TiNb包覆NiTi形状记忆复合材料的马氏体相变与力学行为的相关性研究

The currently developed shape memory alloys cannot perform a combination of no cytotoxicity, high critical stress for inducing martensite (σSIM) and large recoverable strain, which cannot meet the demand of biomedical fields for comprehensive properties. Recently, in our preliminary study, a new class of nontoxic TiNb-sheath/NiTi-core (TiNb/NiTi) shape memory composites with the above comprehensive properties has been fabricated through a series of metallurgy method. More importantly, it has been preliminarily found that the mechanical behavior of these TiNb/NiTi composites with different structural stability is closely related to their martensitic transformation. It is of academic importance to clarify the relative scientific issues, and of engineering significance to develop novel shape memory materials for biomedical applications. Based on our preliminary research work, this project aims to fabricate different structural stability TiNb/NiTi composites by adjusting composition and metallurgical process, and to investigate the relationship between martensitic transformation and mechanical behavior. Meanwhile, the related origin of high σSIM and large recoverable strain in high structural stability TiNb/NiTi composite will be revealed and the physical origin of narrow stress hysteresis and large linear superelasticity in low structural stability TiNb/NiTi composite will be discovered. On this basis, the relationship between martensitic transformation and mechanical behavior in different structural stability TiNb/NiTi composites will be clarified. It is expected to obtain high-quality academic achievement by the success of this project.

目前的形状记忆合金难以同时满足生物医用领域对其无细胞毒性、高的应力诱发马氏体相变临界应力（σSIM）及大的可恢复应变量的综合要求。最近，我们通过冶金复合首次制备出兼具上述综合性能的TiNb包覆NiTi（TiNb/NiTi）形状记忆复合材料。并初步发现，不同稳定性TiNb/NiTi的马氏体相变与其力学行为密切相关。阐明与此相关的科学问题具有重要学术价值，对研发具有重要应用前景的新型生物医用形状记忆材料具有重要指导意义。本项目拟在前期探索研究的基础上，通过调控成分及冶金工艺，制备不同稳定性的TiNb/NiTi复合材料，并通过对其马氏体相变与力学行为的深入研究，搞清高稳定性TiNb/NiTi实现高σSIM及大可恢复应变量的相关机理，揭示低稳定性TiNb/NiTi兼具窄应力滞后及大线性超弹的物理机制，阐明TiNb/NiTi中母相稳定性-马氏体相变-力学行为之间的内在相关性，可望获得高水平学术成果。

目前，无论单一的NiTi合金还是β钛合金均难以同时满足生物医用领域对形状记忆材料在细胞毒性及力学行为方面的综合要求。在本项研究中，我们通过冶金复合工艺制备出一系列兼具优良生物相容性和力学行为匹配的TiNb包覆NiTi（以下简记为TiNb/NiTi）形状记忆复合材料。研究表明，不同稳定性的TiNb/NiTi形状记忆复合材料的力学行为与两类应力诱发马氏体相变（即β↔α″和B2↔B19′相变）高度相关。其中，高稳定性Ti72.8Nb27.2/Ni50.9Ti49.1形状记忆材料较大的可恢复应变量主要源于两类应力诱发马氏体相变（即均匀的β↔α″和局部化的B2↔B19′）产生的相变应变及其自身固有的弹性变形，而高的应力诱发马氏体相变临界应力（σSIM）则主要归因于其内层的Ni50.9Ti49.1组元。低稳定性的Ti72.8Nb27.2/Ni50.3Ti49.7形状记忆材料经预变形处理后能够兼具大的线性超弹及窄的应力滞后。进一步的研究表明，Ti72.8Nb27.2/Ni50.3Ti49.7材料的线性超弹主要归因于其固有的弹性变形（包括β，α″，B2及B19′相的弹性伸长与恢复），两类均匀（即非局部化）且完全可逆的β↔α″和B2↔B19′应力诱发马氏体相变，以及B19′马氏体板条中细小的(001)复合孪晶的共同作用。由于位错和晶界等微观结构因素对马氏体相变的阻碍作用，这两类（即β↔α″和B2↔B19′）可逆的应力诱发马氏体相变能够均匀地发生在整个拉伸加载-卸载过程中，导致Ti72.8Nb27.2/Ni50.3Ti49.7形状记忆材料呈现线性超弹（而非具有“应力平台”特征）变形。同时，这两类相变在加载-卸载过程中能够通过应力诱发结构转变时产生的相变应变赋予Ti72.8Nb27.2/Ni50.3Ti49.7材料大的可恢复应变量。此外，由于Ti72.8Nb27.2/Ni50.3Ti49.7形状记忆材料中细小的微观组织（如细小的晶粒尺寸及纳米孪晶结构等）对马氏体相变尺度的限制，导致β↔α″和B2↔B19′两类应力诱发马氏体相变均仅能发生在微小的区域内（即相变尺度较小），有助于Ti72.8Nb27.2/Ni50.3Ti49.7形状记忆材料在加载-卸载过程中实现窄的应力滞后。相关研究成果有助于为涉及多类型马氏体相变的新型形状记忆复合材料的设计与研发提供理论指导。