Abstract High entropy shape memory alloys (HESMAs) represent a relatively young class of functional materials. They show a reversible martensitic phase transformation which allows to exploit shape memory effects at relatively high temperatures. HESMAs represent ordered complex solid-solutions. Their high temperature phase is of B2 type, and various elements (e.g. Ni, Cu, Ti, Zr, Hf) occupy sites in specific sub-lattices. In the present work, we study the processing and the functional properties of HESMAs. We study effects of chemical complexity on solidification microstructures and martensitic transformations. Binary, ternary, quaternary, quinary and senary model alloys were investigated using advanced microstructural and thermal characterization methods. The results show that element partitioning during solidification results in a redistribution of individual alloy elements in dendritic/interdendritic regions. Surprisingly, the atomic ratios of the two groups of elements which occupy the Ni- (first group: Ni, Cu and Pd) and Ti-sub-lattice (second group: Ti, Zr, Hf) are maintained. This allows the material to form martensite throughout its heterogeneous microstructure. The effect of chemical complexity/composition on martensite start temperatures, MS, is discussed on the basis of valence electron concentrations, cV. Some of the alloys fall into MS(cV)-regimes which are uncommon for classical Ni-Ti-based shape memory alloys. In the present work, a new HESMA of type NiCuPdTiZrHf was identified which has the potential to provide maximum shape memory strains close to 15%.

中文翻译：

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高熵形状记忆合金的化学复杂性、微观结构和马氏体转变

摘要 高熵形状记忆合金 (HESMA) 代表了一类相对年轻的功能材料。它们显示出可逆的马氏体相变，这允许在相对较高的温度下利用形状记忆效应。HESMA 代表有序的复杂固溶体。它们的高温相是 B2 型的，各种元素（如 Ni、Cu、Ti、Zr、Hf）在特定的亚晶格中占据位置。在目前的工作中，我们研究了 HESMA 的加工和功能特性。我们研究化学复杂性对凝固微观结构和马氏体转变的影响。使用先进的显微结构和热表征方法研究了二元、三元、四元、五元和三元模型合金。结果表明，凝固过程中的元素分配导致单个合金元素在枝晶/枝晶间区域重新分布。令人惊讶的是，占据 Ni-（第一组：Ni、Cu 和 Pd）和 Ti-亚晶格（第二组：Ti、Zr、Hf）的两组元素的原子比保持不变。这允许材料在其异质微观结构中形成马氏体。在价电子浓度 cV 的基础上讨论了化学复杂性/成分对马氏体起始温度 MS 的影响。一些合金属于 MS(cV) 体系，这对于经典的 Ni-Ti 基形状记忆合金来说并不常见。在目前的工作中，确定了一种新的 NiCuPdTiZrHf 型 HESMA，它有可能提供接近 15% 的最大形状记忆应变。