The feasibility to fabricate ultra-fine-grained AZ91 Mg alloy is investigated with a newly presented severe plastic deformation method entitled cyclic contraction/expansion extrusion. In this method, an initial cylindrical AZ91 Mg billet is placed into a die and the moving punch causes the large deformations by extruding the material in two different regions entitled contraction and extrusion. The evolution of AZ91 microstructure and mechanical properties during cyclic contraction/expansion extrusion was studied through different experimental observations. The microstructure observations showed the ultra-fine-grained structure of AZ91 at the end of the third pass where the average grain size of 600 nm obtained from the initial value of 130 µm. The tensile tests showed that the ultimate tensile strength, yield strength, hardness, and elongation of AZ91 cyclic contraction/expansion extrusion-processed samples are increased significantly. Discontinuous dynamic recrystallization has a main role in the grain refinement of Mg alloys during hot deformations. The evolution of grains in microlevel is analyzed by the cellular automaton finite element method in the DEFORM software environment. The macroscopic flow parameters including effective plastic strain, stain rate, and temperature were calculated in finite element. By tracing these data in defined domain of cellular automaton, the discontinuous dynamic recrystallization of material is analyzed through a devised cellular automaton finite element post-processing step. The imposed plastic strain and variation of dislocation density are the two main driven forces in discontinuous dynamic recrystallization of AZ91 samples during cyclic contraction/expansion extrusion processing. The experimentally observed grains and the cellular automaton finite element predicted microstructure were reasonably in good agreement.

中文翻译：

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使用实验和微机械元胞自动机有限元方法表征通过循环收缩/膨胀挤压加工的 AZ91 镁合金

用新提出的称为循环收缩/膨胀挤压的严重塑性变形方法研究了制造超细晶粒 AZ91 镁合金的可行性。在这种方法中，将初始圆柱形 AZ91 镁坯放入模具中，移动的冲头通过在两个不同的区域（收缩和挤压）挤压材料来引起大变形。通过不同的实验观察研究了循环收缩/膨胀挤压过程中 AZ91 微观结构和机械性能的演变。微观结构观察表明，在第三道次结束时 AZ91 具有超细晶粒结构，其中从初始值 130 µm 获得 600 nm 的平均晶粒尺寸。拉伸试验表明，极限拉伸强度、屈服强度、硬度、AZ91 循环收缩/膨胀挤压加工样品的伸长率显着增加。不连续动态再结晶在热变形过程中镁合金的晶粒细化中起主要作用。在DEFORM软件环境下，采用元胞自动机有限元方法分析晶粒在微观层面的演化。在有限元中计算了宏观流动参数，包括有效塑性应变、染色速率和温度。通过在元胞自动机的定义域中跟踪这些数据，通过设计的元胞自动机有限元后处理步骤分析材料的不连续动态再结晶。施加的塑性应变和位错密度的变化是 AZ91 样品在循环收缩/膨胀挤压加工过程中不连续动态再结晶的两个主要驱动力。实验观察到的晶粒和元胞自动机有限元预测的微观结构相当吻合。