固溶处理及时效对镍钛形状记忆合金组织演化及力学性能的影响

将镍钛形状记忆合金Ni50.9Ti49.1（摩尔分数）在1123 K固溶处理2 h，然后分别在573、723和873 K时效2h。采用透射电镜、高分辨率透射电镜、扫描电镜和压缩实验，系统研究固溶处理和时效对镍钛合金组织演化及力学性能的影响。结果表明：固溶处理有助于消除原始镍钛样品中的Ti2Ni相，但不能消除TiC相。固溶处理导致镍钛合金中原子排列的有序畴界。在所有时效镍钛样品中，Ni4Ti3析出相、R相和B2奥氏体相共存于室温下的镍钛基体上，然而在873 K时效的镍钛样品中，可以观察到马氏体孪晶。在573和723 K时效的镍钛样品中，细小密集的Ni4Ti3相均匀分布在镍钛基体上，而且与B2基体保持共格关系。然而，在873 K时效的镍钛样品中， Ni4Ti3相尺寸非常不均匀，和B2基体保持共格、半共格和非共格关系。在723 K时效的条件下，细小均匀的Ni4Ti3相阻碍位错运动，导致最大的位错滑移临界分切应力，因此镍钛样品表现出最高的屈服强度。

Effect of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi shape memory alloy

As-received nickel-titanium （NiTi） shape memory alloy with a nominal composition of Ni50.9Ti49.1 （mole fraction,%） was subjected to solution treatment at 1123 K for 2 h and subsequent aging for 2 h at 573 K, 723 K and 873 K, respectively. The influence of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi alloy was systematically investigated by transmission electron microscopy （TEM）, high resolution transmission electron microscopy （HRTEM）, scanning electron microscopy （SEM） and compression test. Solution treatment contributes to eliminating the Ti2Ni phase in the as-received NiTi sample, in which the TiC phase is unable to be removed. Solution treatment leads to ordered domain of atomic arrangement in NiTi alloy. In all the aged NiTi samples, the Ni4Ti3 precipitates, the R phase and the B2 austenite coexist in the NiTi matrix at room temperature, while the martensitic twins can be observed in the NiTi samples aged at 873 K. In the NiTi samples aged at 573 and 723 K, the fine and dense Ni4Ti3 precipitates distribute uniformly in the NiTi matrix, and thus they are coherent with the B2 matrix. However, in the NiTi sample aged at 873 K, the Ni4Ti3 precipitates exhibit the very inhomogeneous size, and they are coherent, semi-coherent and incoherent with the B2 matrix. In the case of aging at 723 K, the NiTi sample exhibits the maximum yield strength, where the fine and homogeneous Ni4Ti3 precipitates act as the effective obstacles against the dislocation motion, which results in the maximum critical resolved shear stress for dislocation slip.