稀土变质过共晶铝硅合金中纳米相与硅相界面行为、组织调控及硅相形核机制研究

Primary Si crystals reveal several morphologies, such as star-like(five-folded), polygonal and plate-like shape. Moreover, the morphology of eutectic Si is also complex, including coarse acicular and flake shape during conventional casting processes. The morphology of primary and eutectic seriously limits the further application in automobile engine parts. Therefore, the project is proposed to produce hypereutectic Al-Si alloys with fine coral-like fibrous eutectic Si and fine passivated primary Si of the edges and angles. The project mainly focus on the fundament of scientific issues about the interface behaviors and nucleation mechanism of Si phases. The interface behaviors of nanometer precipitation phase and Si phases, microstructure adjustment and mechanisms of nucleation and growth will be researched. The morphology evolution of Si phases before and after rare earth modification will be analyzed. From the perspective of morphology and distribution of the precipitation nanoscale rare earth phase, the interface relationship between the rare earth and Si phases will be primarily investigated, and the correlation function of heterogeneous nucleation of Si phases and rare earth phase will be analyzed. Subsequently, the effect of nanometer precipitation phase on the mechanisms of heterogeneous nucleation and growth will be further studied. Furthermore, the evolution of Si morphology will be analyzed. Eventually, the precipitation kinetics of nanoscale rare earth phase, the relationship between precipitated phase and nucleation of Si phase will be revealed. The study results will contribute to improving the modification mechanism and microstructure regulation mechanism of hypereutectic Al-Si alloy. Research on the project has important scientific significance and application value.

常规铸造过共晶Al-Si合金在凝固过程中易形成粗大针片状共晶硅和形态复杂的初生硅及硅相的基体上的局部偏聚，制约了其在汽车发动机部件上的应用。因此，本项目提出稀土变质Al-Si合金,制备具有珊瑚状纤维共晶硅和细小初生硅的Al-Si合金，围绕纳米稀土相的析出与硅相界面行为、组织调控及硅相形核机制的基础科学问题开展研究。探究稀土元素在熔体中的存在形式、凝固过程中纳米稀土相的析出行为、硅相与稀土相的界面关系；对比分析稀土变质前后合金中硅相形貌的演变过程，从析出稀土相的形态及分布角度重点研究稀土相与硅相界面之间的相关性，分析硅相的异质形核与稀土相的关联作用，近一步探讨稀土/纳米稀土相对硅相异质形核作用和生长方式的影响，分析硅相形态的演变规律；揭示稀土相的析出动力学及硅相形核和生长机制。项目的研究有助于完善Al-Si合金的变质机制和组织调控机制，具有一定的科学意义和应用价值。

过共晶Al-Si合金因具有良好的流动性、低密度、低的热膨胀系数和高的比强度，被作为传统铸铁的理想替代材料，用来生产汽车发动机活塞、缸体缸盖等零部件，以减轻自重，降低油耗，实现节能环保的目的。然而，传统铸造的Al-Si合金组织中，存在着粗大的α-Al枝晶和粗大不规则的Si相，且在Si相尖端容易形成应力集中，降低了合金的力学性能，从而制约了改过共晶Al-Si合金的工业化应用。为了解决由于微观组织的粗大影响合金力学性能的问题，本项目提出“稀土变质Al-Si合金”制备具有珊瑚状纤维共晶硅和端部钝化的细小粒状初生硅的Al-Si合金。本项目以过共Al-20Si合金为研究对象，研究了稀土Sm、稀土Yb、稀土Y以及RE/Al-5Ti-1B复合对铸造Al-Si合金中α-Al、初生Si和共晶Si形貌及尺寸的影响。.本项目的研究结果表明：稀土元素Sm、Yb对Al-20%Si合金组织中初生Si相和共晶Si相有优良的变质作用。与未变质时Al-20Si合金的组织相比，当Sm的含量为0.6%时，初生Si相全部转变为端部和边缘钝化的细小块状，均匀分布于共晶基体上，且其平均尺寸由未变质时的108μm减小到35μm；共晶Si完全变质为细小的纤维状，片层间距大幅度减小。当Yb的含量为0.5%时，初生Si的形态转变为细小均匀的块状，且初生Si的平均尺寸减小到48μm，相应的共晶Si转变为细小的珊湖状纤维组织。稀土Y对过共晶Al-20Si合金中的Si相有着明显的变质效果。当稀土Y的添加量为0.8%时，不规则星状和粗大块状初生Si被细化为细小的规则块状，与未变质的Al-20Si合金相比，其平均尺寸从89µm减小到33µm，减小了62.9%，长宽比由2.07减小为1.27，减小了38.6%；共晶Si由未变质时粗大的板片状或长针状变质为细小的纤维状组织。Y/Al-5Ti-1B对Al-20Si合金中的Si相也有着显著的变质效果。当0.6%的稀土Y和1.0%的Al-5Ti-1B变质添加时，初生Si从未变质时不规则的星状或粗大的块状细化为均匀细小的规则块状，平均尺寸减小到29µm，长宽比减小到1.47；粗大的板片状或长针状共晶Si被变质为多分枝的细小纤维状，平均圆整度从未变质时的7.8减小到2.32，减小了70.3%；合金的抗拉强度提高到154MPa，延伸率增加到1.79%，且合金的断裂模式由脆性断裂逐渐地转变为韧脆混