SLM生物医用近β型钛合金马氏体相变与残余应力松弛机理研究

Surgical implants prepared by selective laser melting of near-beta biomedical titanium alloy have important applications in orthopedics and dentistry due to its excellent biomechanical adaptability, however, the residual stresses formed during manufacturing process have adverse effects on its functionality and stability. Controlling the residual stresses is the critical research topic to ensure the quality of implants. In this project, Ti-Nb near-beta titanium alloys with different content of beta stable elements are selected, with the aim of quantifying the effect of martensitic transformation on residual stress, the constitutive relationship between martensitic transformation, temperature and residual stress is studied. Firstly, the three-dimensional residual stress field inside the sample is measured by neutron diffraction, secondly, the crystallographic information of martensite is obtained by synchrotron radiation diffraction, thirdly, the martensite twins and their interfacial structures are studied, and the texture and martensite variants selection are explored. Finally, the coupling effect of martensite transformation and microscale thermal and mechanical processes is analyzed by using finite element simulation technology, and the internal relationship between residual stress relaxation and its influencing factors is revealed, in order to realize the combination of alloy composition design, processing technology and residual stress adjustment, which has scientific significance in the field of biomedical titanium alloy product design and additive manufacturing.

采用激光选区熔化技术制备的生物医用近β型钛合金外科植入体具有良好的生物力学适配性，在骨科、齿科领域有重要的应用，然而在制造过程中形成的残余应力对其功能性和稳定性有不利影响，控制残余应力是保证植入体品质的研究热点和难点。本项目选择具有不同β稳定元素含量的Ti-Nb系近β型钛合金，以量化马氏体相变对残余应力的松弛作用为出发点，围绕马氏体相变、温度和残余应力之间的本构关系开展研究。首先利用中子衍射精确测量样品内部三维残余应力场；然后利用同步辐射衍射获取马氏体的晶体学信息；接着对马氏体孪晶及其界面结构进行研究，探求马氏体变体的织构和析出规律；最后结合有限元模拟技术分析马氏体相变与材料热、机械微观物理过程的耦合作用，揭示残余应力松弛现象和影响因素之间的内在联系，以期实现合金成分设计、加工工艺和残余应力调控多环节的有机结合，在生物医用钛合金产品设计和增材制造领域具有显著的科学意义。

本项目研究针对激光3D打印态、淬火态、冷变形态以及不同时效状态的TiNb合金开展了详细的实验研究，研究测试手段涵盖常规金相分析、扫描电子显微分析、电子背散射衍射、常规X射线衍射、同步辐射X射线衍射、透射电子显微分析等表征技术，针对TiNb合金微观组织演变、固态相变、热物理性能演变、衍射弹性常数及残余应力分析等问题开展了具有开创性或探索性的研究，取得了详实的实验数据。重要实验结论如下：（1）近β型TiNb合金经快冷形成的α"马氏体物相经短时受热至奥氏体转变温度以上（~300℃），经过切变型马氏体逆转变为β相，随即β相中发生扩散型等温α"iso相变，纳米尺寸的α"iso物相弥散分布于β相基体上，新相与母线存在惯习关系。（2）近β型TiNb合金经快冷形成的α"马氏体物相本征弹性模量处于较低水平，经冷变形形成的织构对弹性模量作用有限，材料在中低温度区间（~250℃）经历较长时间等温处理，会诱发受Nb元素扩散控制的β→α"iso 以及β→ω相变，材料发生显著的弥散强化，其弹性模量与形成ω物相的含量成正比，等温时间和温度是优化材料综合弹性性能的关键因素。（3）近β型TiNb合金经快冷形成的α"马氏体物相在较低温度（~150℃）经历较长时间等温处理，会诱发少量α"物相发生逆马氏体转变为β相，其余α"物相发生Nb元素调幅分解，形成贫Nb的α"物相及β相两相复合组织，材料的弹性模量变化较小，强度随时效时间延长而略有增加，在等温时效处理初期材料塑性有少量增加，随后随等温时间延长，材料塑性劣化。