d-metal Heusler磁相变合金NiMnTi(Co)的多相变路径弹热效应研究

Elastocaloric effect is one of emerging functionalities of magnetic phase transformation materials. Due to its potential applications in the efficient and eco-friendly solid-state cooling devices, elastocaloric effect has attracted a lot of attention in the field of condensed matter physics and material science. We observed that different stress loading condition, ambient temperature, magnetic field and intrinsic phase transformation features of the alloys result in different crystal structures of martensite, consequently in different martensitic transformation paths in d-metal Heusler alloys NiMnTi(Co). This proposal aims to reveal the underlying mechanism of influence of martensitic transformation path on the elastocaloric effect including adiabatic temperature change, critical stress, hysteresis and reversibility, on the basis of the theories of martensitic transformation and ferromagnetics. Moreover, this proposal intends to enhance the elastocaloric effect of NiMnTi(Co) alloys. The results of this project will be of important significance in terms of in-deep understanding of the key factor affecting elastocaloric effect of magnetic martensitic transformation materials and provide important references for enhancing the elastocaloric effect in magnetic phase transformation materials.

弹热效应是磁相变材料展现的新功能之一，因其在环保高效的固态制冷技术中具有潜在应用而成为凝聚态物理和材料领域的研究热点。本项目以新型高韧性的d-metal Heusler磁相变合金NiMnTi(Co)为研究对象，以马氏体相变晶体学理论和铁磁学理论为基础，针对其相变路径对材料自身相变特征和所受应力、温度、磁场等外界物理环境激励的依赖关系，研究不同相变路径的发生条件及其对于绝热温变、相变临界应力、滞后损耗等弹热性能的影响机制，以期在NiMnTi(Co)合金中获得大弹热效应。本项目将为深入理解影响弹热效应的关键因素和提高磁相变合金弹热效应的制冷能力和相变可逆性提供科学基础。

本项目以新型高韧性all-d-metal Heusler磁相变合金NiMnTi(Co)为研究对象，通过实验和理论计算，系统研究了其相变行为和弹热效应，阐明了d-d共价、相变路径、合金微观组织结构等材料自身相变特征和所受应力、磁场等物理外场对该合金绝热温变、相变临界场、可逆性等弹热性能影响机制。.（1）借助原位数字图像关联DIC应变云图和红外成像技术相结合的手段，对比研究了NiMnTi和NiCoMnTi无序多晶的弹热效应，建立了微观化学键强度、应变梯度以及相变可逆性之间的关联。（2）通过定向凝固法制备NiCoMnTi择优取向多晶，降低相变临界应力至40MPa，单位应力下的绝热温变达到0.31K/MPa。通过原位TEM和原位DIC揭示了临界应力得以降低的两重原因在于奥氏体到多重调制马氏体的相变路径和001择优取向织构带来的晶粒之间的协调变形。（3）得益于Ni35.5Co14.5Mn35Ti15的大相变体积变化（-1.6%）和小相变滞后（6K），获得了1kbar压力下24.2J/kgK的低场大压热效应，超过所报道的其他磁相变合金。（4）利用其磁弹反耦合的特点，提出“磁场辅助卸载压力”的多卡效应实现路径，获得了加载循环下可逆的熵变，解决了一级相变滞后所带来的能量转换效率低的问题。.本项目的实施不仅获得了具有良好弹热效应的NiMnTi(Co)合金，也为理解all-d-metal Heusler磁相变合金的相变机理和调控这类合金的弹热效应提供重要数据和科学基础，推动我国固态制冷领域的发展。