WE71镁合金板材制备与组织性能研究

发布时间：2018-09-08 06:37

【摘要】：变形稀土镁合金具有较好的室温、高温力学性能和抗腐蚀性能,符合航空航天、武器装备和汽车轻量化的要求,具有广阔的应用和发展前景。目前变形稀土镁合金主要采用锻造和挤压的方式进行塑性变形,其产品规格和形状受到很大的限制。而轧制作为最经济高效的塑性加工方法,在金属材料塑性加工领域得到广泛的应用,镁合金板材轧制的研究也越来越受到重视。随着镁合金板材需求的增多,对板材的强度提出了更高的要求,致使开发高强度的宽幅镁合金板材成为必然。根据目前稀土镁合金的相关文献可知,稀土元素质量百分比在10%以上的合金,强度高,塑性低,变形热加工窗口窄,只能通过特定的加工方式制备,不适合开发高强度的镁合金板材。因此,在本课题组对EW75、WE93、WE91和WE83合金研究的基础上,通过适当的降低稀土含量,设计开发出一种塑性较好、强度较高和热加工窗口较宽的稀土镁合金,为WE71合金。其兼顾了强度和塑性,为开发高强度稀土镁合金宽幅板材提供了可能。本文以Mg-7Y-1Nd-0.5Zr(WE71)合金为研究对象,对铸态、均匀化态、变形态等不同状态合金的组织、结构和性能进行了系统的研究。为研制高性能的稀土镁合金板材提供实验基础和理论依据。确定铸态WE71合金微观组织是由a-Mg基体、“骨骼状”的共晶相、富Y的方块相和Zr颗粒组成。经过均匀化热处理后,共晶组织基本回溶到基体中,均匀化态合金的组织主要由a-Mg基体和富Y的方块相组成,方块相的结构和成分与铸态合金中的相同。均匀化后的合金进行直接挤压,挤压过程中合金发生了完全动态再结晶,合金的微观组织由平均晶粒尺寸为18μm的细小等轴晶粒组成。研究了均匀化态WE71合金的热模拟变形过程中的组织演变规律。晶界是主要的动态再结晶的形核位置。变形量较小时,晶界发生迁移形成锯齿状,具有非连续动态再结晶的特征。变形量为60%时,亚晶界通过吸收位错不断增加其取向差,形成大角度晶界,属于连续动态再结晶的特征。均匀化态合金的变形激活能Q为212.37kJ/mol,本构方程可以表为：ε=3.337×1012[sinh(0.01108|σ|)]445 exp[-(212.37 ×103)/8.314T]。铸态合金轧制后,共晶组织破碎,成点链状或线状分布,促进动态再结晶的形核,细化晶粒：组织中产生大量的孪晶,道次间退火,发生了孪晶诱导再结晶的现象,细化组织,改善塑性,降低变形抗力,有利于下一道次的轧制变形。将动态再结晶与静态再结晶有机结合,调控合金组织与性能。铸态WE71合金轧制后共晶组织的比表面积增加、晶界面积增加、位错密度增加,加快了溶质原子的扩散,缩短了均匀化时间。经过轧制+均匀化后合金的晶粒尺寸明显小于铸态直接进行均匀化合金的晶粒尺寸。说明轧制+均匀化的工艺制度在消除共晶组织和细化晶粒方面具有良好的效果。挤压态合金通过轧制变形后,获得了较为均匀的显微组织。随着轧制温度的升高,基面织构的最大极密度先增加后减小,在450℃轧制时达到最大值,为7.175。随着道次变形量的增大,发生动态再结晶区域越大,细化了晶粒,改善了组织的均匀性。随着总变形量的增大,发生动态再结晶区域面积越来越大,显微组织由细小的等轴再结晶晶粒组成,说明要获得更加细小的再结晶晶粒,在热轧过程中必须采用较大的总变形量进行轧制。在保证轧制行为有效的前提下,通过对组织和性能的考察,得出WE71合金的最佳轧制工艺为：温度为500℃,道次变形量为10%,总变形量约为50%。研究了WE71合金的时效沉淀析出序列及其强化机制。通过对WE71合金在175℃时效过程进行研究,发现了含Y元素较高的稀土镁合金在低温时效过程中,析出相形核和长大过程缓慢。WE71合金在200℃等温时效过程中的主要沉淀相为β'和β相。β'相是以(1010)为惯习面的底心正交结构,晶格常数为a=O.64nm,b=2.22nm,c=0.52mn。β'相与基体处于完全的共格关系,β'相与基体的共格关系及其不同变体间相互交错成网状是其在200℃时效具有较高的热稳定的主要原因。T5态合金的高温拉伸试验表明,随着温度的升高,合金的屈服强度迅速降低,而延伸率增加；随着保温时间的延长,强度值下降,而延伸率增加。本研究发展的理论和制订的新工艺将很好的指导高强度稀土镁合金板材的轧制,很大程度上提高了轧制成形性和生产效率,具有重大生产和学术的意义。
[Abstract]:Wrought rare earth magnesium alloys have good mechanical properties at room temperature, high temperature and corrosion resistance, which meet the requirements of aerospace, weaponry and automotive lightweight, and have broad application and development prospects. As the most economical and efficient plastic processing method, rolling has been widely used in the field of metal plastic processing, and the research of magnesium alloy sheet rolling has been paid more and more attention. According to the related literatures of rare earth magnesium alloys, the alloys with the mass percent of rare earth elements more than 10% have high strength, low plasticity and narrow deformation hot working window, which can only be prepared by specific processing methods, and are not suitable for developing high strength magnesium alloy sheets. On the basis of this, a kind of rare earth magnesium alloy with good plasticity, high strength and wide hot working window was designed and developed by reducing the content of rare earth properly, which is called WE71 alloy. The microstructure, structure and properties of as-cast and deformed WE71 alloys were systematically studied. The experimental and theoretical basis was provided for the development of high-performance RE-Mg alloy sheets. It was determined that the microstructure of as-cast WE71 alloy was composed of a-Mg matrix, skeletal eutectic phase, Y-rich cubic phase and Zr particles. After heat treatment, the eutectic structure