【作者】 王晓军；

【导师】 吴昆；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2008， 博士

【摘要】 本文探索了SiCp/AZ91镁基复合材料的搅拌铸造工艺,并对复合材料开展了高温压缩和热挤压变形行为研究。采用光学显微镜、扫描电镜和透射电镜等方法,研究了铸态复合材料的显微组织和界面结构、高温压缩和热挤压变形过程中颗粒和基体显微组织的演变规律,分析了复合材料在高温压缩变形的变形机制,采用原位拉伸技术研究了铸态复合材料的断裂机制,并采用中子衍射技术研究了复合材料挤压织构的演变规律,并对铸态和挤压态的复合材料的力学性能开展了研究。镁基复合材料不宜采用液态搅拌铸造工艺,但是特别适合半固态搅拌铸造工艺;采用半固态涡流搅拌铸造工艺可以制备尺寸细小的颗粒增强镁基复合材料,且所制备的复合材料颗粒分布均匀和空隙率较小。显微组织研究表明SiCp在基体晶界附近偏聚,呈“项链状”颗粒分布,这是搅拌铸造复合材料一种典型特征。SiCp在AZ91合金熔体中稳定性较好,没有发生界面反应。随着颗粒尺寸的减小和体积分数的升高,铸态复合材料的屈服强度明显升高。复合材料的弹性模量随着体积分数升高而升高,但是随颗粒尺寸的变化不明显。动态SEM原位拉伸试验结果表明:“项链状”颗粒分布导致微裂纹主要以界面脱粘机制在颗粒偏聚区内形成,且裂纹扩展倾向于经过颗粒偏聚区域;“项链状”颗粒分布及其所致的弱界面是搅拌铸造复合材料的断裂强度不高的主要原因,必须通过热变形消除这种颗粒分布和改善界面结合。采用带有门槛应力修正的幂指数方程计算得到复合材料的高温压缩变形真激活能为91 kJ/mol,应力因子n=5,表明复合材料的高温压缩变形的控制机制为晶界位错攀移机制。本文压缩试验数据和W.D. Nix等人建立的晶界位错攀移蠕变模型(slip band模型)吻合较好;通过显微组织观察证实SiCp及其“项链状”分布使得本文复合材料的高温压缩变形条件更加符合slip band模型,并建立了适合本文复合材料的变形条件slip band模型。压缩温度和应变速率对复合材料的基体的动态再结晶(DRX)、位错和孪晶有重要影响。室温压缩表明在SiCp附近位错塞积严重,所形成的颗粒变形区是DRX的优先形核部位;高温压缩变形时,DRX首先在有颗粒偏聚的铸态晶粒的晶界区域发生,逐渐向原始晶粒内部区域延伸;“项链状”颗粒分布导致复合材料的DRX机制为“项链状”再结晶机制。热挤压能够消除铸态复合材料的颗粒偏聚和改善颗粒分布;挤压温度越高和挤压比越大,越有利于改善颗粒分布。挤压过程中SiCp可能发生断裂,挤压温度越低,挤压比越大,颗粒尺寸越大,颗粒越容易发生断裂。而且颗粒断裂对局部颗粒含量具有敏感性。SiCp能够促进DRX形核,降低基体的DRX温度。挤压过程中,SiCp对晶粒长大存在两种作用:首先,SiCp能够促进DRX晶粒的长大;但是当晶粒长大到与颗粒相接触时颗粒又能够阻碍DRX晶粒的长大。挤压态复合材料中基体的织构为(1010)纤维织构。挤压温度越高和挤压比越大,复合材料中基体的(1010)纤维织构越强。SiCp的加入没有改变基体的主要织构组分,但是对基体织构的强度有两种影响:当体积分数为5%,复合材料的织构强度高于单一基体合金的织构;当体积分数大于10%时,复合材料的基体织构变得越来越弱。复合材料中基体织构的强度不随颗粒尺寸单调变化,而是在10μm时出现一个织构强度的峰值。热挤压显著提高了复合材料的力学性能。在250-350℃温度区间内,复合材料的屈服强度和断裂强度都随着挤压温度的升高而升高;350R5挤压的复合材料的力学性能比350R12挤压的低,这与合金的力学性能随挤压温度和挤压比的变化规律相反。挤压过程中基体的显微组织和织构的演化不是挤压态复合材料力学能的决定性因素,SiCp在挤压过程中的演变(颗粒分布和颗粒断裂)才是主导因素。随着颗粒体积分数的升高和颗粒尺寸的减小,挤压态复合材料的屈服强度和断裂强度都明显升高。更多还原

