Abstract The asymmetrical rolling (ASR) process with shear deformation is considered as a promising technology to adjust/improve through-thickness microstructure homogeneity and integrated properties of high-strength aluminium alloy plates. But the advantages may come with caveats that are the subject of our research. In this paper, the room-temperature low cycle fatigue properties and fracture behaviour of the ASR-ed AA7050 aluminium alloy plates are compared with the symmetrical rolling (SR) one. It is shown that after either type of rolling the plates exhibit similar low-cycle fatigue lives but the SR-ed one displays a better cyclic deformation ability and slightly higher fatigue lives at high strain amplitudes. It is demonstrated that the severe surface-localized deformation contributes to the formation of slip relief on the surface and subsequently initiates micro-cracks that are propagated via transgranular and/or intergranular fracture modes along with obvious fatigue striations. Recrystallised grains with coarse grain boundary precipitates and wide precipitate-free zones near the upper/bottom layers as well as numerous and larger secondary particles in the ASR-ed plates may cause early crack initiation for a short crack initiation life. However, the SR-ed plate with more frequent subgrains near surface layers, numerous fine subgrains and less indissoluble particles could possess better crack initiation/propagation resistance and cyclic loading behaviour. Fine subgrains with higher microhardness/strength can facilitate passing of dislocations or slip bands into adjacent grains so as to delay crack propagation such as via energy-intensified transgranular fracture for extending fatigue life. Properly balancing the through-thickness strain/deformation distribution and the formation of recrystallization/indissoluble particles via implementing a feasible ASR process becomes a critical issue to achieve fracture-resistant microstructures for high-strength aluminium alloy plates. The underlying causes/mechanisms regarding the differences of microstructures and mechanical behaviour are revealed and discussed based on modelling through-thickness temperature/strain distribution and detailed microstructure characterization.

中文翻译：

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非对称轧制高强7050铝合金板的室温低周疲劳与断裂行为

摘要 具有剪切变形的非对称轧制（ASR）工艺被认为是调整/改善高强度铝合金板全厚度组织均匀性和综合性能的一种有前景的技术。但优势可能伴随着我们研究的主题。在本文中，ASR 处理的 AA7050 铝合金板与对称轧制 (SR) 板的室温低周疲劳性能和断裂行为进行了比较。结果表明，在任一类型的轧制后，板材都表现出相似的低周疲劳寿命，但 SR-ed 显示出更好的循环变形能力和在高应变幅下略高的疲劳寿命。结果表明，严重的表面局部变形有助于在表面形成滑移释放，随后引发通过穿晶和/或晶间断裂模式传播的微裂纹以及明显的疲劳条纹。具有粗晶界析出物和靠近上/底层的宽无析出区的再结晶晶粒以及 ASR 板中大量和较大的二次颗粒可能导致早期裂纹萌生，从而导致裂纹萌生寿命短。然而，SR-ed 板在表层附近具有更频繁的亚晶、众多的细亚晶和较少的不溶性颗粒，可以具有更好的裂纹萌生/扩展阻力和循环加载行为。具有更高显微硬度/强度的细亚晶粒可以促进位错或滑移带进入相邻晶粒，从而延迟裂纹扩展，例如通过能量强化穿晶断裂来延长疲劳寿命。通过实施可行的 ASR 工艺正确平衡全厚度应变/变形分布和再结晶/不溶性颗粒的形成，成为实现高强度铝合金板抗断裂微观结构的关键问题。基于全厚度温度/应变分布建模和详细的微观结构表征，揭示和讨论了有关微观结构和机械行为差异的根本原因/机制。