Study On Oxidation Behavior And Mechanical Properties Of TiAl Alloy Heat-exposed Microstructure

Compared with traditional high-temperature structural materials,TiAl intermetallic compounds have higher specific strength and specific modulus,as well as excellent corrosion and oxidation resistance.It is expected to replace nickel-based super alloys at 600?750?.In the new GE9X engine developed by the United States General Company(GE),3D printing technology is used to obtain high-quality TiAl alloy turbine,realizing the large-scale application of TiAl alloy.At the same time,TiAl alloy has also been successfully used in automobile exhaust valves and turbochargers.Difficulty in processing is the primary problem that limits the wider application of TiAl alloys.Meanwhile insufficient oxidation resistance and performance degradation after thermal exposure are other shortcomings of TiAl alloys.In this thesis,three kinds of industrial TiAl alloys,namely Ti-45Al-2Nb-2Mn-1B(45XD),Ti-47Al-2Nb-2Cr(4822),Ti-43.5Al-4Nb-1Mo-0.5B(TNM-0.5B)have been fully studied.Samples were obtained by investment centrifugal casting.The 700?oxidation test and the room temperature tensile test after heat exposure were designed.TNM-0.5B alloy was the main analysis object.The research discussed:the product type and morphology evolution of the alloy surface during 700? exposure,and changes in the mechanical properties of the alloy after thermal exposure.Main conclusions are as follows:Exposured at 700? for 500h showed obvious differences in the weight gain of three alloys.Among them,TNM-0.5B alloy gained the least weight,followed by 45XD,and 4822 alloy gained the most weight.Before and after thermal exposure,the internal microstructure of three alloys are changed insignificantly,and the alloy has good structural stability after long-term thermal exposure at 700?.At the initial stage of thermal exposure of TNM-0.5B alloy,the alloy surface appears to be "embossed".In this study,the oxidation rate of different phases in the microstructure are different.After research and comparison:?o phase can continue to provide Ti during oxidation process due to it's Ti enrichment.Finally,surface oxidation products are more concentrated,the outermost layer is prismatic TiO2;while the equiaxed y phase is rich in Al and poor in Ti.so the oxidation rate is slow.and the oxidation surface is a composite product of TiO2+Al2O3;the oxidation rate of ?2 phase is intermediate.After the TNM-0.5B alloy was exposed for 200h,a Nb-rich layer was observed at the front of the oxidation structure,and the Nb2Al product peak was detected in the XRD spectrum.The appearance of the Nb-rich layer effectively hindered the diffusion of O elements into the matrix and slowed the subsequent oxidation rateTNM-0.5B alloy exposed at 700? for 500h,the orientation of the edge lamella structure has an effect on the thickness of the oxidation-affected layer(Al-rich zone).When the lamella orientation is perpendicular to the edge,the thickness of the Al-poor zone can reach 5-7?m,while the orientation and the edge are at an angle of 45°,the thickness of the oxidation-affected layer is only 2-4 ?m.After three TiAl alloys exposed at 700? for 500h,the tensile properties at room temperature decreased to varying degrees,and the fracture analysis was all embrittlement.Among them,the plasticity of TNM-0.5B alloy decreased significantly after heat exposure for 2h.After 200h heat exposure,serious embrittlement happens.Due to the influence of lamella orientation on the oxidation diffusion path of the microstructure front,at the front of the matrix will form an influence layer with uneven thickness,forming interlaced gaps at the surface,which is easy to induce early crack initiation and propagation.The macroscopic crack propagation after thermal exposure of the alloy has the following characteristics:First,easy to tear and propagate along the(?2+?)lamellar interface.Second,easy to propagate inward along the grain boundary.Third,crack is easy to change direction at the large rod-shaped boride and then expand along the direction of boride.When TNM-0.5B alloy is not exposed,the ?" phase is black and fine dots,randomly distributed in the ?o phase.As the exposure time increases,the Mo element in the ?" phase diffuses into the matrix,and the Nb element in the matrix moves into?" phase,?o phase grows up;heat exposure 100h,?o phase is round rice grain;heat exposure 200h,?o is elongated fusiform;heat exposure 500h,the number of ?o in ?0 phase matrix decreases,and the transition of ?o??o+? occurs.