镁合金枝晶组织三维表征与相场模拟研究

发布时间：2018-12-21 17:09

【摘要】：枝晶组织作为凝固过程中最常见的微观组织,对产品的最终性能有着重要的影响。目前,对具有密排六方晶体结构(hcp)的镁合金的三维枝晶形貌、取向选择等仍不清楚,因此,对镁合金三维枝晶组织进行研究意义重大。本文的主要目的是探究镁合金枝晶组织的生长。论文采用同步辐射X射线断层扫描技术对 Mg-Sn、Mg-Gd、Mg-Al、Mg-Ca、Mg-Ba、Mg-Y 以及 Mg-Zn 合金中α-Mg枝晶组织进行三维表征,利用EBSD技术标定枝晶的优先生长方向,最后采用三维相场方法模拟镁合金的枝晶形貌及其演变过程。通过以上实验和模拟研究得到三个方面结果。对于绝大多数镁合金而言,α-Mg都呈现十八分支的枝晶形貌,其中六个分支在基面上沿着1120方向生长,另外十二个分支分居于基面上下两侧沿着1123方向生长。其中,1120主干可能伴有4个或6个二次晶臂,1123主干可能有3个或5个二次晶臂。虽然都具有十八个分支结构,含不同固溶元素的镁基合金枝晶形貌仍存在一定的差异。另外,本文还运用三种相关理论对1120和1123两个优先方向存在的可能性进行了解释。在Mg-Zn合金中,枝晶的优先生长方向会发生转变。当Zn含量小于等于20wt.%时,α-Mg(Zn)呈现十八分支形貌,沿着1120和1123方向生长;当Zn含量大于等于45wt.%时,α-Mg(Zn)呈现十二分支形貌,沿着1123方向生长,并伴有3个二次晶臂;当Zn含量在20～45wt.%之间时,α-Mg(Zn)枝晶生长并不稳定,有呈藻状生长的趋势,主干方向为1121,并有3个1121和2个1012的二次晶臂。本文还研究发现α-Zn枝晶组织呈现伪十二加二分支的生长形态,这种独特的枝晶形貌也为Mg-Zn合金枝晶生长的取向转变行为提供了支撑。基于三维表征所得到的镁合金枝晶形貌,本文建立了镁合金枝晶的各向异性生长模型,模型中包括ε1、ε2和ε3三个各向异性系数,其中ε1主要改变的是枝晶在c轴的生长,ε2控制枝晶的整体生长倾向,ε3影响枝晶在基面1120方向上的生长。模拟结果能很好地展现α-Mg枝晶十八分支、十二分支形貌以及取向转变行为,并与实验结果吻合良好。
[Abstract]:As the most common microstructure in solidification process, dendritic structure has an important effect on the final properties of the product. At present, the 3D dendritic morphology and orientation selection of magnesium alloy with dense hexagonal crystal structure (hcp) are still unclear, so it is of great significance to study the three-dimensional dendritic structure of magnesium alloy. The main purpose of this paper is to study the growth of dendritic structure of magnesium alloy. The dendrite structure of 伪-Mg in Mg-Sn,Mg-Gd,Mg-Al,Mg-Ca,Mg-Ba,Mg-Y and Mg-Zn alloy was characterized by synchrotron radiation X-ray tomography. The preferential growth direction of dendrite was calibrated by EBSD technique. Finally, the dendritic morphology and evolution process of magnesium alloy were simulated by three-dimensional phase field method. Through the above experimental and simulation studies, three aspects of the results are obtained. For most magnesium alloys, 伪-Mg exhibits dendritic morphology of 18 branches, in which six branches grow along the 1120 direction on the base plane and the other 12 branches grow along the 1123 direction on the upper and lower sides of the base plane. Among them, 1120 trunk may be accompanied by 4 or 6 secondary crystal arms, and 1123 trunk may have 3 or 5 secondary crystal arms. Although all of them have 18 branching structures, there are still some differences in dendritic morphology of magnesium based alloys with different solid solution elements. In addition, three theories are used to explain the possibility of the existence of 1120 and 1123 priority directions. In Mg-Zn alloy, the preferred growth direction of dendrite changes. When the content of Zn is less than 20 wt.%, 伪-Mg (Zn) exhibits 18 branching morphology and grows in the direction of 1120 and 1123. When the content of Zn is greater than 45wt.%, 伪-Mg (Zn) exhibits 12 branching morphology and grows along the direction of 1123, with three secondary crystal arms. When the content of Zn is between 20 and 45 wt.%, the dendritic growth of 伪-Mg (Zn) is unstable, with a trend of algal growth, with a main trunk direction of 1121, and three secondary arms of 1121 and 1012. It is also found that the dendritic structure of 伪-Zn exhibits the growth morphology of pseudo-dodecanax. This unique dendritic morphology also supports the orientation transformation behavior of dendritic growth of Mg-Zn alloy. Based on the dendritic morphology obtained by 3D characterization, the anisotropic growth model of dendrite is established in this paper. The anisotropy coefficients of 蔚 1, 蔚 2 and 蔚 3 are included in the model. The main change of 蔚 1 is the growth of dendrite on the c axis. 蔚 2 controls the overall growth tendency of dendrites, and 蔚 3 affects the growth of dendrites in the 1120 direction of the base plane. The simulation results show that the 伪-Mg dendrite has 18 branches, 12 branching morphology and orientation transition behavior, which is in good agreement with the experimental results.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.22

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