In this paper, the effects of Co addition on the microstructural evolution and mechanical properties of Ti-Nb based shape memory alloys were systematically studied. The results showed that the addition of Co into Ti-Nb shape memory alloy can tailor its average bond length of atoms and average d orbital energy level, further changing the phase constituents. With increasing Co content from 0.5 at.% to 5.0 at.%, the evolution of phase composition for Ti-Nb based shape memory alloys was as follows: α´´ + β → β + ω → β → α´´ + β → β. In contrast, the volume fraction of α´´ martensite phase in Ti-Nb shape memory alloys with 2.0 at.% Co was obviously higher than that in Ti-Nb shape memory alloys with 0.5 at.%. Moreover, the self-accommodation configuration with a V-shaped and triangular martensite variants were typical morphologies in Ti-Nb-Co shape memory alloy in order to the minimum energy theory. On the basis of solid solution strengthening, the yield strength and maximum tensile strength continuously increased, as the Co content is increased from 0.5 at.% to 3.0 at.%. However, the reduction of yield strength and maximum tensile strength was observed in Ti-Nb-Co shape memory alloy with 5.0 at.% Co owing to the formation of metastable β parent phase. Meanwhile, the largest value of micro-hardness (421 HV) can be optimized by tailoring 1.0 at.% Co as a result of solid solution strengthening and precipitation strengthening.

本文系统研究了Co添加对Ti-Nb基形状记忆合金显微组织演变和力学性能的影响。结果表明，在Ti-Nb形状记忆合金中添加Co可以调整其原子平均键长和平均d轨道能级，进一步改变相组成。随着 Co 含量从 0.5 at.% 增加到 5.0 at.%，Ti-Nb 基形状记忆合金的相组成演变如下：α´´ + β → β + ω → β → α´´ + β → β。相比之下，含2.0 at.% Co 的Ti-Nb 形状记忆合金中α'' 马氏体相的体积分数明显高于含0.5 at.% 的Ti-Nb 形状记忆合金。而且，根据最小能量理论，具有 V 形和三角形马氏体变体的自调节配置是 Ti-Nb-Co 形状记忆合金中的典型形态。在固溶强化的基础上，随着 Co 含量从 0.5 at.% 增加到 3.0 at.%，屈服强度和最大抗拉强度不断提高。然而，由于亚稳态 β 母相的形成，在含 5.0 at.% Co 的 Ti-Nb-Co 形状记忆合金中观察到屈服强度和最大抗拉强度降低。同时，由于固溶强化和析出强化，可以通过定制 1.0 at.% Co 来优化显微硬度的最大值 (421 HV)。随着 Co 含量从 0.5 at.% 增加到 3.0 at.%，屈服强度和最大抗拉强度不断提高。然而，由于亚稳态 β 母相的形成，在含 5.0 at.% Co 的 Ti-Nb-Co 形状记忆合金中观察到屈服强度和最大抗拉强度降低。同时，由于固溶强化和析出强化，可以通过定制 1.0 at.% Co 来优化显微硬度的最大值 (421 HV)。随着 Co 含量从 0.5 at.% 增加到 3.0 at.%，屈服强度和最大抗拉强度不断提高。然而，由于亚稳态 β 母相的形成，在含 5.0 at.% Co 的 Ti-Nb-Co 形状记忆合金中观察到屈服强度和最大抗拉强度降低。同时，由于固溶强化和析出强化，可以通过定制 1.0 at.% Co 来优化显微硬度的最大值 (421 HV)。