纳米SiC颗粒增强Mg-9Al-lSi复合材料的ECAP变形组织及高温蠕变行为研究

发布时间：2018-01-11 06:23

本文关键词：纳米SiC颗粒增强Mg-9Al-lSi复合材料的ECAP变形组织及高温蠕变行为研究　出处：《太原理工大学》2017年硕士论文　论文类型：学位论文

【摘要】：高温稳定相Mg_2Si增强Mg-Al-Si复合材料是最早为汽车动力系统量身打造的抗蠕变镁基复合材料。然而,铸态Mg-Al-Si复合材料中网状Mg17Al12相和粗大汉字状Mg_2Si相会严重割裂基体,导致力学性能大幅降低,限制了其广泛应用。因此,改善第二相的尺寸和形貌对Mg-Al-Si复合材料性能的提高有重要意义。本文通过半固态搅拌+超声波分散技术制备了纳米SiC颗粒含量为1wt.%的Mg-9Al-1Si(记为Mg-9Al-1Si-1SiC)复合材料,并对复合材料进行ECAP变形。采用OM、SEM、XRD、EDS和TEM等检测手段,分析了纳米SiC颗粒对Mg-9Al-1Si复合材料组织的影响,以及固溶态Mg-9Al-1Si-1SiC复合材料不同道次变形后显微组织的变化规律;并对复合材料的室温力学性能和高温蠕变性能进行了系统研究。研究结果发现,铸态的Mg-9Al-1Si复合材料中添加1wt.%纳米SiC颗粒之后,基体晶粒、Mg17A112和Mg_2Si相都得到了明显细化,但Mg17A112相仍然呈网状分布,Mg_2Si相仍然呈汉字状形态;纳米SiC颗粒在基体中分布较为均匀,而在Mg17Al12和Mg_2Si相周围呈团簇分布。对铸态Mg-9Al-1Si和Mg-9Al-1Si-1SiC复合材料直接进行4道次ECAP变形,基体晶粒显著细化,Mg17Al12和Mg_2Si相明显破碎。与挤压态Mg-9Al-1Si复合材料相比,挤压态Mg-9Al-1Si-1SiC复合材料的基体晶粒更加细小,碎化的Mg17Al12和Mg_2Si颗粒较小且分布更加均匀;另外ECAP变形后,纳米SiC颗粒分布得到改善,使其表现出较高的力学性能。然而,铸态Mg-9Al-1Si-1SiC复合材料直接ECAP变形后,组织中仍然存在少量尺寸较大的块状Mg17Al12相,在室温拉伸过程中,块状Mg17Al12相附近容易造成应力集中,成为微裂纹萌生的根源,将会阻碍力学性能的进一步提高。在挤压之前对Mg-9Al-1Si-1SiC复合材料进行固溶处理来消除Mg17Al12相,再对固溶态Mg-9Al-1Si-1SiC复合材料进行不同道次ECAP变形发现:不同道次变形后析出的Mg17Al12均为细小的颗粒状,2道次变形后析出的Mg17Al12颗粒的数量最多;随着变形道次增加,Mg_2Si相逐渐碎化且分布更加均匀;基体的平均晶粒尺寸在2道次变形后最小。复合材料的抗拉强度和伸长率随着变形道次增加而逐渐升高,4道次变形后分别为296MPa和8.8%;而屈服强度在2道次变形后最高,4道次变形后有所降低。在473K/70MPa蠕变条件下,铸态Mg-9Al-1Si-1SiC复合材料抗蠕变性能高于铸态Mg-9Al-1Si复合材料;而对于挤压态Mg-9Al-1Si-1SiC复合材料,其抗蠕变性能明显低于铸态复合材料,并且晶粒尺寸越小,稳态蠕变速率越高,抗蠕变性能越差。在(448～498K)/(70～90MPa)的蠕变条件下,铸态Mg-9Al-1Si-1SiC复合材料的应力指数为5.51～6.89,蠕变激活能为86～111kJ/mol,蠕变机制为受扩散控制的位错攀移机制和第二相颗粒增强机制的共同作用。
[Abstract]:High-temperature stable phase Mg_2Si reinforced Mg-Al-Si composites are the first creep resistant magnesium matrix composites designed for automotive power systems. The network Mg17Al12 phase and the coarse Chinese character Mg_2Si phase in the as-cast Mg-Al-Si composites will seriously split the matrix, which leads to a significant reduction in mechanical properties, which limits its wide application. It is important to improve the size and morphology of the second phase for improving the properties of Mg-Al-Si composites. In this paper, semi-solid stirring is used to improve the properties of Mg-Al-Si composites. Mg-9Al-1Si with the content of 1wt.% SiC nanoparticles was prepared by ultrasonic dispersion technique. It is described as Mg-9Al-1Si-1Si-1SiC) composite material. The composite materials were deformed by ECAP. The methods of OMSEMXRDX DS and TEM were used. The effect of nanometer SiC particles on the microstructure of Mg-9Al-1Si composites was analyzed. And the change of microstructure of solid solution Mg-9Al-1Si-1SiC composites after different pass deformation; The mechanical properties of the composites at room temperature and creep properties at high temperature were systematically studied. The results showed that after adding 1wt.