搅拌摩擦加工镁合金细晶强化行为表征

Friction stir processing (FSP) is a new fine grained material fabricated technology. FSPed fine grained magnesium alloy has lower stress concentration factor k in the Hall-Petch equation, especially exhibits inverse relationship. This result is mainly attributed to grain orientation, grain boundary orientation and alloying element. Object of this study firstly focuses on solid solution magnesium alloy. The fine grained magnesium alloy with different grain orientation and grain boundary orientation is fabricated using FSP. TEM, SEM and EBSD technology are used to analyze the grain size, grain orientation and grain boundary orientation evolution. The relationship between grain size, high angle grain boundary fraction and friction stress, stress concentration factor is obtained on the base of Hall-Petch equation fitted from mechanical property data. The effect of alloy element on the grain refinement strengthening is studied by ageing treatment controlling the alloy element distribution in heat-treatable strengthening magnesium alloy. On the base of above studies, the effect of the grain orientation and high angle grain boundary on the plastic deformation in fine grained magnesium alloy is investigated. The reason of lower k value and even inverse Hall-Petch relationship is interpreted. The effect of the alloy element on the grain refinement strengthening is revealed. Critical condition and control methods of the grain refinement strengthening key factor are obtained, and the grain refinement strengthening equation of magnesium alloy with widely applicability is created.

搅拌摩擦加工（FSP）是一项新型细晶材料制备技术。FSP细晶镁合金Hall-Petch方程中应力集中因子k值较低，甚至出现反Hall-Petch关系现象。晶粒取向、晶界取向和合金元素是决定此现象的关键因素。本研究先以固溶体镁合金为对象，采用FSP技术制备出具有不同晶粒取向和晶界取向的细晶镁合金。采用TEM、SEM、EBSD等技术获得晶粒尺寸、晶粒取向和晶界取向的演化统计数据。通过力学性能数据拟合Hall-Petch方程，建立晶粒取向、高角度晶界分数与摩擦应力和应力集中因子的关系。再以时效控制可热处理强化镁合金中合金元素分布，研究合金元素对细晶强化作用规律，加深对晶粒取向和高角度晶界在细晶镁合金塑性变形中作用的理解，揭示造成k值较低和反Hall-Petch关系的本质原因，探讨合金元素对细晶强化行为作用机理，寻求细晶强化关键因素的临界条件和控制手段，建立适用性更广的镁合金细晶强化方程。

本项目采用AZ31镁合金轧制板材和可热处理强化AZ80镁合金轧制板材为研究对象，结合搅拌头设计，选择合适的搅拌摩擦加工（FSP）工艺参数，制备了具有均匀等轴晶粒的细晶组织；利用EBSD对FSP加工前后的镁合金织构、晶界取向差、施密特因子进行了检测分析，并对FSP加工后的镁合金的显微硬度及室温拉伸性能进行了测试，探讨了晶粒尺寸、织构和第二相对细晶镁合金力学行为的影响。得出的主要研究结论如下：AZ31镁合金FSP后搅拌区获得平均晶粒尺寸为1.56～5.95μm的均匀等轴细晶组织，显微硬度随平均晶粒尺寸的减小而增大；经过FSP加工，镁合金大角度晶界比例由86.2%下降为54.0%～59.9%，基面法向由垂直于PD方向转变为与PD方向夹角为60°～68°，织构强度由8.22增大到65.92～69.91；加工后的镁合金在PD、TD两个方向上的屈服强度和晶粒尺寸的关系符合Hall-Petch关系，但拉伸性能在两个方向上表现出明显的各向异性；实验表明当晶粒尺寸一样，织构的方向易于滑移时，屈服强度值会降低，说明晶粒尺寸、晶界取向、织构都能影响屈服强度值；加工后镁合金在PD方向的变形行为主要为基面滑移，伴随拉伸孪生，变形过程表现为较低的屈服强度值和持续稳定的加工硬化率；在TD方向的变形行为为柱面滑移，变形过程表现为较高的屈服强度值和快速下降的加工硬化率；孪生是镁合金表现出不同加工硬化行为的原因。织构、晶粒尺寸都会对孪生变形产生影响，孪生变形又进一步对加工硬化行为产生影响。.AZ80镁合金FSP加工后搅拌区获得平均晶粒尺寸1.65μm～9.22μm的均匀等轴细晶组织，显微硬度和屈服强度的变化均符合Hall-Petch关系；SZ区基面织构c轴沿垂直方向（ND）向FSP加工方向（PD）偏转30°～40°，导致室温拉伸性能沿横向（TD）和PD出现了显著的各向异性；FSP试样沿TD和PD的拉伸变形机制分别以柱面滑移和基面滑移为主，TD拉伸试样的第三阶段的软化速率是PD拉伸试样的4～6倍；时效温度对显微硬度的影响规律：240℃峰值时效时间较短，180℃次之，120℃最长；180℃时效时，细晶镁合金的第二相体积分数、硬度值和屈服强度均随时效时间延长呈现先增加后平稳的趋势，延伸率随时效时间逐渐降低。