The Role of Supercompatibility on the Fatigue of Shape Memory Alloys

形状记忆合金(SMA)表现出两种不同的性质，对许多应用都很有吸引力。首先，形状记忆效应是许多固态致动器的基础，它依赖于奥氏体和马氏体之间可逆的、热诱导的、高能量密度的相变。其次，它们表现出超弹性，例如用于自我膨胀的医疗植入物或弹性热能冷却，这是基于可逆的应力诱导的奥氏体-马氏体相变。在这两种情况下，具有大本征应变的一阶相变导致了大的功输出和大的热变。它们的可逆性得益于两相的相容。在新器件中实施SMA的关键是它们的疲劳特性，特别是在高周应用中。一般而言，疲劳涉及两个方面：功能疲劳和结构疲劳，前者描述材料的功能特性随循环的变化，后者指材料的完整性。通常，这两种类型的疲劳是紧密相连的。在我们以前的工作中，我们已经列出了我们认为在形状记忆合金的应力诱导相变(超弹性)中控制功能和结构疲劳的最重要的因素：晶体相容性、晶粒度(众所周知)和析出物。我们已经证明，如果调整成分以满足所有这三个因素，那么具有前所未有的抗功能疲劳能力的合金是可能的。然而，我们的结果显示，这些因素中的一个或多个可以稍作妥协，但仍会出现显著的功能疲劳。这是一个非常重要的发现，因为完美的晶体兼容性--所谓的超兼容性--是一个非常严格的条件，到目前为止，只有在两种特定的合金中才能几乎理想地满足这一条件。例如，满足高精度超相容的合金具有特殊的抗功能疲劳性能，但仅满足这些相容条件的合金也是如此，但具有较小的晶粒度和有利的细小共格析出物阵列。因此，我们的主要目标是：验证不同因素的影响，确定满足超低疲劳组分所需的精度，从而为未来寻找超低疲劳SMA提供方向。这些目标将适用于金属SMA，尽管预计它们应适用于广泛类别的固-固相变。

Shape memory alloys (SMAs) show two distinct properties that are attractive for many applications. First, the shape memory effect is the basis for many solid-state actuators, which relies on a reversible, thermally induced, high energy density phase transformation between austenite and martensite. Second, they exhibit superelasticity used for example in self-expanding medical implants or in elastocaloric cooling, which is based on a reversible, stress-induced austenite-martensite phase transformation. These first order phase transformations with large eigenstrains in both cases result in large work output and large enthalpy changes. Their reversibility is aided by the compatibility of the two phases. Essential for the implementation of SMAs in new devices is their fatigue characteristics, especially for high-cycle applications. In general, fatigue concerns two aspects: functional fatigue, which describes the cycle-dependent changes of their functional properties and structural fatigue, which refers to the integrity of the material. Commonly, both types of fatigue are closely interconnected.In our previous work we have laid out what we believe are the most important factors governing functional and structural fatigue in stress-induced phase transformations (superelasticity) in shape memory alloys: Crystallographic compatibility, grain size (that was well known) and precipitation. We have shown that, if compositions are tuned so that all three of these factors are fulfilled, alloys with unprecedented resistance to functional fatigue are possible. However, our results have revealed that one or more of these factors can be compromised a little and still remarkable functional fatigue is achieved. This is a very important finding as the perfect crystallographic compatibility – the so-called supercompatibility -- is a very restrictive condition which was up to now only found to be almost ideally fulfilled in only two specific alloys. For example, alloys satisfying supercompatibility to high accuracy have exceptional resistance to functional fatigue, but so do alloys that satisfy these compatibility conditions only approximately, but that have small grain size and a favorable array of fine coherent precipitates. Establishing this as a general finding would significantly increase the possibility to identify ultra-low fatigue compositions thus offering a wider selection to meet other criteria as e.g. transformation temperatures and strain or biocompatibility.Thus, our main objectives are: to verify the influence of the different factors, to determine the required accuracy in satisfying this supercompatibility and thus to derive directions for a future search of ultra-low fatigue SMAs. These objectives will be applied here in the context of metallic SMAs, although it is expected that they should be applicable to broad classes of solid-solid phase transformations.