基于NiTi合金相变潜热的绿色弹热制冷有望取代传统蒸气压缩制冷技术，但材料各项制冷指标之间难以平衡的问题仍是制约其实用化的瓶颈。本文研究了微观组织对NiTi合金弹热制冷性能的影响；指出了具有均匀微观组织特征的传统NiTi作为弹热制冷剂的不足；提出并完善了梯度晶粒结构NiTi制冷剂的概念；通过梯度结构的优化设计，明显提升了NiTi合金的弹热制冷潜力，同时阐释了潜在的物理机制。论文主要内容如下： 探明了NiTi合金弹热制冷性能的晶粒尺寸效应。结合冷轧和后续退火制备出一系列具有不同平均晶粒尺寸的近均质NiTi合金，展示了材料弹热制冷性能的晶粒尺寸相关性，并将其归因于马氏体相变差异；阐述了均质NiTi合金中不同制冷指标之间的关系并完善了弹热材料制冷潜力的评价标准；明确了微观晶粒尺寸与宏观制冷指标的关联，并优化晶粒尺寸使材料具备良好的综合制冷性能。 结合冷轧和单道次激光扫描热处理成功制备出梯度晶粒结构NiTi合金，并证明其拥有平衡不同制冷指标的潜力。表征了材料的微观组织并解释其成因；阐释了梯度结构晶粒NiTi良好的强度和延展性与均匀的马氏体相变特征；验证梯度结构效应有利于提高制冷能力与效率、降低能耗、拓宽有效工作温域和增强耐久性。 基于实验观测和机理分析，探明了梯度晶粒结构分布对NiTi相变行为和弹热制冷性能的影响。通过冷轧和多道次激光面扫描，沿试样厚度方向引入了晶粒尺寸梯度；建立了激光加工参数和所生成梯度结构的关联，并量化了激光热源对材料的渐进退火作用；通过梯度结构设计，实现了马氏体相变行为的主动调控和弹热制冷性能的明显优化；揭示了梯度结构NiTi中的跨尺度相变机理和弹热效应增强机制。 通过在预优化的纳米晶基体上构筑梯度晶粒结构，激发了NiTi合金中利于弹热制冷的基体增强和协同增强双机制，使材料同时拥有大潜热、低能耗、高能效和抗疲劳特性，最终显著提升了综合弹热制冷性能。表征了冷轧NiTi合金中的组成和含量，评估了纳米晶优化所产生的基体增强作用；验证了梯度晶粒结构NiTi合金中远优于常规弹热材料的制冷性能；打破了传统弹热材料中大潜热和低能耗之间的权衡，并刷新了现有固态制冷剂候选材料制冷效率的记录；揭示并量化了梯度晶粒结构NiTi合金中由异质层力学性能差异所引起的协同增强作用。
Green elastocaloric cooling exploiting the latent heat of NiTi is promising to replace the conventional vapor-compression refrigeration technology, but the difficulty in balancing the various refrigeration metrics of NiTi is still a bottleneck restricting its practical application. In this work, the influence of microstructure on the elastocaloric cooling performances of NiTi alloy is investigated. The shortcomings of common NiTi with homogeneous microstructures as an elastocaloric refrigerant are disclosed. A novel concept of gradient-grained-structured NiTi refrigerant is proposed and perfected. The cooling potential of NiTi is significantly enhanced by optimizing the gradient structure, and the underlying physical mechanisms are elucidated. The main conclusions of this thesis are as follows: Effects of grain size on the elastocaloric cooling performance of NiTi are clarified. Homogeneous NiTi alloys with different grain sizes are prepared through cold rolling and subsequent annealing, and the NiTi shows significantly grain-size dependent cooling properties resulting from the difference in martensitic transformation. The relationship between various refrigeration indicators is discussed and the criteria for evaluating the cooling potential of elastocaloric materials are perfected. The correlation between microscopic grain size and macroscopic refrigeration metrics is established, and a suitable grain size yielding favorable comprehensive cooling performance is selected. Gradient-grained-structured NiTi is fabricated for the first time by combing cold rolling and single-track laser scanning annealing, and the material’s potential to balance different refrigeration indexes is demonstrated. The material’s microstructures are characterized and explained. The good strength and ductility as well as the uniform martensitic transformation characteristic of the gradient-grained-structured NiTi are presented and analyzed. It is found that the gradient structure effect is conducive to improving the cooling capacity and efficiency, reducing energy dissipation, widening the effective working temperature span and enhancing durability. Based on experimental measurement and mechanism analysis, the effects of gradient-grained-structured distribution on the martensitic transformation and the elastocaloric cooling performance of NiTi are ascertained. The grain-size gradient is introduced along the thickness direction of the sample after cold rolling and laser surface scanning annealing. The correlation between the laser processing parameters and the generated gradient structure is established, and the progressive annealing effect from the laser heat source on the material is quantified. Tunable martensitic transformation characteristics and improved elastocaloric cooling performances are realized by tailoring the gradient structure. Trans-scale martensitic transformation behavior and the elastocaloric strengthening mechanism in the gradient-grained-structured NiTi are revealed. By constructing a gradient-grained structure on a pre-optimized nanocrystalline substrate, matrix strengthening and synergetic strengthening mechanisms contributing to elastocaloric cooling in NiTi are activated. As a result, the gradient-grained-structured NiTi simultaneously possesses large latent heat, low energy dissipation, high energy efficiency and fatigue resistance, which drastically improves the comprehensive elastocaloric cooling performance. The components and amounts of the cold-rolled NiTi are characterized, and the matrix strengthening produced by nanocrystalline optimization is evaluated. It is confirmed that the cooling potential of the gradient-grained-structured NiTi is far superior to that of conventional elastocaloric materials. The longstanding trade-off between large latent heat and low energy dissipation is overcome, which contributes to a record-breaking cooling efficiency in existing solid refrigerant candidates. The synergetic strengthening effect generated by interlayer mechanical incompatibility is revealed and quantified.
