘要：

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 RB19′. Among these phase transformations B2 B19′ and RB19′ 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 transforma- tion. 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 phasetransformation 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 herringbone microstrueture 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