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

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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.