【作者】 张静；

【导师】 于化顺； 姜石峰；

【作者基本信息】 山东大学 ， 材料加工工程， 2011， 博士

【摘要】 Al-Si系合金是铸造铝合金中品种最多,用量最大的合金,广泛应用于航空、汽车、仪表及机械等工业领域。但水平连续铸造Al-Si合金铸锭存在Si相粗大、组织不均匀和缩孔等缺陷,严重损害了材料的力学性能。本文从金属组织遗传性原理出发,使用同种成分的细晶组织材料(Fine-grained Structural Materials,简称FSM)作为中间合金来优化水平连铸Al-Si合金组织,得到了组织均匀、细化、无缩孔缺陷,力学性能良好的合金铸锭。研究讨论了FSM中间合金含量和工艺参数对合金组织和力学性能的影响,并利用电子背散射衍射(EBSD)、扫描电镜(SEM)、透射电镜(TEM)、高分辨电镜(HRTEM)、差式扫描热分析(DSC)等手段探讨了FSM中间合金对共晶Si相的变质机理。通过对Al-12%Si和Al-18%Si合金进行DSC分析确定了水平连铸Al-12%Si和Al-18%Si合金FSM中间合金的加入温度分别为720℃和740℃,并确定了制备FSM Al-12%Si和Al-18%Si中间合金的工艺参数,即将Al-12%Si和Al-18%Si合金熔体分别过热到900℃和950℃后水冷铜模铸造加二次水冷,得到了初晶Si细化、共晶Si高度分枝的凝固组织。加入同种成分FSM中间合金制备的水平连铸Al-12%Si和Al-18%Si合金铸锭铸锭横截面上缩孔面积减少,显微组织细化,组织均匀性得到改善,力学性能也有所提高。其中初晶a-A1二次枝晶臂间距减小,共晶Si由粗大的针片状转变为高度分枝的纤维状,初晶Si尺寸减小。DSC分析可知中间合金的加入可以明显改善合金凝固过程对冷却速度的敏感性,铸锭合金表面和中心凝固曲线上过冷度差异明显减小。但是,中间合金存在最佳加入量,在结晶器一次冷却条件下,Al-12%Si和Al-18%Si的最佳加入量为30%,二次水冷却条件下Al-18%Si最佳加入量为15%。对共晶Si变质前后的生长机理进行了分析,结果表明未变质共晶Si深腐蚀后表面和侧面都呈明显的板片状特征,生长端面以凸角特征为主,可以观察到少量的凹角端面。并且生长端面能够观察到比较规则的生长台阶。大量TEM观察并未发现Si晶体内部有典型的孪晶缺陷,因此未变质共晶Si中孪晶凹角生长机制(TPRE)并不是共晶Si生长的主要机制,而是台阶生长机制导致Si的板片状形貌。Si中的位错分解产生两个不完位错,并且位错中间夹着一定宽度的层错便可以成为Si生长有效的台阶源。EBSD分析较大范围显示了Al、Si两相的共晶结构。FSM中间合金变质后,共晶Si形貌由针片状转变为纤维状,相邻两相间由大角度晶界转变为小角度亚晶界为主。共晶Si形核的位置由初晶a-Al枝晶转变为熔体中弥散分布的Si原子集团,共晶团中Si相与Al相显示出更为紧密的耦合生长关系,{110}Si和{100}Al的择优取向关系变得明显。FSM中间合金不仅使共晶Si形貌发生了变化,也改变了共晶Si内在晶体缺陷的性质及分布。共晶Si内部出现两套斜交的孪晶系,密度和孪晶间距存在明显差异。其中一套厚孪晶密度较大,并且一直延伸到生长末端。而另一套则为微孪晶,或堆垛层错。Si枝晶的结合处可以明显观察到堆垛层错的存在,并且与Si分枝的方向垂直。即共晶Si中高密度层错决定了Si的高密度分枝,Si分枝则以另一套厚孪晶TPRE机制继续长大。对纤维状共晶Si进行HRTEM分析发现FSM中间合金变质后的Si中不仅出现了大量的孪晶和层错,还弥散分布着许多黑色小颗粒,尺寸在10到20nm之间。对纳米颗粒进行FFT变换,发现纳米颗粒面间距约为正常Si相的3倍。从能量和晶体学角度分析,确定纳米颗粒为加入的中间合金未完全熔化形成的Si-Si原子集团。通过对共晶Si形貌和内部缺陷分析确定FSM中间合金变质共晶Si的生长机理为组织细化的Al-12%Si合金加入熔体后吸收热量熔化,由于Si-Si键能较大,因此形成大量的Si-Si原子集团弥散分布在熔体中。从金属组织遗传角度来看,这些原子集团就是Al-Si合金的遗传因子,它们保留了中间合金的细晶组织特征,使得最终得到组织细化的凝固组织。从形核和生长理论分析,这些原子集团中晶格畸变最小的,与Si晶体结构最为接近的原子集团可以成为Si形核时的最佳衬底。大量过剩的原子团簇富集在固液界面前沿,阻碍了台阶的部分运动。并且由于结构有所差异,因此Si原子面在堆垛过程中需要通过层错、孪晶等缺陷来协调与原子集团的取向差异。而层错、孪晶等缺陷的产生使得共晶Si不断调整结晶位向,从而形成三维空间上高密度分枝的珊瑚状形态。更多还原

