Sn在TiAl粉末烧结过程的浸润行为及其合金化机理

Due to the low sintering activity and densification ability of prealloyed powders, TiAl based alloy parts fabricated by pressureless sintering always exhibit undesirable mechanical properties. Then the low melting-point Sn based alloys are designed to be used as a sintering aid to achieve liquid-phase assisted sintering of TiAl powders. The additive will improve the sintering performance of the alloy powders, and hence the sintering process will be strengthened. Together with the positive effects of Sn micro-alloying, the comprehensive properties of sintered alloys are expected to be enhanced. The study focuses on the following aspects. Firstly, the wetting behavior of the interface between the sintering aid and TiAl alloy will be investigated based on the variation mechanism of the contact angles. Accordingly, a kinetic theoretical model of wetting for Sn-Al/TiAl system can be established. The dissolution, diffusion and precipitation behavior of intermetallic compounds (γ-TiAl and α2-Ti3Al) will be described as passing through the liquid phase boundary (Sn based alloy), so that the migration of the intermetallic compounds during sintering can be illuminated. The occurrence state of Sn in γ and α2 phases will be intensively investigated including the crystal sites and the laws of electronic structure variation. Consequently, the comprehension performance control theory of Sn micro-alloying TiAl based alloys will be proposed. In summary, the research aims to clarify the activated sintering mechanism of TiAl alloy powders, establish the densification kinetics model of sintering, and finally realize the near net-shape of powder metallurgy alloy components with high performance. The study results will enrich the activated sintering theory of intermetallics, and provide theoretical foundation for the fabrication of light high-temperature alloy parts with complex shape.

因TiAl基合金粉末烧结活性低、致密化困难等，导致常压烧结态TiAl合金件的力学性能不理想。为此本项目提出低熔点Sn基合金辅助液相烧结的研究思想，以改善TiAl合金粉末烧结活性并微合金化提升烧结TiAl合金综合性能。重点研究烧结剂和TiAl合金的表面浸润行为，探究其浸润角影响规律，建立烧结剂在TiAl合金表面润湿的动力学理论模型；描述烧结过程中固相γ-TiAl和α2-Ti3Al等金属间化合物通过液态晶界相（Sn基合金）的互溶扩散析出特征，阐明金属间化合物迁移活化机制；研究Sn在γ相和α2相的赋存状态，阐明Sn原子晶位及其对晶体电子结构的影响，提出Sn微合金化TiAl合金综合性能控制理论。研究旨为揭示TiAl合金粉末活化烧结机制，建立活化烧结致密化动力学模型，以实现高性能粉末合金的近终形制备。本研究将丰富金属间化合物活化烧结理论，为复杂形状粉末轻质高温合金零件制备提供理论支撑。

粉末冶金是制备TiAl合金部件有效技术手段之一，但TiAl合金粉末烧结活性低、致密化困难。为此本项目采用低熔点Sn基合金辅助液相烧结，获得高致密度TiAl合金材料。研究Sn在TiAl合金表面润湿规律，建立Sn辅助液相烧结行为，并提出Sn微合金化TiAl合金综合性能控制理论。主要研究内容及结论如下：1）真空座滴法研究不同Al含量Sn-xAl（x=0-50at.%）中间合金在TiAl基合金表面润湿变化规律，确定了不润湿（<950℃）、快速铺展（950~1150℃）和平衡润湿（>1150℃）三个阶段。并提出微量Al改善Sn液滴对TiAl基体润湿机制，即TiAl2和Ti6Sn5相组成的前体膜和TiAl3和Ti6Sn5相组成的润湿界面反应层共同促进Sn对TiAl基体的润湿。2）对比研究Sn和Sn-Al粉末添加在不同烧结工艺条件下烧结密度变化规律。同样Sn添加量的Sn-Al粉末较Sn粉末在1100~1350℃烧结时致密度提高，微观结构为近片层和双态组织。室温拉伸实验结果表明Sn元素对其常温力学性能尤其塑性具有改善作用，1at.%Sn含量TiAl合金抗拉强度和延伸率分别为519.7MPa和1.33%，较铸造4822合金分别提升17.7%和46.2%。3）利用第一性原理研究0-7.4at.%Sn对γ和α2两相晶体结构和形成能的影响，同时真空电弧熔炼炉制备不同Sn含量Ti-50Al合金纽扣锭进行实验验证。结果表明：Sn元素掺杂会造成明显晶格畸变，增加γ与α2相晶胞体积。Sn在γ相中占据Al位，而在α2相中占据Ti位。Sn添加使γ相稳定性降低，α2相稳定性明显提升。Sn主要固溶于α2相，含量约为γ相中的两倍。4）0-7.4at.%Sn添加量下TiAl合金弹性模量、态密度和电子结构的第一性原理计算表明Sn含量0~1.8at.%，γ和α2两相G/B值均下降；Sn含量1.8~7.4at.%时，两相G/B值均上升。VRH判据表明，1.8at.%Sn含量以下改善材料塑性。纽扣锭室温压缩实验结果表明添加1.8at.%Sn时，TiAl合金展现出最优的力学性能，其抗压强度和压缩率较无Sn添加时分别提高11.2%和27.5%。.项目实现Sn掺杂TiAl合金粉末辅助液相强化烧结技术，建立了Sn微合金化TiAl合金体系，为后续复杂形状TiAl合金粉末近终成形技术奠定了理论和技术基础。