【作者】 陈双双；

【导师】 刘志光；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 硕士

【摘要】 γ-TiAl基合金由于低密度、高强度以及良好的高温强度和抗蠕变性,目前被认为是具有应用潜力的高温结构材料。但是在室温条件下表现出的低塑性和韧性,一直以来限制着它们的实际应用。本文首先采用真空钮扣炉制备了不同Nb含量的Ti-46Al-xNb-2Cr-0.3Y(x=2,4,6)母合金锭,接着利用快速凝固-熔体旋转法(Melt-Spinning)制备了同成分的合金薄带,研究合金元素Nb和快速凝固对合金组织和性能的影响。研究的目的是探索提高TiAl合金的室温塑性和高温性能的途径。利用金相电镜(OM)、扫描电镜(SEM)及电子探针(EPMA)对铸态合金的显微组织研究发现,合金呈现典型的枝晶组织,在枝晶间均匀地分布着白点状的Y2O3颗粒相以及网纹状的Al2Y化合物。对合金的XRD分析表明:随着Nb含量的增加合金中的α2-Ti3Al相含量减少,这是因为Nb的合金化降低了合金中Ti/Al的比例,促进了γ相的形核所致。由于α2-Ti3Al相含量的减少,显微硬度也随着Nb含量的增加而降低。利用扫描电镜(SEM)和透射电镜(TEM)对快速凝固合金薄带的显微组织进行了观察。发现快速凝固使合金的显微组织明显细化,成分更加均匀。基体主要由块状组织和层片组织构成,存在着大量的析出物、孪晶、位错等微观结构。相分析表明,合金仍然由γ-TiAl和α2-Ti3Al相构成,不过在快速凝固条件下合金中的α2-Ti3Al相含量明显增多。快速凝固合金经过退火处理后,发现层片组织不断地被γ晶粒吞食,造成层片的减少和消失。随着退火温度的提高,合金中的层片组织越来越少,在970℃退火处理以后,合金几乎全部由块状组织构成。对铸态、快速凝固态和热处理态合金的γ-TiAl及α2-Ti3Al相的晶格参数的研究发现,三种合金在快速凝固条件下γ和α2相晶格参数a、c都要比铸态条件的低。快速凝固合金薄带经过热处理后,γ相和α2-Ti3Al相的晶格参数表现出相似的规律,其中a减少,而c变大。理论表明,晶格参数的减小会对TiAl材料电子云状态及位错滑移特点产生影响,有利于提高TiAl合金的室温塑性。更多还原

【Abstract】 γ-Based TiAl alloys are currently considered as potential structural materials for elevated temperature applications due to their low density, high strength and excellent creep resistance. Factors impeding their practical application are poor ductility and toughness at ambient temperature. The present paper used argon arc melting to prepare Ti-46Al-xNb-2Cr-0.3Y ingot with different Nb contents (x=2,4,6 at.%), then used rapid solidification technique–melt spinning to prepar alloy ribbon which have the same contents. The influence of Nb addition and rapid solidification technique on the microstructure and mechanical properties of the alloys was investigated intensively, in order to explore a new method to enhance RT ductility and improve HT properties of TiAl based alloys.The microstructure of the cast alloys were studied by OM, SEM, EPMA. It was found that the alloys appeared typical dendritic microstructure, Al2Y phase and Y2O3 particles distribute homoegeneously on the interdendritic. XRD results showed that the amount ofα2-Ti3A phase in alloy reduced with the Nb element increasing, as a result of reducing the proportion of Ti/Al caused by nucleation ofγphase. The micro-hardness also showed a decreasing tendency with Nb content increasing because of the reducing content ofα2-Ti3A phase in alloy.SEM and TEM observations are performed to to investigate the solidification microstructure . It was found that the microstructure of alloy ribbon was refined and component became more homoegeneously by rapid solidification technology. The matrix are composed of massive structure and lamellar structure,which contains a lot of precipitated phases, twins and dislocations microstructure. There were still two phases in alloy ribbons, includingγ-TiAl andα2-Ti3Al phasse, yet the amount ofα2-Ti3Al phase obviously increased by rapid solidification. After annealling, the lamellar structure diwindle and disappear and was continuously swallowed byγgrains. With the increasing annealling temperature, the amount of lamellar structure in alloys became less. When annealling temperature was above 970℃, the matrix was composed of only massive structure. Investigation of lattice parameters of the cast, rapid solidification and annealled alloys revealed that the lattice parameters (c,a) ofγandα2 phase decreased compared with those in the cast alloy. After annealling, crystal parameters (c,a) ofγandα2 phased show the similar tendency, in which a axis decreased while c axis increased. The above changes in TiAl base alloys is attributed to the electron cloud conditions and sliding characters of dislocations which are beneficial to improving the RT ductility for TiAl base alloy.更多还原