Mg-Zn-Y-Zr合金搅拌摩擦加工组织演变机理及强韧化研究

发布时间：2019-05-18 07:53

【摘要】：稀土镁合金相比于传统镁合金,具有高强度、耐高温和耐腐蚀等特点,在轻量化领域具有十分广阔的应用前景。目前稀土镁合金制备以铸造为主,提高其强度和塑性是当前推广镁合金使用的重要课题之一。搅拌摩擦加工作为一种大塑性变形技术,具有操作简便、高效和环保等优点,可实现材料的局部或整体改性,提高材料的服役性能。本文对搅拌摩擦加工制备高性能Mg-Zn-Y-Zr合金进行了研究,阐述了搅拌摩擦加工参数和材料初始状态对搅拌摩擦加工区域镁合金宏微观组织、力学性能和断裂行为的影响,分析了搅拌摩擦加工材料的时效析出行为及其对力学性能的影响,揭示了搅拌摩擦加工强韧化镁合金材料的内在机制,并实现了高强高韧镁合金的制备。对铸态Mg-Zn-Y-Zr合金进行搅拌摩擦加工,通过改变行进速度研究不同程度热机耦合作用对铸态粗晶镁合金的组织改善和性能提升效果。搅拌摩擦加工后,搅拌区分为上部呈现纤维状晶粒的纤维区和中下部呈现细小等轴晶的细晶区,随着行进速度的提高,细晶区面积逐渐减小,但减小速率快速下降。搅拌摩擦加工后镁合金组织显著细化,铸态粗晶发生剧烈的动态再结晶,晶粒细化至微米级别,铸态组织中粗大的晶间网状共晶相破碎成细小颗粒并弥散分布。在剧烈的热机耦合作用下,材料的元素扩散速率大幅提升,使被破碎的I相(Mg3Zn6Y)颗粒快速转变为W相(Mg3Zn3Y2)颗粒,形成芯部为残余I相,边缘为W相的芯壳结构。搅拌摩擦加工过程中剧烈的塑性变形使细晶区形成镁基面平行于搅拌针表面的环形基面织构。细晶、弥散颗粒和环形基面织构均有利于材料强度和塑性的提升。随着行进速度的提高,细晶区中晶粒尺寸减小,弥散颗粒含量提高,整体力学性能提升,在800 rpm-200 mm/min工艺参数下屈服强度、抗拉强度和延伸率分别达到171MPa、300 MPa和27%,相比于铸态母材分别提高了92%,53%和145%。对铸态和固溶态合金搅拌摩擦加工的热机影响区进行组织分析,表征不同部位微区组织特征研究搅拌摩擦加工过程中的微观组织演变行为,随着搅拌摩擦加工过程中热机耦合作用的增强,粗晶镁合金中先后发生孪生动态再结晶、颗粒促进形核以及连续和不连续动态再结晶从而使晶粒细化。由于固溶态合金内第二相含量较少,且大量固溶原子造成溶质拖曳效应显著降低动态再结晶程度,使固溶态合金搅拌摩擦加工细晶区材料的平均晶粒尺寸偏大,强度低于相同参数下的铸态搅拌摩擦加工细晶区材料。对于800 rpm-200 mm/min工艺参数下的铸态和固溶态合金搅拌摩擦加工细晶区材料,250℃下分别时效处理8 h和10 h可达峰时效状态。由于大量热稳定性良好的W相弥散颗粒和沿晶界析出W相纳米颗粒的阻碍作用,晶粒长大缓慢,在峰时效状态下仍保持微米级细晶组织。峰时效下在晶粒内部析出β1'杆状沉淀相,进一步提高材料的强度且保持较高的塑性。铸态合金搅拌摩擦加工细晶区峰时效材料的屈服强度、抗拉强度和延伸率分别达到201 MPa、330 MPa和24%,较铸态母材分别提高了126%、68%和118%,实现了高强高韧镁合金的制备。
[Abstract]:Compared with the traditional magnesium alloy, the rare earth magnesium alloy has the characteristics of high strength, high temperature resistance and corrosion resistance and the like, and has a wide application prospect in the field of light weight. At present, the preparation of rare-earth magnesium alloy is dominated by casting, and the strength and plasticity of the rare-earth magnesium alloy are one of the most important topics to be used to promote the use of the magnesium alloy. The stirring and friction processing is used as a large plastic deformation technology, and has the advantages of simple and convenient operation, high efficiency, environmental protection and the like, and can realize the local or integral modification of the material and improve the service performance of the material. In this paper, a high-performance Mg-Zn-Y-Zr alloy was prepared by friction stir processing, and the effects of the friction processing parameters and the initial state of the material on the microstructure, mechanical properties and fracture behavior of the magnesium alloy macro-microstructure in the friction processing area were described. The aging and precipitation behavior of the friction-friction material and its effect on the mechanical properties are analyzed. The internal mechanism of the high-strength and high-toughness magnesium alloy is also realized. The microstructure of the as-cast Mg-Zn-Y-Zr alloy and the effect of mechanical coupling on the microstructure and the performance of the as-cast coarse-grained magnesium alloy were studied by changing the traveling speed. After the stirring and friction processing, the fiber area and the middle-lower part of the fibrous crystal grains are divided into fine crystal areas with fine equiaxed grains in the upper part, and the area of the fine crystal area is gradually reduced along with the increase of the traveling speed, but the reduction rate is reduced rapidly. After friction processing, the microstructure of the magnesium alloy was refined, and the as-cast crude crystal was recrystallized, the