Mg-Nd-Sm-Zn-Zr合金微观组织演变及力学性能研究

发布时间：2018-07-16 23:25

【摘要】：镁稀土合金因其比强度高、耐腐蚀性能和生物相容性优良,引起国内外越来越多的重视,但现有的高强镁稀土合金大都采用价格昂贵的重稀土元素作为主要强化元素,大大提高了这类合金的成本。为拓展镁稀土合金应用领域,急需开发一种低成本高性能的镁稀土合金。本文提出采用价格较低的轻稀土元素Nd和Sm联合强化镁合金的新思路,同时配合少量的Zn和Zr元素进一步改善组织。目前,基于混合轻稀土的镁合金研究尚处于起步阶段,Mg-Nd-Sm-Zn-Zr合金的微观组织及力学性能尚不清楚。因此,本文系统开展了Mg-Nd-Sm-Zn-Zr合金成分优化,深入研究了合金在固溶和时效过程组织演变,考察了正向挤压联合等通道转角挤压及其随后时效处理对合金组织和力学性能影响。为优化Mg-Nd-Sm-Zn-Zr合金成分,依据本课题组前期大量的相关研究工作,在已确定的Sm、Zn、Zr元素的最佳含量的基础上,系统研究了稀土元素Nd对Mg-x Nd-2.0Sm-0.4Zn-0.4Zr(0≤x≤2.5)合金铸态、固溶态和时效态的组织和性能的影响,揭示了合金的断裂机制。结果表明:铸态合金主要由α-Mg晶粒和共晶β相组成,β相主要呈网状分布在晶界附近。铸态合金经固溶处理后,β相溶解于α-Mg基体中,形成过饱和固溶体,经随后的时效处理,在α-Mg的基体中形成大量的GP区、β′相和β1相,片状β′相和β1相的惯析面主要为棱柱面。Nd元素可显著增强析出强化效果,Nd元素含量为2.0 wt.%时,时效态合金可获得较高的强度,屈服强度和抗拉强度分别为154 MPa和261 MPa,延伸率为5.8%。铸态合金经固溶处理和时效处理后的拉伸断裂行为由沿晶断裂向穿晶断裂转变。在合金成分优化的基础上,通过调控固溶温度和时间,考察了固溶温度和时间对Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr合金组织和性能的影响。结果表明:随着固溶温度的增加和固溶时间的延长,合金的晶粒尺寸逐渐增大,β相逐渐减少,强度先升高后降低,当固溶温度高于515℃且固溶时间超过8 h,β相基本分解。确定了合金的最佳固溶处理工艺为515℃×8 h。基于优化的固溶处理工艺,进一步研究了时效温度和时间对Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr合金组织和性能的影响。结果表明:随着时效温度的逐渐增加,β′相和β1相的尺寸逐渐增大,当时效温度过高,析出物的密度明显降低。合金的强度随时效温度的升高,先升高后降低,延伸率与强度的变化相反。随着时效时间的延长,合金依次经历初始欠时效、半峰时效、峰时效和过时效阶段。欠时效阶段随时效时间的延长β′相和β1相的尺寸和密度逐渐增大,峰时效阶段β′相和β1相的密度最高,过时效阶段β1相显著粗化且密度降低。首次发现时效态合金中存在基于纳米粒子和粗大片状β1相的棋盘形双相共构析出物。合金经190℃×18 h时效处理后可获得较高的强度,屈服强度和抗拉强度分别为152 MPa和266 MPa,延伸率为6.1%。为缩短Mg-Nd-Sm-Zn-Zr合金的时效处理时间,在时效前对固溶态合金进行冷压缩预变形处理。结果表明:合金的硬度达到峰值所需时效时间仅为10 h,时效时间缩短近一半,且硬度峰值更高。发现预变形合金存在新型基面析出物βB′相,基于基面析出的βB′相及棱柱面析出的β′相所共同形成的封闭结构,可显著增强时效强化效应。利用HAADF-STEM等分析测试技术,确定Mg-Nd-Sm-Zn-Zr合金的时效序列为:SSSS→GP区→β′相→β1相→β相,析出物主要以与基体结构相似度逐步降低的顺序析出,析出物与基体的共格关系和位向关系随析出物的转变逐渐减弱。采用正向挤压联合等通道转角挤压技术制备了高性能Mg-Nd-Sm-Zn-Zr合金。结果表明:随着Nd元素含量的增加,挤压态合金的晶粒尺寸逐渐减小,强度逐渐提高,延伸率逐渐降低。挤压态合金经190℃×18 h时效处理后,析出了大量的析出物,且随着Nd元素含量的增加,析出物数量逐渐增加,强度逐渐升高,塑性逐渐降低。Nd元素含量达到2.0 wt.%时强度较高,屈服强度和抗拉强度分别为187 MPa和315 MPa,延伸率为8.5%。综上,本文设计并制备了低成本高性能Mg-Nd-Sm-Zn-Zr合金,系统研究了合金的组织演变和力学性能。开发的铸态和固溶态合金具有中等强度和塑性,可广泛应用于汽车、3C等领域;经时效处理的合金具有较高的强度,在航空等轻量化领域具有广阔的应用前景;采用正向挤压联合等通道转角挤压技术制备的高强度、高塑性的合金,在医疗等领域的应用潜力巨大。因此,该研究为扩大镁稀土合金的应用,提供学术理论依据,具有重要的应用价值。
[Abstract]:Because of its high specific strength, corrosion resistance and good biocompatibility, magnesium rare earth alloys have attracted more and more attention at home and abroad. However, the existing high strength magnesium rare earth alloys are mostly used as the main intensities of heavy rare earth elements, which greatly improve the cost of this kind of alloy, so it is urgent to develop the application field of magnesium rare earth alloys. A low cost and high performance magnesium rare earth alloy. In this paper, a new idea of combining Nd and Sm with low prices of light rare-earth elements to strengthen magnesium alloys is proposed. At the same time, a small amount of Zn and Zr elements are used to further improve the microstructure. At present, the research on magnesium alloys based on mixed light rare earth is still in the initial stage, the microstructure and force of the Mg-Nd-Sm-Zn-Zr alloy Therefore, the composition optimization of Mg-Nd-Sm-Zn-Zr alloy was systematically carried out in this paper. The microstructure evolution of the alloy in the solid solution and aging process was deeply studied. The influence of the forward extrusion combined equal channel angular extrusion and the subsequent aging treatment on the microstructure and mechanical properties of the alloy was investigated. In order to optimize the composition of Mg-Nd-Sm-Zn-Zr alloy, this course was based on this lesson. On the basis of the optimal content of Sm, Zn and Zr elements, the effects of rare earth element Nd on the structure and properties of Mg-x Nd-2.0Sm-0.4Zn-0.4Zr (0 < < x < 2.5) alloy are systematically studied on the basis of the optimum content of the determined elements. The results show that the alloy's fracture mechanism is revealed. The results show that the cast alloy is mainly based on the alpha -Mg. The crystalline grain is composed of the eutectic beta phase and the beta phase is mainly in the vicinity of the grain boundary. After solid solution treatment, the cast alloy is dissolved in the matrix of alpha -Mg to form a supersaturated solid solution. After the aging treatment, a large number of GP regions, beta 'phase and beta 1 phase are formed in the matrix of alpha -Mg. The inertial surface of the beta' phase and beta 1 phase is mainly.Nd element of the prism surface. When the content of Nd element is 2 wt.