is basically dissolved back into the matrix. The microstructure of homogenized alloy is mainly composed of a-Mg matrix and Y-rich cubic phase. The structure and composition of the cubic phase are the same as those of as-cast alloy. The homogenized alloy undergoes direct extrusion, and the alloy undergoes complete dynamic recrystallization during extrusion. Microstructure evolution of homogenized WE71 alloy during thermal simulation deformation was studied. Grain boundary is the main nucleation site of dynamic recrystallization. When deformation is small, grain boundary migrates to form serrated shape, which is characterized by discontinuous dynamic recrystallization. The deformation activation energy Q of the homogenized alloy is 212.37 kJ/mol, and the constitutive equation can be expressed as follows: e = 3.337 *1012 [sinh (0.01108 | _ |)] 445 exp [-(212.37 | 103) / 8.314T]. After rolling, the eutectic structure of the as-cast alloy is broken and the point chain is formed. Distribution in shape or linearity promotes nucleation of dynamic recrystallization and refines grains: a large number of twins are produced in the microstructure, annealing between passes, twin-induced recrystallization occurs, refines microstructure, improves plasticity, reduces deformation resistance, and is conducive to the next rolling deformation. The specific surface area of eutectic structure, intergranular area and dislocation density of as-rolled WE71 alloy increase, which accelerates the diffusion of solute atoms and shortens the homogenization time. The extruded alloys obtained uniform microstructure after rolling deformation. With the increase of rolling temperature, the maximum extreme density of basal texture first increases and then decreases, reaching a maximum value of 7.175 when rolling at 450 C. The larger the dynamic recrystallization region is, the finer the grain is, and the more homogeneous the microstructure is. With the increase of total deformation, the area of dynamic recrystallization region becomes larger and larger. The microstructure consists of fine equiaxed recrystallized grains, which indicates that to obtain finer recrystallized grains, a larger total transformation must be adopted during hot rolling. On the premise of guaranteeing the effective rolling behavior, the optimum rolling process of WE71 alloy is obtained by investigating the microstructure and properties of the alloy. The optimum rolling process is as follows: the temperature is 500 C, the pass deformation is 10%, and the total deformation is about 50%. The aging precipitation sequence and strengthening mechanism of WE71 alloy are studied. The main precipitation phases of WE71 alloy during isothermal aging at 200 C are beta and beta phases. The main reason for the high thermal stability of the alloy aged at 200 C is the complete coherence of the matrix. The tensile test at high temperature shows that the yield strength of the alloy decreases rapidly and the elongation increases with the increase of temperature. The theory developed in this study and the new technology formulated will guide the rolling of high strength RE-Mg alloy sheets, greatly improve the formability and production efficiency, and have great production and academic significance.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.22;TG339

【参考文献】