grain was refined to the micron level, and the coarse intercrystalline eutectic phase in the as-cast structure was broken into fine particles and dispersed. The element diffusion rate of the material is greatly improved under intense heat engine coupling, and the crushed I-phase (Mg3Zn6Y) particles are rapidly converted into W-phase (Mg3Zn3Y2) particles, and the core is a residual I-phase, and the edge is a W-phase core shell structure. The intense plastic deformation during the stirring and friction processing causes the fine crystal region to form an annular base surface texture which is parallel to the surface of the stirring needle. The fine crystal, the dispersion particle and the annular base surface texture are both favorable for the material strength and the plasticity of the material. As the advancing speed is improved, the grain size in the fine crystal region is reduced, the content of the dispersion particles is improved, the overall mechanical property is improved, the yield strength, the tensile strength and the elongation at the process parameters of 800 to 200 mm/ min reach 171 MPa,300 MPa and 27%, respectively, and compared with the as-cast base material, 53 per cent and 145 per cent. The heat engine-affected zone of the as-cast and in-state alloy stirring and friction processing is carried out for tissue analysis, and the microstructure evolution behavior of the micro-structure in the stirring and friction processing process is characterized by the characteristics of the micro-area microstructure of different parts, and the heat engine coupling effect is enhanced in the stirring friction processing process, In the coarse-grained magnesium alloy, the twin dynamic recrystallization occurs, and the grains promote the nucleation and the continuous and discontinuous dynamic recrystallization so as to refine the crystal grains. because the second phase content in the solid-state alloy is less, and a large amount of solution atoms cause the solute drag effect to obviously lower the dynamic recrystallization degree, the average grain size of the solid-state alloy stirring and friction processing fine crystal region material is large, And the strength is lower than the as-cast stirring friction processing fine crystal region material under the same parameters. For as-cast and high-temperature alloy stirring and friction processing of the fine crystal zone material under the process parameters of 800 rpm to 200 mm/ min, the aging conditions for 8h and 10h are respectively treated at 250 & deg; C for 8 h and 10 h. As the W-phase dispersion particles with good thermal stability and the barrier function of the W-phase nano-particles are precipitated along the grain boundary, the grain growth is slow, and the micron-level fine crystal tissue is still maintained under the state of the peak aging. And the 1 'rod-like precipitated phase is precipitated in the crystal grains under the peak aging, so that the strength of the material is further improved and the plasticity is maintained. The yield strength, tensile strength and elongation of the high-strength and high-toughness magnesium alloy are respectively improved by the yield strength, the tensile strength and the elongation of the high-strength and high-toughness magnesium alloy.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG306;TG146.22

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