【作者】 侯秀丽；

【导师】 曹占义；

【作者基本信息】 吉林大学 ， 材料学， 2012， 博士

【摘要】 近年来，随着材料减重、降低能耗和环境保护意识的增强，镁合金逐渐成为高技术和民用工业领域的理想替代材料。而变形镁合金的开发与应用也正在受到越来越多的重视。稀土元素是镁合金的主要合金化元素之一，添加稀土元素不仅可以细化镁合金的组织，还能提高镁合金的抗氧化性和抗蠕变性能，因此能够明显增加镁合金的使用温度范围。而且稀土元素会影响镁合金在变形加工过程中的再结晶行为及织构的演变，产生对合金织构的弱化作用，这对改善变形镁合金塑性的各向异性，提高其综合性能具有重要意义。本论文以Mg–Gd–Y–Nd–Zn(Zr)系合金为研究对象，对其进行了热挤压、轧制和自由锻造等变形加工，研究了不同变形加工参数对合金微观组织、织构和力学性能，以及时效硬化行为的影响作用，并讨论了稀土元素及其化合物对合金组织和力学性能的作用机制。首先，采用挤压变形技术制备了Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr合金板材和棒材，发现该合金的成形性能良好。合金在挤压加工过程中发生了完全的再结晶，组织得到明显细化，且挤压合金在不同加工平面上的组织分布比较均匀。对挤压合金在200℃至300℃进行了时效热处理，合金表现出了明显的时效硬化行为。通过对峰时效态合金的TEM分析发现合金基体中生成了大量的β′析出相，这些析出相沿α-Mg基体的{1120}棱柱面析出，具有正交晶体（bco）结构，并且与α-Mg基体保持半共格的位相关系，能够有效阻碍基面位错的滑移。拉伸测试表明，β′析出相能够明显提高峰时效态合金的室温和高温强度。对挤压合金的织构分析显示，合金的基面呈现平行于挤压方向分布的特征，这导致挤压态和峰时效态合金在室温下沿挤压方向上的压缩屈服强度/拉伸屈服强度的比值均低于1。但随着温度的升高，织构的影响作用减弱，合金的压缩屈服强度和拉伸屈服强度的相对大小发生了逆转，压缩屈服强度/拉伸屈服强度的比值升高并大于1，这主要是受合金显微组织（即β′第二相或溶质原子气团）的影响。而且位错与β′相和溶质原子气团之间不同的相互作用导致了挤压态和峰时效态合金在高温下压缩屈服强度/拉伸屈服强度的比值的偏离。其次，对Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr合金进行了自由锻造加工。结果表明，锻造加工明显细化了合金的晶粒，且锻造合金平行于拔长方向和垂直于拔长方向平面上的晶粒尺寸差别不大。拉伸测试显示，锻造合金在平行于拔长方向上的强度要高于垂直拔长方向上的强度，但垂直拔长方向的拉伸曲线在屈服之后，表现出了较高的应变硬化效果，而且具有较高的延伸率。此外，研究了Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr合金高温热压缩过程中的孪生变形和动态析出行为。结果表明，在热压缩过程中孪生变形是协调合金塑性变形的主要增补机制，尤其是{1011}-{1012}型双孪生变形。而且合金中发生了大量的动态析出，这种动态析出阻碍并最终抑制了动态再结晶的发生。该合金在热压缩过程中动态析出的矩形片状β′相与经过T6热处理析出的β′相形貌完全不同，主要原因是由于弹性应变能的降低，但它们与镁基体的位相关系仍然保持不变。对Mg6.5Gd1.3Nd0.7Y0.3Zn合金进行了挤压和轧制加工，制备出了厚度分别为2mm、8mm的挤压板和3mm的轧制板材，合金的成形性能良好。经过变形加工，合金的晶粒明显细化，而且粗大的第二相被破碎为小颗粒，在2mm和8mm挤压板材合金中，这些颗粒呈现平行于挤压方向的带状分布特征，在轧制合金中这种带状分布特征明显减弱。通过对变形合金的织构分析显示，挤压和轧制合金均呈现出较弱的<1010>ED纤维织构特征，且{0001}基面极图以板材的法线方向为轴朝板材垂直方向劈开形成两个弱的峰值组元。变形合金这种弱化的织构与固溶态的稀土元素和Mg–RE化合物对合金在变形加工过程中动态再结晶的影响作用有关。拉伸测试表明，挤压和轧制合金在板材ED/RD方向和TD方向存在不同的拉伸各向异性，这主要是受合金织构和第二相颗粒分布的共同影响。通过对合金微观组织的分析，讨论了Mg–Gd–Y–Nd–Zn(Zr)系合金中稀土元素和变形加工对合金力学性能的主要强化机制。稀土元素对提高合金力学性能的主要作用机制包括固溶强化、细晶强化和时效析出强化，其中析出强化的作用效果最好。经过挤压、锻造和轧制等变形加工，进一步提高了合金的细晶强化作用，而且还增加了形变强化作用。更多还原

