Search for compatible Zirconia-based shape memory ceramics

金属形状记忆合金（SMA）表现出两种令人感兴趣的特性：用于固态致动器的形状记忆效应和用于自膨胀医疗植入物的超弹性。它们基于奥氏体和马氏体之间可逆的、热的或应力诱导的一级相变，具有较大的内应力，导致较大的应变和焓变。可逆性是通过两相的相容性来实现的，这导致了界面局部应力和迟滞的减少。这种相容性可以用变换矩阵的平均特征向量理想地必须为1的条件来表示。由于金属SMA中使用的元素，在腐蚀性化学条件下或极端温度下使用是困难的。这激发了一类新的形状记忆材料，它由金属氧化物组成，被称为形状记忆陶瓷（SMC）。尽管Swain在1986年首次发现了一种合适的氧化物（氧化镁稳定的ZrO2），但迄今为止缺乏相容性导致了裂纹的发展和循环时的严重疲劳。在之前自己的研究中，关于金属SMA相容性的发现被转移到它们在（Y0.5Ta0.5O2）1-x (Zr0.5Hf0.5O2)x体系中的陶瓷对应物上。主要结果是相变迟滞与成分的显著依赖，马氏体变异体的应力诱导取向与迟滞的强烈依赖，但没有接近金属SMA的性质。对平均特征值的依赖也有所不同：给定两种可能的转换机制，结果表明，在两种转换的兼容性同样很好地实现的组合中，定义了最低迟滞点（等距离条件）。本应用的主要目标是寻找具有低迟滞和良好形状记忆性能的SMC。该搜索基于包含大约70种不同的zro2基SMC的数据集，包括转变温度和晶体学数据。相应的工作假设是，对于SMC，两个晶格对应的平均特征值必须尽可能接近于1，并且这些条件导致两个晶格参数关系，建议寻找减少四方性的掺杂剂。除了这个主要目标之外，我们还将研究基于可用物理参数数据集的机器学习方法是否适用于识别在迟滞和转换可逆性方面具有显着改善性能的SMC成分。

Metallic shape memory alloys (SMA) show two properties of interest: the shape memory effect used for solid state actuators and the superelasticity e.g. for self-expanding medical implants. They are based on a reversible, thermal or stress-induced phase transition of the first order between austenite and martensite phases with large internal stresses, which lead to large strains and enthalpy changes. The reversibility is achieved through the compatibility of the two phases, which leads to a reduction in the local stresses at the interface and the hysteresis. This compatibility can be expressed by the condition that the mean eigenvector of the transformation matrix ideally has to be one.Due to the elements used in metallic SMA, use under corrosive chemical conditions or at extreme temperatures is difficult. This motivates a new class of shape memory materials, which consist of metal oxides and are called shape memory ceramics (SMC). Although Swain first identified a suitable oxide (MgO-stabilized ZrO2) in 1986, the lack of compatibility to date leads to the development of cracks and severe fatigue upon cycling.In prior own research the findings regarding compatibility in metallic SMA were transferred to their ceramic counterparts in the system (Y0.5Ta0.5O2)1-x (Zr0.5Hf0.5O2)x. The main results were a significant dependence of the transformation hysteresis on the composition and a strong dependency of the stress-induced orientation of the martensitic variants on the hysteresis, without however approaching the properties of metallic SMA. The dependence on the mean eigenvalue also differs: Given the two probable transformation mechanisms, the results suggest that the composition, in which the compatibility for both transformations is equally well fulfilled, defines the point of lowest hysteresis (equidistance condition).The main goal of this application is the search for SMC with low hysteresis and good shape memory properties. This search is based on a data set containing approximately 70 different ZrO2-based SMC with the transformation temperatures and the crystallographic data. The corresponding working hypotheses are that for SMC the mean eigenvalues ​​of both lattice correspondences must be as close as possible to one and that these conditions lead to two lattice parameter relations that suggest searching for dopants that reduce the tetragonality.In addition to this main objective we will investigate whether a machine learning approach based on the available data set of physical parameters will be appropriate to identify compositions for SMC with significantly improved properties in terms of hysteresis and transformation reversibility.