% SiC particles into the as-cast Mg-9Al-1Si composites. The matrix grains of Mg17A112 and Mg_2Si phase were refined obviously, but the Mg17A112 phase was still distributed in the form of network, and the phase of mg _ 2Si was still in the shape of Chinese characters. The distribution of SiC nanoparticles in the matrix is more uniform. The as-cast Mg-9Al-1Si and Mg-9Al-1Si-1SiC composites were directly ECAP for 4 times. Deformation. The matrix grains were significantly refined and the Mg17Al12 and Mg_2Si phases were obviously broken, compared with the extruded Mg-9Al-1Si composites. The matrix grains of extruded Mg-9Al-1Si-1SiC composites are smaller and the particles of Mg17Al12 and Mg_2Si are smaller and more evenly distributed. In addition, after the deformation of ECAP, the distribution of SiC nanoparticles was improved, which resulted in higher mechanical properties. After direct ECAP deformation of the as-cast Mg-9Al-1Si-1SiC composites, a small number of larger bulk Mg17Al12 phases still exist in the microstructure, and during the tensile process at room temperature. It is easy to cause stress concentration near the block Mg17Al12 phase, which is the root of microcrack initiation. It will hinder the further improvement of mechanical properties. The Mg-9Al-1Si-1SiC composites were treated with solid solution before extrusion to eliminate the Mg17Al12 phase. The results of ECAP deformation showed that the Mg17Al12 precipitated after the different passes were fine granular. The number of Mg17Al12 particles precipitated after the second pass deformation is the highest; With the increase of deformation pass, the Mg2Si phase is gradually broken and distributed more evenly. The average grain size of the matrix is the smallest after two times of deformation. The tensile strength and elongation of the composites increase gradually with the increase of the number of the deformation passes. After 4 times of deformation, the tensile strength and elongation of the composites are 296MPa and 8.8, respectively. However, the yield strength decreases after the maximum of 4 passes after two passes of deformation, and the creep condition of 473K / 70 MPA is the same as that under the condition of 473K / 70MPa. The creep resistance of as-cast Mg-9Al-1Si-1SiC composites is higher than that of as-cast Mg-9Al-1Si composites. However, the creep resistance of extruded Mg-9Al-1Si-1SiC composites is obviously lower than that of as-cast composites, and the smaller the grain size, the higher the steady creep rate. The worse the creep resistance is, the worse the creep property is under the creep condition of 448 / 498KT / 70 / 90MPa. The stress index of as-cast Mg-9Al-1Si-1SiC composites is 5.51kJ / mol and the creep activation energy is 86111kJ / mol. The creep mechanism is the joint action of dislocation climbing mechanism controlled by diffusion and the second phase particle enhancement mechanism.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB333

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