%, the aging state alloy can obtain higher strength, the yield strength and tensile strength are 154 MPa and 261 MPa respectively. The tensile fracture behavior of the cast alloy after solid solution treatment and aging treatment is changed from intergranular fracture to transgranular fracture, and the composition of the alloy is superior to the alloy. The effect of solid solution temperature and time on the microstructure and properties of Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr alloy was investigated by adjusting the temperature and time of solid solution. The results showed that the grain size of the alloy increased gradually with the increase of the solid solution temperature and the time of solid solution. The phase of the alloy gradually decreased, the strength first increased and then decreased, when the solid solution temperature was the solution temperature. Higher than 515 C and the solid solution time exceeding 8 h and the basic decomposition of beta phase. The optimum solution treatment process of the alloy was 515 C 8 h. based on the optimized solution treatment process. The effect of aging temperature and time on the microstructure and properties of Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr alloy was further studied. The results showed that the increasing of aging temperature, beta ' The size of the phase and beta 1 phase increases gradually. When the aging temperature is too high, the density of the precipitates decreases obviously. The strength of the alloy increases first and then decreases with the aging temperature. The elongation is the opposite of the strength. With the aging time prolonging, the alloy goes through the initial under aging, half peak aging, peak aging and over aging stage. Under aging stage. The size and density of beta 1 phase and beta 1 phase increased gradually with the aging time. The density of beta and beta 1 phase was the highest at the peak aging stage. The phase of beta 1 was coarsely coarsened and the density decreased in the over aging phase. It was found for the first time that there was a disk shaped double phase co structure precipitate based on nanoparticles and large large beta 1 phase in the aging state alloy. The alloy was 190 C 18 h. High strength is obtained after aging, yield strength and tensile strength are 152 MPa and 266 MPa respectively. The elongation is 6.1%. to shorten the aging time of Mg-Nd-Sm-Zn-Zr alloy, and the cold compression predeformation treatment is done to the solid solution alloy before aging. The results show that the aging time required for the hardness of the alloy is only 10 h, and the aging time is only 10. The time shortening is nearly half and the peak hardness is higher. It is found that the pre deformed alloy has a new base surface precipitate of beta B '. The closed structure formed by the precipitated beta B phase of the base surface and the beta phase precipitated from the prism surface can significantly enhance the aging hardening effect. The aging method of HAADF-STEM is used to determine the aging of the Mg-Nd-Sm-Zn-Zr alloy. The sequence is: SSSS, GP region, beta 'phase, beta 1 phase to beta phase. The precipitates precipitate mainly in the order of gradually decreasing the similarity between the matrix structure and the matrix structure. The relationship between the precipitates and the matrix and the orientation relation gradually weaken with the transition of the precipitates. The high performance Mg-Nd-Sm-Zn-Zr alloy was prepared by the forward extrusion combined with equal channel angular extrusion technology. The results show that with the increase of the content of Nd elements, the grain size of the extruded alloy gradually decreases, the strength increases and the elongation gradually decreases. A large number of precipitates have been precipitated in the extruded alloy after 190 C 18 h aging treatment, and with the increase of the content of Nd elements, the number of precipitates gradually increases, the strength increases gradually, and the plasticity gradually reduces.Nd yuan. The strength of the element is 2 wt.%, the yield strength and tensile strength are 187 MPa and 315 MPa respectively, and the elongation is 8.5%.. The low cost and high performance Mg-Nd-Sm-Zn-Zr alloy is designed and prepared. The microstructure evolution and mechanical properties of the alloy are systematically studied. The developed and solid solution alloys are of medium strength and plasticity, and can be widely used. The pan should be used in the fields of automobile, 3C, and other fields. The aging alloy has a high strength and has a broad application prospect in the field of Aeronautical light weight. The high strength and high plasticity alloy prepared by forward extrusion combined with equal channel angular extrusion technology has great potential in the application of medical and other fields. Therefore, this study is to expand the combination of magnesium rare earth and rare earth. The application of gold provides academic theoretical basis and has important application value.
【学位授予单位】：哈尔滨理工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG146.22

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