定向凝固NiAl-Cr(Mo)合金细小胞状共晶组织形成机理研究

To overcome the shortcomings of poor room-temperature fracture toughness and insufficient elevated-temperature strength in NiAl alloy, prepared NiAl-based eutectic in-situ composite by directional solidification technology is an effective method. Generally, cellular eutectic microstructure is considered to have a very detrimental effect on the mechanical properties of the composite, so slow withdrawal rate is often adopted to obtain planar eutectic microstructure. Nevertheless, a perfect cellular eutectic microstructure was discovered in directionally solidified NiAl-Cr(Mo) eutectic alloy in previous researches. In this case, no coarse and irregular microstructure exits at the cell boundaries, and the thickness of the lamellae is almost uniform, the mechanical properties of alloy are markedly improved. Based on the above results, Ti and Nb elements are added simultaneously into directionally solidified NiAl-Cr(Mo) hypereutectic alloy in this study, trying to find out the mechanisms of phase formation and microstructure selection, then the interface undercooling, the interface morphology and the characteristics of cellular microstructure are researched under the condition of cellular growth. On this basis, perfect cellular eutectic microstructure is prepared controllably at a fast withdrawal rate and the corresponding formation law and formation mechanism are explored. Finally, perfect cellular eutectic microstructure with large volume fraction and regular Cr(Mo) strengthening phase as well as uniformly dispersed precipitated phase would be obtained in the alloyed NiAl-Cr(Mo) hypereutectic alloy after heat treatment, the relationships of the alloying elements and the process parameters with the microstructure and the mechanical properties are built. The research could enrich the solidification theory and markedly increase the production efficiency and the mechanical properties of alloy.

针对NiAl合金室温塑性低和高温强度不足的缺点，采用定向凝固技术制备NiAl基共晶复合材料是对其进行强韧化的有效方法。但之前的研究一般认为胞状共晶组织会对复合材料力学性能产生不利影响，故往往采用较低的抽拉速率以获得平界面共晶组织。申请人前期发现定向凝固NiAl-Cr(Mo)合金在较快抽拉速率下形成了一种细小胞状共晶组织，胞界处无粗大和不规则组织，片层厚度均匀，合金力学性能明显提高。本申请以此为基础，拟向定向凝固NiAl-Cr(Mo) 过共晶合金中添加Ti、Nb元素，研究合金相形成和组织选择机理及胞状生长时界面过冷度、界面形貌及胞状组织特征，在较快抽拉速率下可控制备出细小胞状共晶组织并探究其形成规律和机理；经热处理后获得含大体积分数规则强化相和均匀弥散沉淀相的细小胞状共晶组织；建立合金元素和工艺参数与微观组织及力学性能之间的关系。研究结果可望丰富共晶凝固理论，显著提高合金生产效率和力学性能。

NiAl-Cr(Mo)共晶合金具有高熔点、低密度、高热导性和优异的抗氧化性能，但室温塑性低和高温强度不足限制了其实际应用。本项目向NiAl-Cr(Mo)过共晶合金中添加Nb、Ti和V元素，研究合金元素和抽拉速率对共晶合金中相的形核和竞争生长、微观组织演变、生长界面形态以及沉淀相形貌的影响规律；然后对试样进行热处理，研究热处理对合金微观组织形态的影响；最后对合金进行力学性能测试，研究合金的室温和高温断裂行为以及强化机制。结果表明添加Nb元素后合金由NiAl+Cr(Mo)共晶及Cr2Nb相组成，随Nb含量的增大，Cr2Nb相逐渐增多。定向凝固NiAl-Cr(Mo)-Nb合金在抽拉速率为6μm/s时，通过初生相和共晶的竞争生长，最终在稳态生长时均获得了全共晶凝固组织。添加Ti元素后，合金中出现了Ni2AlTi相，由于此时合金成分偏离共晶点较大，定向凝固NiAl-32Cr-6Mo-3Nb-3Ti合金中已得不到全共晶组织，且随抽拉速率的增大初生相含量增多。V元素全部固溶在Cr(Mo)相中，没有在合金中形成新相。热处理后合金的相组成未发生变化，但析出相弥散分布，共晶组织变得较为均匀。Nb和Ti原子在定向凝固生长界面前沿的富集较为严重，对生长界面前沿过冷度的影响非常明显，造成了共晶两相的生长不稳定性。对NiAl-32Cr-6Mo合金中细小胞状共晶组织的研究发现，相对于粗胞状生长时的共晶胞尖端过冷度，细小胞状共晶生长时胞尖端过冷度最小，界面稳定性最高。定向凝固NiAl-32Cr-6Mo-3Nb和NiAl-32Cr-6Mo-1Nb-1Ti合金具有目前在NiAl共晶系合金中最佳的室温和高温压缩强度，Cr2Nb和Ni2AlTi相的沉淀强化以及Nb、Ti原子的固溶强化明显提高了合金的力学性能。同时，与铸态合金相比，含规则强化相和全共晶组织的定向凝固合金强度显著增大。所有合金在室温下均表现出伪解理断裂，而在1000℃时表现出典型的塑性断裂特征。本项目的研究使合金的力学性能得到较为明显地提高，有助于早日达到应用要求。