搅拌摩擦加工超细晶镁合金塑韧化机制研究

Grain refinement is an effective method for improving the strength of magnesium alloy. However, lower strain hardening capacity of the ultra-fine grained (UFG) material during plastic deformation process, due to lower storage capacity for intragranular dislocation, results in lower plasticity and toughness at room temperature. The characteristic of grain boundary and grain orientation are two key factors in room temperature plasticity and toughness of the UFG magnesium alloy. In this project, firstly, friction stir processing (FSP) will be used to fabricate the UFG solid solution magnesium alloy with different grain boundary characteristic and grain orientation. Microstructure characteristic of the UFG, such as grain boundary, grain orientation, and dislocation configuration, will be investigated using TEM, SEM, EBSD and XRD et al.. Meanwhile, the effect of grain boundary and grain orientation on the plasticizing and toughening mechanism will be investigated separately. Furthermore, the distribution of the second phase in heat-treatable strengthening magnesium alloy will be changed by ageing treatment, and then the effect of the second phase on the plasticity and toughness of the UFG magnesium alloy will be studied. Based on the above intensive investigation, quantifiable constitutive relation between the strain hardening and elongation will be revealed, and the effect of high-angle grain boundary and grain orientation of the UFG magnesium alloy on the plasticizing and toughening mechanism will be further interpreted. Meanwhile, new methods of improving plasticity and toughness of the UFG magnesium alloy via adjusting the microstructure and external deformation condition will also be discussed. The results of this project will provide experimental data and theoretical basis for the toughened optimal design on the microstructure of the UFG magnesium alloy.

细晶强化是提高镁合金强度的有效方法，但由于超细晶内部存储位错的能力较低，造成塑性变形过程中的应变硬化缺失，从而导致超细晶室温塑韧性较差。晶界特征和晶粒取向是决定超细晶镁合金室温塑韧性的关键因素。本项目先以固溶体镁合金为对象，采用搅拌摩擦加工（FSP）技术制备出具有不同晶界特征和晶粒取向的超细晶镁合金。采用TEM、SEM、EBSD、XRD等技术对超细晶晶界特征、晶粒取向、位错组态等进行表征，通过实验手段分离晶界特征、晶粒取向对塑韧性的单独作用机制。再以时效控制可热处理强化镁合金中的第二相分布，分析第二相对超细晶镁合金塑韧性作用规律。通过这些深入研究，揭示超细晶镁合金应变硬化与延伸率的本构量化关系，阐明高角度晶界和晶粒取向对超细晶镁合金塑韧化作用机制，探讨通过调控微观结构和外界变形条件提高超细晶镁合金塑韧性的方法，从而为超细晶镁合金材料的增韧结构优化设计提供实验依据和理论指导。

搅拌摩擦加工（FSP）技术是一项新型的强塑性变形技术，在加工过程中可同时实现材料微观组织的细化、均匀化和致密化，可提高材料的综合强塑性能。本文采用FSP技术制备了超细晶镁合金，系统研究了超细晶镁合金的室（高）温变形行为及机制，主要研究结论有：.采用水下FSP技术成功制备了超细晶镁合金，超细晶镁合金呈现出再结晶组织，晶粒尺寸随着转速的减小和前进速度的增加而减小。FSP导致超细晶呈现明显的晶粒择优取向。晶粒尺寸与显微硬度关系式为Hv=50.2+22.4d-1/2。织构软化导致超细晶镁合金拉伸强度降低。室温力学行为呈现明显的各向异性。.沿PD方向变形时的主要机制是位错基面滑移，拉伸孪生协调变形。样品具有良好的加工硬化性能和高的延伸率；沿TD方向变形时的主要机制是位错基面滑移和柱面滑移，延伸率较低。晶粒细化导致加工硬化率降低，但对加工硬化第三阶段的动态回复影响较小。初始加工硬化速率和饱和应力的比值与均匀延伸率符合幂函数关系。FSP制备的AZ31镁合金均匀延伸率高达32%，表明FSP是一种制备室温高塑性镁合金的方法。.当变形温度为450℃，应变速率为3×10-3 s-1时，FSPAZ31超细晶镁合金的最大延伸率达到1090.8%。随温度的升高，FSP试样的晶粒尺寸逐渐增大。晶粒粗化是导致超塑性性能降低的主要原因。晶界滑移是超塑性变形的主要机制。.随载荷增大，纳米硬度值降低，呈现出明显的尺度效应。与母材相比，超细晶镁合金具有较高的总位错密度、几何必需位错密度和统计存储位错密度，呈现出高的纳米硬度和抗压屈服强度。纳米力学行为符合Tabor压痕理论。