【摘要】：枝晶組織作為凝固過程中最常見的微觀組織,對產(chǎn)品的最終性能有著重要的影響。目前,對具有密排六方晶體結構(hcp)的鎂合金的三維枝晶形貌、取向選擇等仍不清楚,因此,對鎂合金三維枝晶組織進行研究意義重大。本文的主要目的是探究鎂合金枝晶組織的生長。論文采用同步輻射X射線斷層掃描技術對 Mg-Sn、Mg-Gd、Mg-Al、Mg-Ca、Mg-Ba、Mg-Y 以及 Mg-Zn 合金中α-Mg枝晶組織進行三維表征,利用EBSD技術標定枝晶的優(yōu)先生長方向,最后采用三維相場方法模擬鎂合金的枝晶形貌及其演變過程。通過以上實驗和模擬研究得到三個方面結果。對于絕大多數(shù)鎂合金而言,α-Mg都呈現(xiàn)十八分支的枝晶形貌,其中六個分支在基面上沿著1120方向生長,另外十二個分支分居于基面上下兩側沿著1123方向生長。其中,1120主干可能伴有4個或6個二次晶臂,1123主干可能有3個或5個二次晶臂。雖然都具有十八個分支結構,含不同固溶元素的鎂基合金枝晶形貌仍存在一定的差異。另外,本文還運用三種相關理論對1120和1123兩個優(yōu)先方向存在的可能性進行了解釋。在Mg-Zn合金中,枝晶的優(yōu)先生長方向會發(fā)生轉變。當Zn含量小于等于20wt.%時,α-Mg(Zn)呈現(xiàn)十八分支形貌,沿著1120和1123方向生長;當Zn含量大于等于45wt.%時,α-Mg(Zn)呈現(xiàn)十二分支形貌,沿著1123方向生長,并伴有3個二次晶臂;當Zn含量在20～45wt.%之間時,α-Mg(Zn)枝晶生長并不穩(wěn)定,有呈藻狀生長的趨勢,主干方向為1121,并有3個1121和2個1012的二次晶臂。本文還研究發(fā)現(xiàn)α-Zn枝晶組織呈現(xiàn)偽十二加二分支的生長形態(tài),這種獨特的枝晶形貌也為Mg-Zn合金枝晶生長的取向轉變行為提供了支撐�；谌S表征所得到的鎂合金枝晶形貌,本文建立了鎂合金枝晶的各向異性生長模型,模型中包括ε1、ε2和ε3三個各向異性系數(shù),其中ε1主要改變的是枝晶在c軸的生長,ε2控制枝晶的整體生長傾向,ε3影響枝晶在基面1120方向上的生長。模擬結果能很好地展現(xiàn)α-Mg枝晶十八分支、十二分支形貌以及取向轉變行為,并與實驗結果吻合良好。
[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 偽-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, 偽-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.%, 偽-Mg (Zn) exhibits 18 branching morphology and grows in the direction of 1120 and 1123. When the content of Zn is greater than 45wt.%, 偽-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 偽-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 偽-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 偽-Mg dendrite has 18 branches, 12 branching morphology and orientation transition behavior, which is in good agreement with the experimental results.
【學位授予單位】：清華大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：TG146.22

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