Abstract Surface severe plastic deformation such as severe shot peening has been proposed to manufacture a gradient microstructure with grains smaller than 5 μm near the surface. Subsequent annealing of the heavily cold deformed structure contributes to grain boundary recovery, dislocation rearrangement or annihilation, and consequently ductility enhancement due to the formation of defect-free grains. This structure would be unstable in thermal conditions due to incomplete recrystallization, high density of dislocations, and smaller grain size. In this work, microstructural evolution and thermal stability of a gradient microstructure created via the severe shot peening process in AZ31 is investigated through characterizing the microstructure during isothermal and isochronal heat treatment. In the end, thermal stability of the structure is demonstrated wherein recrystallization started at 150 ∘C from the surface and continued through the thickness until new defect-free grains appeared in all deformed layers. The newly-formed structure stays relatively stable with slow grain growth from temperature 300 ∘C to 450 ∘C that leads to the acceptable value of grain boundary activation energy.

中文翻译：

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冷剧烈表面塑性变形后AZ31梯度微观结构的热行为

摘要 表面严重的塑性变形如严重的喷丸处理已被提出来制造一种在表面附近具有小于 5 μm 晶粒的梯度显微组织。重度冷变形组织的后续退火有助于晶界恢复、位错重排或湮灭，从而由于无缺陷晶粒的形成而导致延展性增强。由于不完全再结晶、高密度的位错和较小的晶粒尺寸，这种结构在热条件下会不稳定。在这项工作中，通过在等温和等时热处理过程中表征微观结构，研究了通过 AZ31 中的剧烈喷丸过程产生的梯度微观结构的微观结构演变和热稳定性。到底，证明了结构的热稳定性，其中再结晶从表面开始在 150 ∘ C 处开始，并继续穿过厚度，直到所有变形层中出现新的无缺陷晶粒。新形成的结构保持相对稳定，晶粒从 300 ∘ C 到 450 ∘ C 缓慢生长，这导致晶界活化能的可接受值。