钛锆基合金表面梯度纳米化及疲劳行为研究

Attempt has been made to manufacture structural components of spacecraft using novel TiZr-based alloy due to its low density, high specific strength, good thermal stability as well as high-energy particle radiation resistance. However, fatigue fracture is very prone to occur for structural components. It’s been estimated that 60~90% of all mechanical failures are resulted from fatigue and it is important to improve the service life of structural components by understanding the fatigue properties and mechanisms of applied materials. Considering the characteristic that fatigue crack always initiate from the surface of materials, the fatigue properties may be enhanced by reasonable modification of material surface. By means of laser shock peening technique, this project aims to explore and optimize the preparation process of novel TiZr-based alloy with gradient nanostructured surface layer, reveal the formation mechanisms and microstructure evolution of surface gradient nanostructure, understand the evolvement rule of nanograin as well as residual stress in gradient nanostructured surface under different temperature and loading conditions, and clarify the fatigue mechanism of novel TiZr-based alloy with gradient nanostructured surface layer under the influence of nanograined microstructure combined with residual stress. It is expected to improve the fatigue resistance of novel TiZr-based alloys by use of surface gradient nanocrystallization, promote the industrial applications of high-performance titanium alloys under harsh environments, enrich the microstructure evolution and fatigue fracture theories of novel TiZr-based alloys, and provide theoretical foundation and technique support for the long service-life design of aerospace structural components.

钛锆基合金因具有密度低、比强度高、热稳定性好以及耐高能粒子辐射等优异性能，已被尝试应用于航天器结构件制造。然而结构件易发生疲劳断裂，据统计机械失效中的60~90%可归结为疲劳问题，了解使用材料的疲劳行为及疲劳失效机理，最大限度的提高构件使用寿命至关重要。鉴于疲劳裂纹多萌生于材料表面的特点，适当的改变表面状态或能改善材料疲劳性能。本课题拟利用激光冲击强化技术，探索并优化钛锆基合金表面梯度纳米化工艺；揭示表面梯度纳米结构的形成机制；掌握表面梯度纳米结构在载荷或温度作用下纳米组织演变与残余应力释放规律；阐明梯度纳米结构合金在表面纳米晶与残余应力协同作用下的疲劳失效机理。以期通过表面梯度纳米化尽可能提高其抗疲劳能力，推动新型钛锆基合金在苛刻条件下的工业化应用，丰富钛锆基合金材料微观组织演化及疲劳断裂理论，为航空航天合金构件长寿命设计提供理论依据和技术支撑。

钛合金因其综合性能良好，是飞机的主要结构材料，也是航空发动机风扇、压气机轮盘和叶片等重要构件的首选材料。因受到温度、荷载等外界条件以及残余应力、粗糙度等自身性能的影响，钛及钛合金零部件在实际应用中易发生疲劳失效现象。为了满足日益严苛的服役环境，当今的首要任务是探索影响钛及钛合金疲劳性能的因素，进一步提高钛合金的疲劳寿命。.为此，本项目以TC11钛合金为研究对象，通过激光冲击和超音速微粒轰击两种表面强化技术进行表面强化，尔后分别在不同温度下进行高周疲劳试验及疲劳裂纹扩展试验，探讨钛合金表面纳米化机制，为钛合金航空构件的先进抗疲劳制造提供试验依据和技术支撑。一些比较重要的结果有：.1. 激光冲击与超音速微粒轰击均能使的钛合金表面形成梯度纳米组织。前者在功率密度为4.8 GW/cm2的条件下虽然能够形成纳米组织，但由于强大的冲击力，同时伴随微裂纹的形成。经超音速微粒轰击后，因表面剧烈塑性变形，TC11合金表层形成晶粒尺寸约为10 nm，最大不超过25 nm。.2. 经超音速微粒轰击后，在应力水平为500 MPa时，室温疲劳寿命平均值由1.0×105周次提高至1.1×106周次，疲劳寿命提高了11倍。疲劳断口均由疲劳源区、裂纹扩展区、瞬断区三部分组成，但在表面纳米晶与残余压应力的协同作用下，SFPB处理后的疲劳源由处理前的表层转移至次表面。.3. 不同温度下高周疲劳试验后，钛合金表层组织的晶粒尺寸仍处于纳米量级，平均尺寸与疲劳加载前相当。同时疲劳试验后，表层残余应力虽有一定释放，但变化并不大，故而合金表层所形成的梯度纳米组织具有良好的稳定性。 .本项目丰富了钛合金材料微观组织演化及疲劳断裂理论，为航空航天合金构件长寿命设计提供了理论依据和技术支撑。