高能多场耦合作用下铝合金疲劳延寿的机理研究

The aim of this project is to address the premature failure behaviour of low cycle fatigue about ultrafine-grained Al alloy and the mechanism of fatigue life improvement by the technology of ultrasonic assisted-ECAP. This study can be used to ensure the longlife and reliable service of some key components used in aeronautics, Astronautics and high-speed rail. The following issues will be focused in the present project: (1) By describing the initialization and distribution laws of bimodal grains under ultrasonic assisted-ECAP, the relationship between bimodal grains and fatigue life of ultrafine-grained Al alloy can be analyzed. (2) Studying the distribution rule of residual stress under ultrasonic assisted-ECAP and the relaxation rule of residual stress under the fatigue loading, and building the mapping relationship between residual stress relaxation and fatigue life of ultrafine-grained Al alloy. (3) By characterizing the fatigue damage of ultrafine-grained Al alloy, the prediction model of fatigue life can be developped considering the interactions of microstrure, residual stress and fatigue damage on the basis of crystal plasticity and damage mechanics theory. In this interdisciplinary program, the mechanology, mechanics, material and numerical analyses are included. Through the accomplishment of this project, the mechanism of fatigue life improvement using the technology of ultrasonic assisted-ECAP can be clarified from the micro scale to macro scale. Also, this project could provide some important insights on the improvement of the long-life design and manufacturing of the key matreials and components in aeronautics, Astronautics and high-speed rail.

面向航空航天与高铁铝合金结构件长寿命可靠运行的需求，针对超细晶铝合金低周疲劳的早期失效特性及超声振动辅助等径角挤压的疲劳延寿机理展开研究。主要包括：（1）表征高能多场耦合作用下双峰尺寸分布晶粒的形成与演变规律，探明在循环载荷作用下微观结构参数与超细晶铝合金疲劳寿命的关系；（2）通过研究高能多场耦合作用下超细晶铝合金残余应力的分布规律与循环载荷作用下的释放机制，构建残余应力效应与超细晶铝合金疲劳寿命的映射关系；（3）表征超细晶铝合金的疲劳损伤因子，综合微观组织和残余应力与疲劳寿命的关系，在晶体塑性力学与损伤力学理论的基础上，建立计及微观组织、残余应力场与疲劳损伤场耦合作用的多尺度疲劳寿命预测模型。本研究体现了机械学、固体力学、材料科学和计算科学等多学科交叉，可望从微观到宏观角度揭示超声振动辅助等径角挤压的疲劳延寿机理，为航空航天与高铁关键材料与零部件的寿命设计方法和制造水平提升提供科学支持。

铝合金在航空航天等交通领域应用广泛，但铝合金强度低；超细晶铝合金虽强度有所改善，但是低周疲劳寿命又恶化。特种能量场辅助塑性成形能显著且定量调控材料的微观组织与力学性能，因此本项目采用超声波振动辅助等径角挤压工艺，主要开展了如下研究内容：（1）超声波振动辅助等径角挤压对铝合金微观组织的调控及延寿机理；（2）超声振动辅助等径角挤压下铝合金的残余应力及延寿机理；（3）多物理场耦合作用下超细晶铝合金疲劳寿命预测模型。取得了以下研究成果：（1）基于Deform-3d软件与正交试验方法设计了超声波辅助等径角挤压装置及制备工艺，经8道次US-ECAP的铝合金晶粒显著细化，平均尺寸在1μm以下；（2）8道次US-ECAP的铝合金中有细长条状，长度20-150nm，宽度20nm左右，沿晶界附近析出析Mg2Si，揭示了铝合金细晶强化与析出强化联合作用的超声波诱导机制；（3）超细晶铝合金在平均压应力下棘轮行为较平均拉应力下明显，借助于EBSD分析技术发现超细晶铝合金在循环平均压应力的再结晶与平均拉应力抵消残余应力的机制；（4）在Gerard所发展的晶体循环塑性本构框架下引入Hall-Petch效应和损伤力学的有效应力的概念，建立一种考虑晶粒尺寸效应与晶内损伤的双峰晶粒尺寸分布超细晶铝合金的晶体循环塑性本构模型；（5）初步探明多晶FCC材料单轴拉伸载荷下微观应力三轴度分布与演化规律。以上研究成果可为金属材料与构件的可靠性设计及寿命分析提供一定的理论依据。