生物医用β型钛合金中新型马氏体孪晶及界面结构的研究

The shape memory effect in biomedical β type titanium alloy is of great importance for realizing the function of dental stents and bone fixed pressurizer, and the optimization of toughness and ductility for long-term service in human body. This project aims at investigating the martensitic transformation twinning and interfacial twinning boundary which would serve for the quantization of shape memory effect and improvement of toughness and ductility. On the one hand, the post thermo-mechanical treatments will be employed for microstructural optimization, with emphasis on solid phase transformation, grain size and crystallographic texture. Combining in situ high energy synchrotron X-ray diffraction (SXRD) which can be used to measure cell parameters, the shape memory effect and mechanical properties could thus be evaluated. On the other hand, EBSD and HTEM will be employed for obtaining orientations of various martensitic variants. Based on the method of crystallographic reconstruction of martensitic lattice, the martensitic transformation twinning will be indexed unambiguously and the formation mechanism of ITB will be explored. The innovation of this project is employment of advanced characterization techniques including the SXRD and HTEM for investigation of martensitic transformation twinning and ITB featured by small size, highly strained lattice and thermodynamics instability. Potential results are vital and meaningful for practical application and benefit to the crystallographic theory of martensitic transformation and theory of deformation twinning.

β型钛合金的形状记忆效应对牙科正畸和骨伤固定、加压效果显著，如何解析该合金中出现的新型马氏体孪晶，阐明与其相关的复杂孪晶界面形成机理，是实现形状记忆精准控制和延长服役寿命的研究难点和热点。本项目以β型钛合金的形状记忆效应量化和塑韧性提高为出发点，对马氏体孪晶及其界面结构进行研究。首先探求中温热处理对固态相变、晶粒尺寸和晶体学织构的调控规律；随后结合同步辐射X射线原位衍射，根据所获得的晶体学信息计算相变应变，进而量化宏观应变，探究塑韧性、形状记忆性能随组织变化的内在联系；最后结合透射电子显微分析确认马氏体相的存在形式，运用极射赤面投影法重构晶格取向，解析马氏体孪晶要素，建立晶格演变的几何模型，最终解释复杂界面结构的形成机理。本项目的原位高分辨衍射分析和晶格取向的重构过程将为小尺寸、不稳定的马氏体相变研究提供新的思路，对新型孪晶及其界面结构形成机理的探索将丰富经典马氏体唯象理论和形变孪晶理论。

本项目以Ti24Nb4Zr8Sn、Ti22Nb和TiNi这三类生物医用形状记忆合金为研究对象，旨在揭示新型马氏体相变孪晶类型和界面结构，为形状记忆效应量化和塑韧性提高提供理论基础。在项目的具体执行过程中，采用冷轧和中温热处理处理调控合金显微组织中的固态析出相，结合原位同步辐射X射线衍射实验确定马氏体相变的晶体学信息。采用高分辨透射电子显微技术获取母相孪晶的取向，借助晶格重构法分析马氏体孪晶类型及界面结构的形成机理，标定了多种新型孪晶和界面结构，主要的新型孪晶包括：Type II-{210}<512>α″马氏体新型孪晶, Type I-{541}<111>β奥氏体新型孪晶, Type II-{013}<531>β奥氏体新型孪晶和界面应力松弛而产生形成的omega相。最终，为了提高形状记忆效应的能量可控性，采用增材制造方法进行梯度形状记忆合金的制备和表征研究，在形状记忆合金中获得了反常应变硬化现象，为进一步提高形状记忆合金可控性奠定了理论基础。