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执行器（液压，气动等）使用密封来防止流体的排出和污垢，湿度和其他外来物质的进入。在这些动态应用中，密封与轴的旋转表面接触，并且必须在工作温度范围内满足一定的最大摩擦、磨损和泄漏要求。车辆的温度通常在-50到150摄氏度之间变化，而卫星的温度范围更广（-100到200摄氏度），这取决于表面是暴露在阳光下还是在阴影下。在较宽的温度范围内使用轴封的主要限制源于轴和密封的膨胀随温度的差异。在低温（冻结条件）下，密封与轴的摩擦很大，而在高温（100摄氏度以上）下，轴与密封的间隙变得非常大，可能会发生过多的泄漏，因此在工作温度范围内保持压力恒定是很有意义的。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（即卡扣）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）。与此同时，密封件应该比安装它的轴磨损得更快，因为轴比密封件更昂贵，更难以更换。这表明需要在工作温度范围内调整性能的新型轴封。在这个研究项目中，我将开发一种新的策略来克服这些限制，包括使用具有定制可逆性和磨损性能的形状记忆合金，通过优化控制微观结构和成分。微观结构，即晶粒尺寸和分布，将通过控制冷却速度和成分在单个加工步骤中进行冷却。与铜相比，通过多种低成本元素（如铁和镍）的共添加来促进奥氏体的孪生倾向，将开发出新的组合物。变形孪晶是在奥氏体中，当材料受到最小应力值时，原子在协同过程中发生的微小运动，导致宏观变形。一旦材料被孪生，施加的力释放，它保持其变形的形状，除非加热到一个基准温度以上，材料恢复到初始位置（即，脱孪生）。有些元素具有降低缠绕所需能量的能力，因此促进了孪生倾向（即，原子在拉伸时移动的便利性）。这样可以控制温度，从而控制轴封接触应力，马氏体转变为奥氏体，从而减小密封直径，从而减少泄漏。为了降低铜基形状记忆合金的成本，使其在执行器行业中比镍钛合金更具吸引力，将采用低成本的小元素。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和工业应用中至关重要。传统上，快速凝固材料的晶粒尺寸是通过调整成分、用一种元素部分取代另一种元素或通过控制冷却速度来优化的。然而，优化这两个参数所产生的协同效应是新颖的，并且具有巨大的潜力来定制形状记忆合金的性能。在这方面，正如我之前观察到的（S. González等人）。科学。Tech. Adv. Mater. 15, 2014)，在一定浓度下添加少量元素（如Fe和Co）可以促进含cu合金的机械驱动马氏体转变。

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.