【作者】 申乐；

【导师】 郝远； 陈体军；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2011， 硕士

【摘要】 Si是Mg-Al系和Mg-Zn系镁合金常用的合金化元素之一,Mg-Al-Si系合金是历史最悠久的耐热镁合金之一。Si在Mg中几乎不固溶,其固溶度为0.0003%。但是Si与Mg能原位反应生成Mg₂Si相,Mg₂Si具有熔点高、密度低、硬度高和热膨胀系数低等优点,是镁合金较理想的增强相。同时,采用原位反应获得自生增强相的自生复合材料,由于增强相在基体内原位反应生成,具有界面洁净无污染,热稳定性好,与基体相容性好,制造成本低等优点,是金属基复合材料的一个重要发展方向。但是采用原位自生法获得的Mg₂Si相呈粗大汉字状,极大的割裂了基体组织,严重降低了合金的室温性能。本文以Mg₂Si/AM60B镁基复合材料为研究对象,研究了MgCO₃、Sr对的Mg₂Si/AM60B镁基复合材料的复合变质工艺,并研究了对其显微组织及力学性能的影响。同时,对复合变质后的复合材料进行了部分重熔实验,对部分重熔过程中的组织演变进行了分析。研究结果表明:在AM60B镁合金中加入Al-Si中间合金可获得原位自生Mg₂Si增强相的镁基复合材料。随着Si含量的增加,Mg₂Si数量增多,其树枝晶形貌也越发达。在790℃加入0.5%Sr后保温30min对Si含量为2.0%的Mg₂Si/AM60B镁基复合材料中的Mg₂Si相具有较好的变质效果,Mg₂Si相形貌由粗大的树枝状或汉字状转为细小的多角形块状。在Sr变质的基础上,再经1.2%的MgCO₃在790℃保温10min进行复合变质处理,使基体晶粒的的平均尺寸由260μm减小到80μm左右,复合变质效果较好。经Sr和MgCO₃复合变质后的Mg₂Si/AM60B镁基复合材料在经过半固态等温热处理后可获得适合触变成形所需的半固态锭料。在复合材料的部分重熔过程中,Mg₂Si相的变化过程较为简单。由带尖角的多边形状变颗粒为边界较圆整光滑的形状,最后大部分变为类球状;而复合材料的基体组织演变过程则可分为以下几个阶段:枝晶间共晶组织的溶解而导致的初期快速粗化阶段、剩余共晶组织的熔化而引起的组织分离阶段、初生固相颗粒部分熔化所引起的球状化阶段以及最后因合并和Ostwald熟化而引起的粗化阶段。此外,复合材料的铸态基体组织的细化效果越好,则其半固态基体组织就越细小圆整,半固态组织的质量也就越高。因此要想获得高质量的半固态组织,对基体进行尽可能好的细化是很有必要的。更多还原

【Abstract】 Si is one of the commonly used alloying elements in the Mg-Al and Mg-Zn based alloys, and Mg-Al-Si alloy is the oldest one of heat-resistant magnesium alloy. The solubility of Si in Mg is only 0.0003% and Si and Mg can generate Mg₂Si phase by in-situ reaction. Mg₂Si has high melting point, low density, high hardness and low coefficient of thermal expansion, and is an ideal reinforcement for magnesium alloys, Simultaneiously, in-situ composites with reinforcing phase generated from reaction have advantages of clean interface, high thermal stability, good compatibility with matrix, low manufacturing cost, is an important development direction of metal matrix composites. But Mg₂Si phase with Chinese characters greatly lacerate the matrix, which reduce the performance of the alloy at room temperature.In this thesis, the modification techqnique of Mg₂Si/AM60B magnesium matrix composite was studied by MgCO₃ and Sr. The effects of the modification on microstructure and mechanical properties was also studied. At the same time, the microstructural evolution of the modified composite was investigated.The results show that the in-situ Mg₂Si/AM60B composite can be produced by adding Al-Si master alloy into AM60B alloy. The number of Mg₂Si increases as the addition amount increases and the dendritic morphology become more developed. The addition of 0.5% Si at 790℃and subsequent hold for 30 min has good modification effect. The morphology of Mg₂Si phase changes from developed dendritic shape or Chinese characters to small polygonal particles. Then treated by 1.2%MgCO₃ at 790℃and held for 10min, the average grain size of matrix is decreased from 260μm to 80μm. Namely, the used complex modification technique has good effect.A semisolid microstructure avaible for thixoforming can be obtained after the modified Mg₂Si/AM60B being partially remelted. During partial melting, the microstructure evolution of the Mg₂Si particles is simple. They evolve from the original polygonal particle to spheroidal particles and at the same time they size also increases. The microstructure evolution of the matrix can be divided into four steps, the initial coarsening due to the dissolution of interdendritic eutectic, structure separation resulted from the melting of the residual eutectic and the penetration of the firstly formed liquid into the recrystallized boundaries, spheroidization due to the partial melting of primary particles and final coarsening attributed to the merging and Ostwald ripening. In addition The more finer the as-cast microstructure is,the more better and rounder the primary particles in the semi-solid microstructure is. Consequently, it is necessary that the as-cast microstructure must be refined to obtain the high-quality semi-solid microstructure.更多还原