Compositional and Microstructural Optimization and Microstructural, Thermal and Mechanical Assessment of NiTi Shape Memory Alloys for Ferroelastic Cooling

在过去的几十年里，替代制冷方法越来越受到关注，因为它们提供了高效率，从而有助于更有效地利用资源和减少温室气体排放。伪弹性NiTi基形状记忆合金是一类很有前途的铁弹性冷却候选材料。在机械加载过程中，应力诱发的马氏体相变允许高达7%的可逆变形。在卸载时，应力诱发的马氏体转变回奥氏体，并恢复其初始形状。正向相变和反向相变都与热效应有关，热效应是加载时马氏体形成时的放热产物，以及卸载时马氏体消失时的吸热反应。在本工作中，我们旨在研究和利用吸热效应来冷却。该项目有四个目标：第一，它的目的是描述在应力诱发马氏体的反向相变过程中控制冷却的基本相变机制(弹性热行为的原子和微观原因)。其次，将开发一种程序，允许量化与装卸相关的弹性热效应。第三，通过合金化和显微组织的调整来优化NiTi基合金的冷却电位。最后，功能疲劳将受到关注，它可能会很好地限制循环运行的铁弹性冷却设备的使用寿命。

In the last decades, alternative refrigeration methods have increasingly received attention because they provide high efficiencies, and thus can contribute to a more efficient use of resources and lower greenhouse gas emissions. Pseudoelastic NiTi-based shape memory alloys (SMAs) represent a promising class of candidate materials for ferroelastic cooling. During mechanical loading, a stress induced martensitic transformation allows for a reversible deformation of up to 7 %. On unloading, stress-induced martensite transforms back to austenite, and recovers its initial shape. Both, forward and reverse transformations are associated with heat effects, an exothermic heat production when martensite forms on loading and an endothermic reaction when it disappears on unloading. In the present work, we aim at studying and exploiting the endothermic heat effect for cooling. The project has four objectives: First, it aims at characterizing the elementary transformation mechanisms which govern cooling during the back transformation of stress induced martensite (atomistic and microstructural reasons for elastocaloric behavior). Second, a procedure will be developed which allows to quantify the elastocaloric heat effects associated with loading and unloading. Thirdly, an effort will be made to optimize the cooling potential of NiTi based alloys through alloying and through tailoring the microstructure. And finally, functional fatigue, which may well limit the service life of a cyclically operating ferroelastic cooling device, will receive attention.