【Abstract】 Al-Si alloys are the most species and the largest amount of casting aluminium alloys, which are widely used in aerospace, automotive, instrumentation, and machinery industries. However, casting defects such as coarse Si phase, macrosegregation and porosity are usually present in the horizontal continuous casting (HCC) Al-Si microstructure. It is detrimental to the mechanical properties of the alloys. This paper use fine-grained structural materials (FSM for short) of the same composition as the master alloy to optimize the microstructure of HCC Al-Si alloys because of the structural heredity. Alloys with fine and uniform microstructure and good mechanical properties have been obtained in present work. The effect of FSM master alloy amount and parameters on microstructure and mechanical properties were studied systematically. The mechanism of modification of FSM mater alloy was also discussed via EBSD, SEM, TEM, HRTEM and DSC.The addition temperatures of Al-12%Si and Al-18%Si FSM master alloy were determined by DSC analysis, which are 720 and 740℃, respectively. FSM Al-12%Si and Al-18%Si master alloys were prepared by thermal speed treatment. The overheating temperatures were 900 and 950℃according to the DSC curves. Water-cooled copper mold and jet water were used to get rapid cooling rate. The microstructure of FSM master alloy consisted of fine primary silicon and fibrous eutectic silicon.HCC Al-12%Si and Al-18%Si billets have fine and uniform microstructure with less center-line shrinkage and better mechanical properties after FSM master alloy addition. The secondary dendrite arm spacing of a-Al and size of primary Si decreased, the morphology of eutectic Si changed from neeld-like shape to fibrous shape. The addition of FSM master alloy made the alloys less sensitive to cooling rate by DSC analysis. However, the optimum of addition amount is 30% for Al-12%Si and Al-18%Si under first cooling condition and 15% for Al-18%Si under second cooling condition. The surface and side face of unmodified eutectic silicon show a flake-like feature after deep etching. The tips of silicon are mainly salient with regular growth steps. Few re-entrant features have been found. There are no obvious SAD patterns for twin spots during TEM observation. It means TPRE is not the dominating mechanism for flake Si, whereas growth steps induced by defects are responsible for the eutectic Si. Stacking fault between two partial dislocations disassemble from a perfect dislocation can be the available growth steps.EBSD may show the crystallographic orientation of the Al-Si eutectic in a large range of sight field. The boundry between two adjacent phases changes from high-angle grain boundry to low-angle grain boundry after modified by FSM master alloy. The nucleation position of eutectic silicon is the primary a-Al in unmodified alloy, while the dispersed Si clusters can be the nuclei for eutectic Si in modified Al-Si alloy. Moreover, Si and Al phase in eutectic grain show more closely coupled growth combination. It shows visible preferred orientation between {110} Si and {100}Al.FSM master alloy changes not only the morphology of the Si phase but also the nature and distribution of the defects inside Si crystals. It usually contains two oblique crossing sets of twins. However, the density and twin distance are of big difference. One set is thick twin, which extend along the growth direction to the growing tip. The other set is density stacking fault perpendicular to the growth direction of the branch. In other words, density stacking fault dominate the branching and the thick twin is responsible for the growth of Si branch by TPRE mechanism.Observed using HRTEM at the fibrous Si, there are not only defects but also dark paticles with the size of from 10 to 20 nm. It is revealed by Fourier transform analysis that the lattice within the nanoclusters is modulated with a period of three times the plane spacing in the regular Si lattice. It is argured that the lattice-modulatied characteristics should be Si-Si cluster resulted from unmelt FSM master alloy.Above the analysis of morphology and defects of eutectic Si, the modification mechanism of FSM Al-Si alloy can be concluded as the unmelt Si-Si nanoclusters after added into HCC melt. From the point of view of structural heredity, these clusters are the genetic factors which reserve the fine-grained microstructural features resulting in the fine solidification structure. From the nucleation and growth mechanism side, the lattice-modulatied Si clusters which have the closest structure with Si can be act as the best nuclei of eutectic Si. The other excess Si clusters enrich on the front of solid-liquid interface which hinder the step growth. Stacking faults and twins are emerged during the Si atoms stacking on the growth plane to adjust the the orientation difference. These defects cause the change of Si crystal orientation, which result in the fibrous morphology of modified eutectic Si.更多还原