Research On The Wettability And Interfacial Interaction Between Ti-Al Molten Alloys And Ceramics

With rapid development of aviation and aerospace industries, it is required that the aerospace materials possess excellent high-temperature properties and low density. Despite a variety of conventional high temperature alloys, such as Ni-based alloy, has been developed to meet the requirement, the inherent high density and limited working temperatures of these alloys restrict their further development. Therefore, it is demanding to design novel structural materials which can potentially be developed that possess high-temperature workability and low density. Ti-Al alloys have been reported that own low density, relatively high specific strength as well as good oxidation resistance and excellent creep resistance in recent decades. Therefore, this type of alloy is being one popular alloy to potentially substitute Ni-base superalloy in the future. Nevertheless, the poor ductility at ambient temperature, and insufficient strength and oxidation resistance at high temperatures have limited its practical applications. With recent advances on metal matrix composite, it has become one indispensable novel material in aerospace, automotive and military field since the excellent comprehensive performances could be achieved by this materials principle design. Thus, to further improve the high-temperature strength and oxidation resistance, the Ti Al-based composite material with adding refractory reinforcements were introduced with taking full advantage of each properties by designing composition and adjusting process parameters.Liquid processing is one common way to manufacture the materials. To initiate the process, a good wettability between metal and ceramic is demanded. In the meantime, the interface microstructure between matrix and ceramic is another important key issue since the performance of interface microstructure during forming process can directly affect the final performance of the composite material. In this study, the effort has been made on a comprehensive understanding of the wettability, interfacial forming process and interface bonding between Ti-Al alloys and ceramic. The output of this study is aimed to provide a meaningful guidance for Ti-Al-based composite materials designing, preparing and machining. This paper focuses on the wettability and interfacial interaction between Ti-Al alloys and three ceramic substrates. The interface bonding characterization, interface stability and electronic structure have been investigated by the first principles simulation. The major results of the present study are summarized as follows:The ?-Ti Al/Ti C0.78 system is a typical reactive wetting system. The apparent equilibrium angle is smaller than 10° and slightly depends on alloy composition when Al concentration is between 40 wt.% to 80 wt.%. The interfacial product is depending on the C activity in molten alloy and oxygen pressure. However, only new interface layer with Ti2 Al C compound is formed between ?-Ti Al and Ti C0.78 due to the dynamics limited. The first-principles results shows that the obvious interaction is occurred at the ?-Ti Al(110)/Ti C(001) interface. The interfacial charge redistribution is rather localized, which forms a strong Al-C polar covalent bond and also Ti-C covalent bond and Ti-Al covalent/metal bond. The formation of Ti2 Al C is ascribed to the combination of Al and C first. The wettability of Ti-Al/Ti C system is mainly depending on interfacial chemical bond. The transformation from non-wetting to wetting is ascribed to the strong interaction of Al and C atom.The apparent equilibrium contact angle of Ti-Al molten alloy on Ti N0.87 substrate is lower than 20° and decreases slightly with increasing of Al concentration. The formation of small amount of Al N at the interface plays an important role on interface bonding for Ti-Al/Ti N system. However, the main factor for controling wettability is the Ti adsorption at the interface. The charge exchange and rearrangement are occurred at the ?-Ti Al(110)/Ti N(100) interface, especially between N atom in Ti N(100) and Al atom in ?-Ti Al(110). There is a mount of charge accumulation along the Al-N direction and the spread of charge accumulation are rather localized, which is the main interaction to strengthen interface bonding. However, it is not obvious to improve the wettability when compared with the ideal adhesion energies before and after relaxation, which shows that the apparent contact angle is decreased relatively slow.For Ti-Al/VN system, the initial apparent contact angle increases from 95° to 120° with the increase in Al content from 40 wt.% to 80 wt.% in the Ti-Al alloys, revealing that the wettability of the VN and the molten Ti-Al alloy at 1758 K depends on alloy composition. The wettability of Ti-Al/VN system is not improved by increasing of Al concentration.The Ti-Al/VN system is a typical dissolution system. However, the wetting of Ti-Al alloy on VN substrate drived by the dissolution of V element is not obvious. The dissolution driving force is from the chemical potenial difference of V element between molten alloy and substrate and the large solubility. The effect of dissolution on contact angle is reflected in the initial stage and processed synchronously with the movement of triple-phase line. With increasing of contact time, the liquid properities is changed with the dissolution of V element and the solid is precipitated at triple-phase line,which cause the liquid solidification.The interaction characteristics for ?-Ti Al(110)/VN(100) system is similar with ?-Ti Al(110)/Ti N(100) system. The effect of Al-N polar covalent bond on interface bonding strengthen is larger than Al-V covalent/ metallic bond and Ti-V covalent bond.The common point exists for above three systems calculated by the first principles. The interface bonding strength is depending on the atom stacking sequence,which is ascribed to the bonding between atoms.The preferred stacking sequence is Al atom of ?-Ti Al(110) above the merallic C/N atoms in ceramic. This interface structure has the largest bonding strength. The calculated ideal adhesion energy for ?-Ti Al(110)/Ti C(001),?-Ti Al(110)/Ti N(100) and ?-Ti Al(110)/VN(100) are 2.233 J/m2?1.899 J/m2 and 1.903J/m2. The higher interface adhesion of ?-Ti Al(110)/Ti C(001) system is ascribed to the high surface energies of Ti C(001) and its stronger trend of bonding. Compared with Al-N polar covalent bond, the interfacial Al-C interaction characterizes the larger electron concentration. The interfacial strength for ?-Ti Al(110)/VN(100) and ?-Ti Al(110)/Ti N(100) is almost same.