钎焊用铝合金材料中原位连续型相变的机理研究

Al-Al laminated composite is a key material in air-cooled radiator, which is composed of Al alloy core layer and Al alloy cladding layer for brazing. 4XXX series Al alloys are selected as the cladding layer, of which a typical one is Al-Si-Mg based 4004 alloy. Mg2Si and Si are two main phases in this alloy. Si phase consumes Mg2Si phase during heat-treatment at elevated temperature, which eventually influences solderability of the cladding layer. According to our preliminary research, phase transformation from Mg2Si to Si is featured by the following two special characteristics: (1) in-situ transformation, (2) continuous decomposition (i.e., continuous transformation). Generally, they both require similarity of crystal lattice structures of the parent and the product phases. Moreover, continuous transformation usually happens in an alloy system with a miscibility gap. Although Mg2Si and Si have very similar atomic structures, their host Al-Si-Mg system does not have any miscibility gap. .This project integrates electron diffraction, crystallography and first principle calculation to study interfacial structure between Mg2Si and Si, the rules of the interface movement and also the atomic segregation around the interface. The aim is to reveal the mechanism of this in-situ continuous phase transformation, which will contribute to an enriched and deepened knowledge of phase transformation in solid, and provide knowledge-based instruction of how to control and improve the performance of Al-Si-Mg based alloys.

铝合金层状复合材料被用于空冷散热器的核心部件，它的钎焊覆层普遍用4XXX系铝合金，Al-Si-Mg系4004合金是其中的代表。Mg2Si和Si是4004合金中的主要第二相，随热处理温度的升高，Mg2Si转变为Si，合金组织发生变化从而影响钎焊性能。初步研究发现，Mg2Si与Si的相变具有2个特殊特征：（1）原位转变；（2）成分连续变化（即连续型转变）。Mg2Si和Si的原子结构十分接近，具有发生原位转变和连续型转变的条件，但4004合金不具有固溶度间隙的相图，连续型转变难以用经典的调幅分解理论解释。本项目应用电子衍射、相变晶体学结合第一性原理计算，重点研究Mg2Si与Si之间的相界面结构，相界面迁移的规律以及相界面附近原子的状态，揭示这种原位连续型相变的机理，丰富并增进固态相变知识，为调控Al-Si-Mg系合金的性能提供科学指导。

Al-Si-Mg系铝合金是轻质高强铝合金中的代表性合金体系之一，Mg2Si和Si是该系列合金中的主要第二相。在热处理过程中会发生Si与Mg2Si之间的相变，合金的组织发生变化从而影响合金的性能。基于上述背景，本项目对Al-Si-Mg系合金中的相变特征和合金组织性能优化开展了系统深入的研究。应用透射电子衍射、相变晶体学计算和第一性原理计算，弄清了Mg2Si与Si之间的相界面结构，揭示了这种原位相变的机理，为调控Al-Si-Mg系合金的性能提供了科学指导。研究发现，Al基体、Mg2Si和Si之间的位向关系（OR）为：<001>//<001>//<110>，<100>//<1-10>// <-11-1>，<010>//<110>//<-112>，其中Mg2Si与Si之间的OR在铝合金中是首次被报道。从相变晶体学出发，用我们发展的近列匹配模型预测了Mg2Si与Si之间可能的8种位向关系。其中，我们观测到的OR具有最高的优先级。第一性原理计算的结果表明Mg2Si/Si的界面能比Mg2Si/Al的界面能低。因此，在形核势垒降低的驱动下，Mg2Si趋向于在Si上形核。.基于理论研究结果，面向航空航天和交通运输对高强度、低密度铝合金的需求，我们研发了几种密度低于纯铝的高强度铝合金，获授权中国发明专利ZL202110951311.6和ZL202010006357.6等。其中，以新型纳米级AlSiLi相为强化相的可时效强化的Al-Si系专利铝合金，时效强化效果显著，在175℃时效2h就达到峰值硬度109 HV，屈服强度和抗拉强度分别为286 MPa和301 MPa，伸长率为7%，合金的密度低于纯铝，实现了低密度、高强度和高塑性的组合，未来有较好的应用前景。