熔体内小粒径TiB2p、Al3Tip低温合成及其协同强化Al-Si合金机制

In situ formed one-single reinforced phase in the Al melt has an obvious tendency for its distribution during the solidification process, which easily leads to the reinforcement segregation, degrading the ductility and toughness of materials. Decreasing the size and improving the dispersion of the reinforcement are beneficial to improving the strength and ductility of the composite. Based on which this project proposes an academic thought that builds a two-phase reinforced mode including in situ small-sized TiB2（on the grain boundary）/Al3Ti（inside grain）to strengthen the Al-Si alloy. On the basis of the previous research, this project plans to adopt the ultrasound assisted mixed salts reaction and the reaction between Ti powders and Al-Si melt at low temperatures to synthesize small-sized TiB2 and Al3Ti particles, respectively, and explore the mechanisms of the nucleation and growth of in situ TiB2 and Al3Ti particles at low temperatures and reveal the dynamic influence mechanism of the ultrasound on the syntheses of TiB2 and Al3Ti. An orthogonal test will be designed and the ultrasound assisted method will be adopted to fabricate Al3Tip+TiB2p/Al-Si composites with different reinforcement contents. We will analyze the mechanism of one-single reinforcement for changing the morphologies and sizes of the α-Al and Si phases in the Al-Si alloy, and further reveal the influence mechanism of the two-phase reinforcements and their distribution mode on the microstructures and mechanical properties of the alloy. Furthermore, the synergistic strengthening mechanism of in situ TiB2 and Al3Ti particles on the mechanical properties of composites will be illuminated. The implementation of this project is expected to provide a novel design method and theoretical guidance for developing the particulate reinforced Al-Si composites with high strength and good ductility.

Al熔体内原位内生单一增强相在凝固过程中的分布具有明显倾向性，易导致增强相偏聚，降低材料塑韧性。增强相粒径减小、分散性提高有利于复合材料强韧性的提高。基于此，本项目提出构建小粒径TiB2（晶界处）/Al3Ti（晶粒内）二元增强模式强化Al-Si合金的学术思想。在前期研究基础上，拟采用超声辅助低温混合盐反应、Ti粉与Al-Si熔体反应分别合成小粒径TiB2和Al3Ti颗粒，探究低温TiB2、Al3Ti形核与生长机制，揭示超声对两者合成动力学影响机制。设计正交试验并采用超声辅助制备不同配比TiB2p+Al3Tip/Al-Si复合材料，分析单一增强相对Al-Si合金中α-Al及Si相形貌尺寸改变作用机制，揭示二元增强体系及其分布对合金组织性能影响机制，并阐明小粒径Al3Ti、TiB2颗粒在复合材料性能中的协同强化机制。本项目的实施为开发高强韧颗粒增强Al-Si复合材料提供新的设计方法及理论指导。

颗粒增强铝基复合材料具有轻质高强的特点，在汽车、航空工业领域具有广泛的应用前景。传统陶瓷颗粒增强铝基复合材料在凝固中陶瓷颗粒多偏聚在α-Al晶界处，易导致材料塑韧性急剧下降，严重制约其推广应用。将具有不同分布倾向（晶内/晶界）的复相增强颗粒引入铝合金基体内，构建晶内晶界双增强铝基复合材料可改善颗粒偏聚现象，有望制备高强韧铝基复合材料。.本项目采用原位内生Al3Ti、TiB2颗粒为增强相。首先通过800°C下Ti粉在A356熔体内的“静置-取样”试验，研究Ti粉在A356熔体内的演变进程，进而揭示Al3Ti颗粒的生成机制。其次，通过超声辅助熔盐/混合盐反应法分别制备Al-Al3Ti、Al-TiB2中间合金，而后经超声辅助重熔稀释工艺制备不同配比(x)wt.%TiB2+(5-x)wt.%Al3Ti/A356(x=0，1，2，3，4，5)复合材料，研究铸态及T6热处理态复合材料微观组织及力学性能，探究了晶内晶界复相增强颗粒对A356合金微观组织改变的作用机制及强韧化合金的作用机理。. 项目得到以下重要结论：（1）Ti粉与A356熔体反应生成Al3Ti颗粒遵循 “扩散反应-剥离”机制。（2）采用超声辅助熔盐反应法可在750℃的Al熔体内制备小尺寸（平均3.1μm）的块状Al3Ti颗粒；（3）利用超声辅助混合盐反应法在900℃的Al熔体内可制备纳米级（平均尺寸约为100nm）TiB2颗粒；（4）采用超声辅助重熔稀释工艺可制备组织、成分可控，气孔、夹杂少的晶内晶界复相增强Al3Ti + TiB2/A356复合材料。重熔后的Al3Ti颗粒平均尺寸为6μm，少量Al3Ti尺寸超过10μm，且Al3Ti作为α-Al异质形核的核心可细化晶粒，多分布在晶粒内部。纳米级TiB2颗粒多与共晶Si共同分布在晶界处，可对共晶Si及α-Al起到包裹作用，使得共晶Si球化并细化α-Al晶粒。（5）2wt.%TiB2+3wt.%Al3Ti/A356为综合力学性能最佳的复合材料，其铸态/热处理态的硬度、屈服强度、抗拉强度和延伸率分别73.6/108.5、129MPa/254MPa、200MPa/310MPa、3.3%/3.8%。（6）两种颗粒对A356的屈服强度贡献机制主要以热错配强化、Orowan强化、载荷转移强化为主。通过公式对TiB2颗粒有效体积分数修正后，理论屈服强度预测值可与实验值良好匹配。