Al-Si铸造合金Si相纳米化析出的物理机制研究

Casting Al-Si alloys are traditional structure materials with low cost, while their applications are limited. If the eutectic Si in casting Al-Si alloys precipitates in the form of nano-scale phase, the problems in processing and applications caused by large size and irregular shape of Si phase will be solved, and great breakthrough of Al-Si alloys used in high property field will be made. In order to determine the conditions of nano-scale eutectic Si precipitation and reveal the physical mechanism, precipitation behavior of Si in supersaturated solid solution and amorphous will be investigated with modern analysis methods in this project. The investigation of Si precipitation in solid state is based on the previous studies, which are about the thermodynamics, kinetics, crystallography and modification of eutectic reaction in liquid Al-Si alloys. However, the research approach in this project is different from that reported previously. Besides, the effects of elements for modification in Al-Si alloys during Si precipitation process will be investigated to reveal the physical mechanisms, including the thermodynamics, kinetics and crystallography. The conditions of thermodynamics, kinetics, crystallography and alloying which meet with nano-scale eutectic Si precipitation will be explored to provide theoretical support for developing nano-scale Si phase precipitation technology and a new generation of Al-Si alloys.

Al-Si铸造合金是成本低廉但应用领域受限的传统结构材料。如果Al-Si铸造合金中的Si相以纳米化方式析出，将会消除由大尺寸不规则形态Si相引起的加工、应用中的各种问题，为Al-Si合金在高性能领域中的应用带来巨大突破。本项目以发现Si相纳米化析出条件以及揭示其物理机制为目标，在前人对Al-Si合金液相共晶反应热力学、动力学、晶体学以及对Si变质处理研究的基础上，同时区别于前人的研究思路，将Al-Si合金处理成非晶态结构和过饱和固溶体，从固态析出方向，利用现代分析手段，研究Si相析出的物理机制；研究作为变质剂的合金元素对Si相析出热力学、动力学、晶体学影响的规律；揭示Si析出过程中形核、长大的热力学、晶体学机制；揭示合金元素对Si析出影响的物理机制；探索满足Si相纳米化析出的热力学、动力学、晶体学条件以及合金化条件；为研发纳米Si相析出技术以及开创新一代Al-Si合金提供理论支持。

本课题分别采用凝固途径及固溶时效途径制备出纳米Si增强Al合金，并研究了Al-Si合金中Si相的纳米化过程、机制及其力学行为，为Al-Si合金中Si相的纳米化及合金的强韧化提供了理论及实验依据。研究表明，Si从Al-Si过饱和固溶体中析出时，Si原子先聚集成Si原子团，然后转变为金刚石结构的等轴状Si相；当Si相中出现不同方向的孪晶及层错时，其生长受扩散控制，Si的形貌保持为等轴状；当Si相中只存在某一个方向的孪晶时，垂直于该孪晶面的方向的生长受界面控制，Si转变为片状。对于快速凝固制备的Al-Si合金，变质剂的添加将降低初生Al及共晶Si的成分过冷度；当冷却速率高于临界值时导致等轴晶转变为柱状晶。对凝固途径及固溶时效途径过程的热力学分析结果表明，通过凝固途径无法获得纳米Si增强块体Al合金，而固溶时效途径则可以实现该块体合金的制备；在纳米Si的析出过程中，其生长速率主要取决于过饱和度及扩散系数，结果显示，存在一个临界过饱和度(0.026)，使扩散系数在该临界值附近发生突变，从而引起生长速率的突变。Si相的纳米化可有效提高Al-Si合金的强度，而其塑性则取决于两种形状的Si析出相的尺寸及比例。围绕等轴状Si析出相的位错环可发生松弛成为棱柱形位错，从而引起位错的积累；而片状Si析出相附近的位错则会随着Si的旋转而消失，从而导致位错的湮灭。当二者达到平衡时，合金具有最佳的均匀延伸率。