【Abstract】 The stir casting processing, hot compression and extrusion behaviors of SiCp/AZ91 magnesium matrix composites were studied in this paper. Optical microscope, scanning electronic microscope (SEM) and transmission electronic microscope (TEM) were employed to observe microstructure and interface structure of as-cast composites and to investigate the microstructure evolution of particle and matrix during hot compression and extrusion. The deformation mechanism for hot compression was analyzed and discussed. The fracture mechanism of as-cast composites was studied using in-situ tensile technique. The evolution of extrusion textures was investigated using neutron diffraction. And the mechanical properties of as-cast and as-extruded composites were studied.Stir casting in liquid condition is not suitable for fabrication of magnesium matrix composites, but compocasting method is very fit. Compocasting with vortex formation is able to fabricate fine particle reinforced magnesium matrix composites with uniform particle distribution and low porosity. Most SiCp are segregated at a microscopic scale near grain boundary regions, which is typical“necklace-type”particle distribution for metal matrix composite fabricated by stir casting. SiCp are very stable in the AZ91 melt, and chemical reactions don’t take place at the interfaces. The yield strength (YS) of as-cast composites increases with decreasing particle size and increasing volume fraction. The elastic modulus of composites increases as volume fraction increases, but particle size doesn’t have significant effect on elastic modulus. The investigations using in-situ SEM tensile technique reveal that“necklace-type”particle distribution results in that the dominant microcrack nucleation mode is interface decohesion in particle segregation regions, and that microcrack propagation tends to pass through particle segregation regions. The“necklace-type”particle distribution, which results in the weak interface between SiCp and matrix, is the main reason for the low ultimate tensile strength (UTS) of as-cast composite fabricated by stir casting, so it is necessary to employ hot deformation to improve particle distribution and interface bonding.According to the power law equation modified by threshold stress, the true activation energy for hot compression deformation of composite is calculated to be 95 kJ/mol, and stress exponent n=5, which indicates that the controlled deformation mechanism is dislocation climb controlled by grain boundary diffusion. Our compression experiment data agree with the slip band model established by W.D. Nix et. al, which is a model for creep based on the climb of dislocation at grain boundaries. Microstructure investigations have demonstrated that SiCp and their“necklace-type”distribution make the composite under study more suitable for the slip band model. A modified slip band model, which is based on the deformation condition of the composite under study, has been established. Compression temperature and strain rate have significant influence on dynamic recrystallization (DRX), dislocation and twins in the matrix. The results of room temperature compression experiments show that dislocations pile up in the matrix near particles, and particle deformation zones (PDZ) which are preferential site for DRX nucleation are formed. During hot compression, DRX takes place first in the regions near particle segregation at the original grain boundaries, and then extends to the interior of original grains. The“necklace-type”particle distribution leads to the“necklace”DRX mechanism of the composites.Hot extrusion can eliminate particle segregation in as-cast composites and improve particle distribution. Higher extrusion temperatures and larger extrusion ratios are favorable for improving particle distribution. SiCp can be cracked during extrusion. Particles are easy to be cracked when low extrusion temperature and large extrusion ratios are employed and the particle size is large. What’s more, particle cracking is sensitive to the local particle content. SiCp can stimulate DRX nucleation and lower DRX temperature of matrix. SiCp influence the growth of DRX grains during extrusion in two ways: SiCp promote the growth of DRX grains, whereas SiCp hinder grain growth when DRX grains grow to the point to get in touch with the surfaces of SiCp.As-extruded alloys and composites exhibit (1010) fiber texture. When SiCp is introduced into a Mg alloy, the main component of texture is not modified, but the intensity of texture evolves with the SiCp content with two ways: in the composite with 5% SiCp the intensity of Mg is higher than in the non-reinforced alloy, whereas for an SiCp volume fraction higher than 10% becomes more and more isotropic. The intensity of Mg texture doesn’t vary monotonically with particle size, and there is a peak of intensity when particle size is 10μm.Hot extrusion significantly improves the mechanical properties of composites. In temperature range from 250 to 350℃, the YS and UTS of composites increase with increasing extrusion temperature. The mechanical properties of 350R5 composites are lower than that of 350R12 composites. The variation of mechanical properties of AZ91 alloy with extrusion temperature and ratio is contrary to that of composites. The evolution of microstructure and texture in matrix during extrusion is not the leading factor of the mechanical properties of as-extruded composite, and the leading factor is the evolution of SiCp during extrusion. Both YS and UTS increase evidently with increasing SiCp volume fraction and decreasing particle size.更多还原