【Abstract】 Recently，due to the emphasis on material light design, reducing energy consumptionand environment protection, Mg alloys are becoming the ideal substitute materials in thehigh-technology fields and civil producing industries. Meanwhile, the research anddevelopment of wrought Mg alloys are attracting more and more attentions. Rare earth (RE)elements are one of the vigorous alloying elements in Mg alloys. The addition of RE elementsin Mg can refine the structure, and can improve the oxidation resistance and creep resistanceas well that expands the service temperature range of Mg applications. Moreover, RE alloyingelements can produce weaker textures in wrought Mg alloys by changing the recrystallizationkinetics during thermo-mechanical processing, which is benefit to reduce the plasticanisotropy and enhance the comprehensive properties of wrought Mg alloys. In thisdissertation, the Mg–Gd–Y–Nd–Zn(Zr) alloy system was chosen as research object, andprocessed by hot extrusion, hot rolling and free forging technologies. The influences ofdifferent processing parameters on the microstructure, texture, mechanical properties andage-hardening behavior of the wrought alloys were systematically investigated. And theaction mechanisms of RE alloying elements and Mg–RE compounds on the microstructureand mechanical properties of these alloys were discussed.Firstly, the Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr alloy was processed by hot extrusiontechnology, and the extruded alloy sheets and bars were produced, respectively. Resultsshowed that this alloy possesses good formability. During thermal-mechanical processing,dynamic recrystallization took place and the structure of wrought alloys was greatly refined.The grain size distributions on different planes of extruded alloy sheets and bars were uniform.The extruded alloy exhibited remarkable age-hardening response from200℃to300℃. TEMobservation of the peak-aged alloy indicated that there are plenty of β′precipitates in α-Mgmatrix. The β′phase precipitated on the {1120} prismatic planes, which has a bco structureand exhibits semi-coherent structure with α-Mg matrix, could effectively block the basal dislocation slip. The results of tensile tests showed that these β′precipitates played animportant role in the improvement of room temperature and high temperature strengthes ofthe peak-aged alloy. Texture analysis of the extruded alloy revealed that the basal planes ofmost grains are distributed parallel to the extrusion direction. Due to the effect of texture, theas-extruded and peak-aged alloys exhibited almost equivalent asymmetry ratios with theCYS/TYS along extrusion direction both lower than1at room temperature. With increasingtemperature, the influence of texture reduced. The relative magnitude between CYS and TYSreversed, and the ratio of CYS/TYS increased higher than1. This was attributed to the effectof microstructure (i.e. β′secondary phase or solute atom clusters) of the alloy. Meanwhile,different interactions between the dislocations and β′secondary phase or solute atom clustersled to the different asymmetry ratios of the as-extruded and peak-aged alloys.Secondly, the Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr alloy was processed by free forgingtechnology. Results showed that grains of the as-forged alloy were greatly refined and thegrain size distributions were similar on the planes parallel and vertical to the drawingdirection. The results of tensile tests revealed that strengths of the alloy specimens along thedrawing direction were higher than those of the alloy specimens vertical to the drawingdirection; while, the tensile stress–strain curves of the alloy specimens vertical to the drawingdirection exhibited remarkable strain hardening after yielding and possessed higherelongations.Furthermore, twinning and dynamic precipitation upon hot compression of theMg–8Gd–2Y–1Nd–0.3Zn–0.6Zr alloy was systematically investigated. Results revealed thatdeformation twinning was an important supplementary deformation mechanism to——accommodate plastic strain of the alloy, especially the {1011}-{1012} double twinning.Dynamic precipitation occurred extensively during hot compression, which hindered or evensuppressed dynamic recrystallization of the alloy. The morphology of β′precipitates obtainedby dynamic precipitation differed from that of the same phase produced by T6heat treatment.But their orientation relation ship with Mg matrix did not change.The Mg6.5Gd1.3Nd0.7Y0.3Zn alloy was processed by hot extrusion and rollingtechnologies. Extruded alloy sheets with thicknesses of2mm,8mm, and rolled alloy sheetwith a thickness of3mm were produced, respectively. Results showed that this alloypossesses good formability. All alloy sheets exhibited fine equiaxed grains, and coarsesecondary phases in the matrix had been crushed into small particles. In the extruded alloysheets, the crushed particles showed a banded structure along extrusion direction; while in therolled alloy sheet, the distribution of crushed particles was more dispersive. Texture analysis revealed that both the extruded and rolled alloy sheets showed a weak <1010>ED fiber texture,and the {0001} basal poles were split in the transverse direction of the sheet, exhibitingtwo-peak intensity maxima. The RE elements both in Mg solid solution and Mg–REcompounds contributed to the texture weakening of alloy sheets by affecting dynamicrecrystallization behavior during thermal-mechanical processing. The results of tensile testsindicated that the extruded and rolled alloy sheets exhibited different tensile planaranisotropies along the ED/RD and TD. It was influenced by both the texture and thedistribution of secondary phase particles.By means of microstructural analysis, the influences of RE alloying elements andthermo-mechanical processing on the strengthening of mechanical properties ofMg–Gd–Y–Nd–Zn(Zr) alloy system were discussed. Solid-solution strengthening,grain-refining strengthening and precipitation strengthening were considered to be the mainstrengthening mechanisms for the RE alloying elements, and among them the precipitationstrengthening took the greatest strengthening effect. After hot extrusion, free forging or hotrolling processing, the grain-refining strengthening was enhanced, and the deformationstrengthening emerged as well.更多还原