Direct microstructure design enabled by additive manufacturing of magnetic shape memory Ni-(Co)-Mn-Sn Heusler alloys for multicaloric applications

在本项目中，Ni（Co）-Mn-Sn Heusler形状记忆合金（SMA）的微观结构和磁性能将专门设计用于使用定向能量沉积（DED）工艺的多热量固态冷却。在微观结构方面，目标是高度纹理化的竹状结构，最终最大限度地提高机械和循环稳定性。通过Co和Fe元素粉末掺杂Ni-Mn-Sn将分别实现对磁性能的设计和提高循环稳定性。在这方面，DED工艺允许少量元素粉末的精确过程合金化，即，Co和Fe。卡塞尔的一种新型DED装置能够对与少量Fe（第一步）和Co（第二步）粉末原位合金化的材料进行高通量表征。DED设备还配备了用于高通量分析的其他仪器，如X射线断层扫描和衍射，以便在加工过程中直接收集数据，最终支持高效的工艺参数识别。为了使所加工的化学成分达到最大的机械和循环稳定性，必须使孔隙率、热裂纹和其他缺陷的发生最小化。后处理热处理将用于均匀化，并进一步影响磁有序。除了机械（伪弹性（PE）和形状记忆效应（SME））和磁性能以及相关的热量效应，这将是深入的特点，DED处理的合金进行了研究，使用多刺激试验台，以最终评估循环multicaloric性能。

In the present project the microstructural and magnetic properties of Ni (Co)-Mn-Sn Heusler shape memory alloys (SMAs) will be specifically designed for multicaloric solid-state cooling using the directed energy deposition (DED) process. With respect to the microstructure, a highly textured bamboo-like structure is aimed for, eventually maximizing mechanic and cyclic stability. Doping of Ni-Mn-Sn by Co and Fe elemental powders will be accomplished to design the magnetic properties and enhance cyclic stability, respectively. In this regard, the DED process allows for a precise in process alloying of small amounts of elemental powders, i.e., Co and Fe. A novel DED setup available in Kassel enables high-throughput characterization of materials being in situ alloyed with small amounts of the Fe (first step) and Co (second step) powders. The DED machine is equipped with further instrumentation for high-throughput analysis, such as X-ray tomography and diffraction in order to collect data directly during processing, eventually supporting an efficient process parameter identification. To achieve maximum mechanical and cyclic stability for the chemical compositions processed, the porosity, the occurrence of hot cracks and other defects must be minimized. Post-process heat treatments will be used for homogenization and further affect magnetic ordering. Besides the mechanical (pseudoelastic (PE) and shape memory effect (SME)) and magnetic properties as well as the associated caloric effects, which will be characterized in depth, the DED processed alloy is investigated using a multi-stimuli test-bed to finally assess the cyclic multicaloric performance.