Effect of Deformation Induced Transformation in B2-Type TiNi Alloys.

近等原子成分的Ti-Ni合金表现出与热弹性马氏体相变相关的形状记忆性能。另一个特殊特征是B2母相和B19‘马氏体相在室温附近的异常塑性。在应力诱发马氏体形成的上限温度Mssiguma以下，Ti-Ni合金的延伸率约为40%~60%。Mssiguma以上延性降低到30%。马氏体应力-应变曲线分为三个阶段。第一阶段观察到了&lt；011&gt；II型孪晶的脱孪生和(100)复合孪晶的形成。第二阶段和第三阶段的变形方式不仅是滑移变形，而且是孪生变形，存在(100)、(001)和(20-1)复合孪晶。B19‘马氏体的塑性是由于形成了具有0.4250的大孪晶剪切量的(20-1)孪晶，并由于形成了三个复合孪晶而增加了独立滑移系。在MS到Mssiguma处形成了基本相同的微结构。在Mssiguma以上，B2母相通过拉伸变形形成应变诱发马氏体。然而，马氏体并不是随着应变的增加而长大的。在马氏体相稳定的温度范围内，Ti-Ni合金表现出良好的塑性。Goo等人提出，具有B_2结构的Ti-Ni合金的塑性是由于形成了{114}B_2化合物孪晶。在本研究中，这种双胞胎来自(20-1)B19‘双胞胎。也就是说，逆转变后的(20-1)B19‘孪晶仍为{114}B2化合物孪晶。这与B2母相和B19‘马氏体相之间的晶格对应是一致的。因此，B2型钛镍合金在室温附近的异常塑性与应力诱发马氏体有关。

Ti-Ni alloys of near-equiatomic composition exhibit the shape memory properties associated with thermoelastic martensitic transformation. The other peculiar characteristic is anomalous ductility around room temperature in both B2 parent and B19' martensite phases. The ductility of Ti-Ni alloys is about 40 to 60% in tensile elongation below Mssiguma which is upper limit temperature for the formation of stress induced martensite. The ductility is reduced to 30% above Mssiguma. The stress-strain curves of martensite is divided into three stages. The de-twinning of <011> Type II twin and the formation of (100) compound twin are observed in the first stage. The deformation mode in second and third stages is not only slip deformation but also twinning deformation with the (100), (001) and (20-1) compound twins. The ductility of the B19' martensite is attributable to the formation of (20-1) twin with large twinning shear of 0.4250 and the increment of independent slip system due to the formation of the three compound twins. The essentially same microstructures are developed at Ms to Mssiguma. Above Mssiguma, strain induced martensite is formed in B2 parent phase by tensile deformation. However, the martensite is not grown with increasing the strain. The Ti-Ni alloys exhibit good ductility at the temperature range where the martensite is stable. Goo et al., have suggested that the ductility of the Ti-Ni with B2 structure is attributable to the formation of {114} B2 compound twinning. In the present study, this twinning is derived from the (20-1)B19' twinning. That is, the (20-1)B19' twinning is remained as the {114}B2 compound twinning after reverse transformation. This Is consistent with the lattice correspondence between B2 parent phase and B19' martensite phase. Therefore, the anomalous ductility of B2-type Ti-Ni alloys around room temperature is related to the stress induced martensite.