深低温应变诱导孪晶钛合金摩擦学行为及其机理

The poor wear resistance of titanium alloy is the bottleneck problem which restricts its high reliability and long life application in the key components of aerospace instrument and equipment. The traditional modifications of wear resistance are mainly surface treatments, which form a thin high wear-resistant surface layer on the surface, while the wear resistance of the matrix is still poor. Based on our early study, which showed that obvious improvement in the wear resistance of titanium alloy was obtained by cryogenic treatment, we proposed the method of cryogenic strain treatment. It can improve the overall wear resistance of titanium alloy through the compound effect of cryogenic temperature and external strain to promote the microstructure modulation. The evolution mechanism in twins is explored on the basis of process parameters optimization. The evaluation of wear resistrance is conducted under the variable working condiction, such as pair materials, speed, loading force and temperature. Combining with the investigation of microstructure, the intrinsic interactions among the parameters of cryogenic strain treatment-microstructures-mechanical properties-tribological behavior-wear mechanisms are revealed. Finally, the improvement mechanisms of cryogenic strain treatment induced twinns on wear resistance in titanium alloy is deeply discussed and illuminated. The implementation of this project can provide new ideas and theoretical guidance for the improvement of wear resistance of titanium alloy.

钛合金耐磨性差的问题是制约其在空天仪器装备关键核心部件高可靠、长寿命应用的瓶颈问题。传统的钛合金耐磨性改性主要采用表面处理手段，在表面形成较薄的高耐磨层，而基体的耐磨性依然较差。本项目基于前期对钛合金进行深低温处理耐磨性得到提升的基础上，提出了深低温应变处理方案，通过深低温温度和外加应变量的复合作用促使微观组织孪晶化调配，从而实现钛合金整体耐磨性的提升。在工艺参数的匹配优化基础上探索孪晶演化机制，在对磨副材料、转速、载荷和温度工况变化条件下对摩擦磨损性能进行评价，结合微观组织结构分析揭示钛合金的深低温应变处理工艺参数-组织结构-力学性能-摩擦学性能-磨损机制的内在关系，最终阐明深低温应变诱导孪晶钛合金对耐磨性的改性机制。本项目的实施能够为钛合金耐磨性的改善提供新的思路和理论指导。

钛合金耐磨性差的问题一直是制约其在航空航天、生物医学领域更加广泛应用的瓶颈问题，本项目针对这一实际需求提出了深低温应变处理方案。首先，针对TC4双相钛合金进行了室温和低温拉伸应变处理，研究了深低温应变处理对钛合金微观组织和耐磨性的影响，结果发现深低温拉伸应变处理对钛合金耐磨性没有明显改善效果。在此基础上提出了深低温剪切应变处理方案，分别针对双相钛合金和纯钛，在滑动磨损过程中改变环境温度和不同的加载力获得不同的剪切应力，通过材料亚表层组织的微观变形规律揭示了耐磨性的变化及其机制。结果表明，室温剪切变形条件下，接触面高温引起的材料软化效应导致亚表层产生明显的变形层，最大可达50μm。深低温剪切变形条件下，亚表层变形层厚度最大仅有20μm，双相钛合金和纯钛在亚表层处形成厚度最大约为15μm的加工硬化层。对于双相钛合金，室温剪切变形容易产生大量孪晶，孪晶的产生与剪切应力大小密切相关，剪切应力越大越容易产生孪晶。而深低温剪切变形后，在α晶粒内部或晶界处产生大量缠结的位错，α晶粒也得到细化。对于纯钛，经室温剪切变形后，位错在α晶粒内以密度较低的网状形态分布，表明晶粒沿着平行于剪切应力的方向协同变形，宏观上表现出明显的塑性变形。而经深低温剪切变形后，α晶粒中位错纠缠形成大量位错墙，一方面抑制了亚表层组织的变形，起到应变强化的作用，另一方面以亚晶界的形式分割α晶粒，促使亚表层组织产生大量亚晶，起到细化晶粒的效果。深低温剪切变形过程引起的亚表层组织变化使得钛合金的磨损机制由室温下的氧化磨损和分层磨损为主导转变为由疲劳断裂引起的不完全分层磨损为主导。深低温下的剪切变形使得TC4双相钛合金低温耐磨性与室温相比分别提高49.7% (100 N)，57.2% (150 N)和36.6% (200 N)，TA2纯钛低温耐磨性与室温相比分别提高了49% (100 N)和66%（150N），两种材料均表现出更低的摩擦系数和表面粗糙度。