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

近等原子组成的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。另一个特殊的特征是B2母相和B19'马氏体相在室温附近的异常塑性。Ti-Ni合金的延展性在低于Mssiguma的拉伸延伸率中为约40%至60%，Mssiguma是应力诱导马氏体形成的上限温度。延展性降低到高于Mssiguma的30%。马氏体的应力-应变曲线可分为三个阶段。在<011>第一阶段，观察到Ⅱ型孪晶的退孪晶和（100）复合孪晶的形成。第二和第三阶段的变形方式不仅是滑移变形，而且还有孪生变形，其中有（100）、（001）和（20-1）复合孪晶。B19'马氏体的塑性主要是由于（20-1）孪晶的形成，孪晶切变高达0.4250，同时由于三个复合孪晶的形成增加了独立滑移系。在Ms至Mssiguma处开发了基本相同的微观结构。在Mssiguma以上，通过拉伸变形在B2母相中形成应变诱发马氏体。然而，马氏体并不随应变的增加而长大。Ti-Ni合金在马氏体稳定的温度范围内表现出良好的延展性。Goo等人，提出B_2结构Ti-Ni合金的塑性是由于{114} B_2化合物孪晶的形成。在本研究中，这种双晶是从（20-1）B19'双晶衍生而来的。也就是说，（20-1）B19'孪晶在逆相变后仍保持为{114}B2化合物孪晶。这与B_2母相和B_19 ′马氏体相的晶格对应关系是一致的。因此，B2型Ti-Ni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.