Ni-rich Ni50.3Ti29.7Hf20 (at.%) high-temperature shape memory alloy (SMA) tubes were thermomechanically cycled under constant torques. Four loading configurations were examined that consisted of a series of ascending stresses (low-to-high stress from 0 to 500 MPa outer fiber shear stress), a series of descending stresses (high-to-low stress from 500 to 0 MPa), and a series of thermal cycles at a constant stress of 500 MPa, all using an upper cycle temperature (UCT) of 300 °C. The last configuration consisted of another series of ascending stress levels using a lesser UCT of 250 °C. It was found that the descending series trial stabilized the material response in fewer cycles than the other loading paths. Similarly, cycling at a constant stress of 500 MPa for approximately 100 cycles reached near stabilization (<0.05% residual strain accumulation). Transformation shear strains were the highest and most stable when cycled from lower-to-higher stresses (ascending series), reaching 5.78% at 400 MPa. Cycling to lesser UCTs of 250 °C (vs. 300 °C) resulted in the highest two-way shape memory effect (TWSME), measuring over 3.25%. This was attributed to the effect of retained martensite and any transformation dislocations that served to stabilize the TWSME at the lower UCT. Results of this study suggest that different training paths might be used, depending on actuator performance requirements, whether the principal need is to maximize transformation strain, maximize the two-way shear strain, or stabilize the response in fewer cycles.

中文翻译：

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Ni50.3Ti29.7Hf20扭矩管的恒扭矩热循环和双向形状记忆效应

富镍 Ni50.3Ti29.7Hf20 (at.%) 高温形状记忆合金 (SMA) 管在恒定扭矩下进行热机械循环。检查了四种加载配置，包括一系列上升应力（从 0 到 500 MPa 外纤维剪切应力从低到高应力）、一系列下降应力（从 500 到 0 MPa 从高到低应力）、以及一系列 500 MPa 恒定应力下的热循环，所有循环上限温度 (UCT) 均为 300 °C。最后的配置包括使用 250 °C 较小 UCT 的另一系列上升应力水平。结果发现，与其他加载路径相比，下降系列试验可以在更少的周期内稳定材料响应。同样，在 500 MPa 恒定应力下循环约 100 个循环达到接近稳定（<0.05% 残余应变累积）。当从较低应力到较高应力（递增序列）循环时，相变剪切应变最高且最稳定，在 400 MPa 时达到 5.78%。循环至 250 °C（相对于 300 °C）的较低 UCT 会产生最高的双向形状记忆效应 (TWSME)，测量结果超过 3.25%。这归因于残余马氏体和任何有助于在较低 UCT 处稳定 TWSME 的相变位错的影响。这项研究的结果表明，可以使用不同的训练路径，具体取决于执行器性能要求，主要需求是最大化转变应变、最大化双向剪切应变，还是在更少的周期内稳定响应。