Microstructure Evolution And Diffusion Mechanism Of Nb/TiAl Alloy Diffusion Bonded Joints

Due to the low density,high elastic modulus,excellent elevated temperature creep resistance and oxidation resistance,TiAl alloys are expected to replace nickel-based alloys as a candidate material for high temperature structural application in the area of aerospace and automotive.However,the industrial application of TiAl alloys is restricted by its poor room temperature ductility and weak high temperature oxidation resistance.Research shows that the addition of Nb can effectively improve the high temperature oxidation resistance and creep properties of TiAl alloy.However,the formation of intermetallic compounds for Nb alloyed TiAl alloys and interdiffusion behavior of Ti-Al-Nb system during high temperature oxidation is not clear,which make it hard to control the microstructure and properties of Nb alloyed TiAl alloys.Therefore,this paper is focus on the formation process and mechanism of intermetallic compounds,and the high temperature diffusion behavior of Ti-Al-Nb system is also investigated.In this paper,Nb/TiAl alloys diffusion couple was prepared by hot pressing sintering technique.The microstructure,composition and phase composition of the interface of Nb/TiAl alloys were investigated by optical microscopy?OM?,scanning electron microscopy?SEM?,X-ray diffraction?XRD?and transmission electron microscopy?TEM?.The growth kinetics of Nb/TiAl alloys diffusion joints was investigated.Then the diffusion behavior and mechanism of Nb/TiAl alloys diffusion system was studied according to Boltzmann-Matano and Dayananda theories.The diffusion joints of Nb/TiAl alloys diffusion couples prepared by hot pressing sintering were well connected,and no cracks and defects existed at the interface of the joints.With the increase of the connection temperature and the holding time,the thickness of the diffusion layer increases.Especially,when the connection temperature is 1350? for 480min,the thickness of the diffusion joint reaches about 139?m,and the interface microstructure changes obviously compared with other conditions.The typical interfacial microstructure of the joint was Nb/AlNb3+AlNb2/AlNb2+Ti2AlNb/Ti2lNb+Ti4NbAl3+Ti₃Al+TiAl/Ti₃Al+TiAl.The joint evolution mechanism contained 4 phases including the temperature rising phase;interface reaction phase;diffusing atoms reaction and cooling phase.Investigation of kinetics associated with Nb jointTiAl alloys bonding process demonstrated that the growth of interface reaction layer obeyed the parabola law governed by the diffusion times.The growth rate is controlled by the diffusion rate.The activation energy of the reaction layer is 434.3KJ/mol.The diffusion fluxes and diffusion coefficients of the elements in the diffusion joint which was connected at a connection temperature of 1350? holding time 480min were calculated by Boltzmann-Matano and Dayananda theories.The results show that the main diffusion coefficient of Al is larger than that of Nb;the main diffusion coefficient of Nb is greater than that of Ti in Nb/TiAl alloys system.In addition,during the diffusion process,Nb atoms inhibit the diffusion of Ti and Al atoms;Ti atoms promote the diffusion of Al and Nb atoms.