块体超细晶高强铝合金原生裂纹对应力腐蚀开裂的影响及作用机理研究

Severe plastic deformation (SPD) technology can significantly improve the strength-to-weight ratio of high strength aluminum alloy to meet the needs of lightweight materials. However, the primary cracks produced by severe deformation have an important influence on the stress corrosion cracking (SCC) sensitivity of aluminum alloys, while previous studies does not fully consider this effect. In view of the above problem, this project achieves the quantitative control of the primary crack in the SPD process, and emphatically discusses the mechanism of its influence on SCC. This project utilizes successively a preheating treatment and a collaborative loaded SPD technology with circumferential torsion and radial extrusion to control the volume fraction and distribution of the primary cracks. The industrial CT technology is used to realize the quantitative characterization and three-dimensional reconstruction of the primary cracks; Coupling the microscopic damage model that considers the intrinsic properties of materials and the crystal plasticity finite element simulation, the quantitative prediction of the primary cracks is realized, and furthermore a "material-process-crack" model is established. Through comparing the SCC sensitivity index of non-SPD samples, non-cracked SPD samples, SPD samples with the primary cracks of different volume fractions and distribution characteristics, as well as observing the grain size, grain boundary precipitates, residual stress and other factors, we eventually determine the influence and mechanism of the primary cracks on the SCC behavior of bulk ultrafine grained high-strength aluminum alloy. This provides a theoretical support for its corrosion and prevention and industrial application.

大塑性变形（SPD）工艺能显著提高高强铝合金的比强度，满足轻量化用材需求。但剧烈变形过程产生的原生裂纹对铝合金抗应力腐蚀开裂（SCC）敏感性有着重要影响，而先前的研究对此考虑较少。针对上述问题，本项目通过对SPD过程原生裂纹的定量控制，重点探讨了原生裂纹对SCC的影响机理。利用坯料预热处理和周向扭转与径向挤压协同加载的SPD技术调控原生裂纹的体积分数和分布特征；采用工业CT技术实现原生裂纹的定量表征和三维重构，耦合含材料内禀特性的细观损伤模型和晶体塑性有限元实现对原生裂纹的定量预测，进而构建“材料-工艺-裂纹”模型。通过对比无SPD试样、无裂纹SPD试样、不同体积分数和不同分布特征SPD裂纹试样的SCC敏感性指标，并基于对晶粒尺寸、晶界析出相和残余应力等因素分析，确定原生裂纹对块体超细晶高强铝合金SCC的影响规律及其作用机理，为块体超细晶高强铝合金的腐蚀与预防及其工业化应用提供理论支撑。

大塑性变形（SPD）工艺在制备超细晶（UFG）材料方面具有明显的技术优势，然而剧烈塑性变形导致材料尤其是高强度材料内部极易产生裂纹（称为原生裂纹）。面向航空工业应用的7050铝合金，经多道次SPD变形后，其等效应变量累积很大，晶粒也被有效细化，但在晶粒内部和晶界处的位错密度仍然较高，原生裂纹大量存在，这对铝合金抗应力腐蚀开裂（SCC）敏感性有着重要影响。针对以上问题，本项目开展对原生裂纹的调控，并研究原生裂纹对铝合金SCC的影响及其作用机理。为预测大塑性变形过程微观裂纹产生机理，开展了基于晶体塑性与延性损伤模型相结合的有限元模拟技术，系统研究了不同加载方式下损伤演化特点，基于损伤最小、累积塑性应变最大的原则，确定简单剪切变形是一种最优方式。基于简单剪切变形在细晶方面巨大优势，设计一种集压扭组合为一体的新型椭圆截面螺旋等通道挤压（ECSEE）模具。通过对铝合金进行前期热处理，如固溶时效等工艺，调控析出相和组织；对大塑性变形工艺制备块体材料进行工业CT扫描，获取微裂纹缺陷的尺寸和分布特征。研究含裂纹块体超细晶样品在晶间腐蚀过程中的腐蚀特征。研究表明，微裂纹的产生使得试样边缘区域出现了由宽到窄的通道，而这一通道使得腐蚀液能够很容易地侵入到试样内部，加剧了腐蚀进程。