Cost-effective temperature adaptive Cu-based shape memory seals through the synergistic effect of co-microalloying and cooling rate control

大多数汽车和卫星微致动器（液压、气动等）利用密封件来防止流体流出和灰尘、湿气和其他外来材料进入。在这些动态应用中，密封件与轴的旋转表面接触，并且必须在工作温度范围内满足最大摩擦、磨损和泄漏的某些要求。车辆的温度通常在-50至150摄氏度之间变化，而卫星的温度范围更宽（-100至200摄氏度），这取决于表面是否暴露在阳光下或阴影中。在较宽温度范围内使用轴封的主要限制来自于轴和密封随温度膨胀的差异。在低温（冷冻条件）下，密封件-轴摩擦高，而在高温（高于100摄氏度）下，轴-密封件间隙变得如此大，以至于可能发生过度泄漏，因此，在工作温度范围内保持压力恒定是有意义的。过度的摩擦和磨损会缩短密封件的使用寿命，而泄漏会导致润滑剂损失（即，卡住）或润滑剂进入（即，外来物质可能进入并损坏发动机和其他部件）。同时，密封件应该比其安装的轴磨损得更快，因为轴比密封件更昂贵且更难更换。这表明需要在工作温度范围内具有调谐性能的新型轴封。在这项研究计划中，我将开发一种新的策略，以克服这些限制，包括使用形状记忆合金与定制的可逆性和磨损性能，通过优化控制的微观结构和组成。微观结构，即，通过控制冷却速率和成分，在单个加工步骤中冷却时，将调整晶粒尺寸和分布。通过与铜相比，相对低成本的元素如铁和镍的多种微量元素共添加，将开发新的成分，以促进奥氏体的孪晶倾向。变形孪晶是当材料受到最小应力值时在奥氏体中的协同过程中发生的原子的小运动，导致宏观变形。一旦材料孪生，并且施加的力被释放，它就保持其变形的形状，除非加热到高于基准温度，对于该基准温度，材料恢复到初始位置（即，解孪晶）。一些元素具有降低孪生所需能量的能力，因此促进孪生倾向（即，原子在受到张力时移动的容易程度）。这使得能够控制温度，并因此控制轴-密封件接触应力，在该温度下马氏体转变成奥氏体，从而导致较小的密封件直径，并因此减少泄漏。为了降低Cu基形状记忆合金的成本并使它们比NiTi合金更适合于致动器工业，将采用低成本的微量元素。因此，了解如何定制这些Cu基形状记忆合金的热机械和摩擦学性能在基础物理冶金学以及工业应用中至关重要。传统上，快速凝固材料的晶粒尺寸已经通过调整成分、通过用另一种元素部分取代一种元素或通过控制冷却速率来优化。然而，通过优化这两个参数产生的协同效应是新颖的，并且具有定制形状记忆合金的性质的巨大潜力。在这方面，正如我以前所观察到的（S。González et al. Sci. Tech.高级材料15，2014），以某些浓度添加一些微量元素（例如Fe和Co）可以促进含Cu合金的机械驱动的马氏体转变。

Most automotive and satellite microactuators (hydraulic, pneumatic, etc) utilize seals to prevent egress of fluids and ingress of dirt, humidity and other extraneous materials. In these dynamic applications the seal is in contact with the rotating surface of the shaft and has to meet certain requirements of maximum friction, wear and leakage over the working temperature range. This temperature normally varies from about -50 to 150 degrees Celsius in vehicles while in satellites the temperature range is wider (-100 to 200 degrees Celsius) depending on whether the surface is exposed to the sun or in the shadow. The main limitation for using shaft seals at a wide temperature range stems from the differences in dilatation with temperature of the shaft and seal. At low temperature (freezing conditions) the seal-shaft friction is high while at high temperature (above 100 degrees Celsius) the shaft-seal clearance becomes so large that excessive leakage may occur and therefore it is of interest to maintain the pressure constant over the working temperature range. Excessive friction and wear would shorten the service life of seals while leakage would result in lubricant loss (i.e., seizure) or lubricant ingress (i.e., extraneous materials could enter and damage engines and other components). At the same time, seals should wear faster than the shaft on which it is mounted since shafts are more costly and difficult to replace than seals. This suggests the need for novel shaft seals with tuned performance over the working temperature range. In this research programme I will develop a new strategy for overcoming these limitations consisting of using shape memory alloys with tailored reversibility and wear performance through optimal control of the microstructure and composition. The microstructure, i.e., grain size and distribution, will be tuned upon cooling in a single processing step by controlling the cooling rate and composition.Novel compositions will be developed through multiple minor element co-addition of relatively low-cost elements such as iron and nickel compared to copper to promote the twinning propensity of austenite. Deformation twinning is a small movement of atoms that occurs in a co-operative process in austenite when the material is subjected to a minimum stress value, resulting in macroscopic deformation. Once the material is twinned, and the force applied released, it keeps its deformed shape unless heated up above a benchmark temperature for which the material recovers to the initial position (i.e., detwinning). Some elements have the ability to decrease the energy required for twining and therefore promotes the twinning propensity (i.e., the ease with which atoms move when strained). This enables to control the temperature, and therefore the shaft-seal contact stress, at which martensite transforms into austenite resulting in a smaller seal diameter and therefore reducing leakage. To decrease the cost of the Cu- based shape memory alloys and make them more appealing for the actuator industry than NiTi alloys, low cost minor elements will be employed. Understanding how to tailor the thermomechanical and tribological performance of these Cu-based shape memory alloys is therefore of utmost importance in fundamental physical metallurgy as well as for industrial applications. Traditionally, the grain size of rapidly solidified materials has been optimized by tuning the composition, through partial substitution of one element by another or by controlling the cooling rate. However, the synergistic effect resulting from optimizing both parameters is novel and has huge potential for tailoring the properties of shape memory alloys. In this regard, as I have previously observed (S. González et al. Sci. Tech. Adv. Mater. 15, 2014), the addition of some minor elements (such as Fe and Co) at certain concentrations can promote a mechanically-driven martensitic transformation of Cu-containing alloys.