The Microstructure And Mechanical Properties Of Ti₂AlC+Al₂O₃ Particles Reinforced TiAl Composites

Owing to the light weight and excellent high temperature mechanical properties,etc.TiAl alloy,as a novel kind of high temperature structural materials,it possesses an promising application in the fields of aerospace and automotive industries.However,the poor ductility at room temperature and insufficient strength at high temperature greatly increase the difficulty of processing large and complex TiAl alloy parts,and also cannot meet the increasingly stringent service environment requirements.Among the methods of improving the properties of TiAl alloy,the compositing method that by introducing ceramic particles,such as Ti₂AlC and Al₂O₃,into TiAl base alloy is considered as an effective method to improve the comprehensive properties of TiAl alloy.In this paper,Ti and Al elemental powders were used as raw materials,and trace C and O elements in stearic acid(C₁₈H₃₆O₂)which was also acted as ball milling process control agent introduced in the mechanical ball milling process,were used as carbon and oxygen sources,（Ti₂AlC+Al₂O₃）_p/TiAl composites with excellent comprehensive properties were successfully fabricated by ball milling and vacuum hot pressing sintering.In this paper,the effects of stearic acid on the microstructure of ball milling process and sintering products were studied.It was found that the powder yield of mechanical ball mill greatly improved by use of steric acid.XRD analysis of sintered products of ball-milled powders with different stearic acid addition showed that the sintered product without adding stearic acid was mainly composed of TiAl and Ti₃Al phases.While for the ball-milled powder with adding stearic acid,the sintering products consisted ofγ-TiAl,α₂-Ti₃Al,Ti₂AlC and Al₂O₃phases.Then,the（Ti₂AlC+Al₂O₃）_p/TiAl composites were characterized by SEM and TEM.Results showed that the composites consisted of equiaxed grain TiAl and Ti₃Al,and the Al₂O₃mainly distributed along the TiAl grain boundary in chain form with Ti₂AlC particles,which can effectively reduce the TiAl grain size.Besides,a small amount of Ti₂AlC particles were dispersed in matrix.With the increase of stearic acid addition,the area in the junction of Ti₂AlC+Al₂O₃that in chain distribution were prone to agglomeration and pores.In addition,the TEM results also showed that the Ti₂AlC/TiAl and Al₂O₃/TiAl interfaces were well combined,and the dislocation density of TiAl grains near Ti₂AlC was obviously lower than that near Al₂O₃particles.Then,the effects of sintering temperature on the microstructure of（Ti₂AlC+Al₂O₃）_p/TiAl composites were studied by SEM and EBSD.The results showed that the dense（Ti₂AlC+Al₂O₃）_p/TiAl composites can be obtained when the sintering temperature is 1150℃and above.At the range from 950℃to 1200℃,the matrix of TiAl composites exhibited equiaxed TiAl and Ti₃Al two-phase microstructure,while equiaxedγ-Tial andγ/α₂lamellar microstructure can be obtained at 1250℃.EBSD analysis showed that the grain size of Ti₂AlC and Al₂O₃increased with the increase of sintering temperature,while the grain size of TiAl decreased firstly and then increased,which was influenced by dynamic recrystallization.Three twin boundaries in TiAl grain boundary:PTB,RB and TTB were characterized,and with the increase of sintering temperature,the content of PTB decreased,while the content of RB and TTB decreased firstly and then increased.The texture analysis showed that TiAl grains had no obvious preferred orientation,the texture intensity of TiAl was weak,and it increased with increasing temperature.Ti₃Al had a certain preferred orientation,and its texture intensity increased with the increase of temperature.During the preparation process,the discontinuous dynamic recrystallization of TiAl was the dominant dynamic recrystallization mechanism.While for Ti₃Al,continuous dynamic recrystallization was the dominant mechanism,and the increased temperature was favorable to the recrystallization process of Ti₃Al.Finally,in the study of mechanical properties of（Ti₂AlC+Al₂O₃）_p/TiAl composites,the specimen,sintering temperature at 1200℃and added 2 wt%stearic acid,had the maximum compressive strength and fracture strain,which were 1674 MPa and 30.7%,respectively.The maximum microhardness of the（Ti₂AlC+Al₂O₃）_p/TiAl composite was 9.1 GPa,which was prepared at 1250℃with 2 wt%additive content of stearic acid.The strengthening mechanism of（Ti₂AlC+Al₂O₃）_p/TiAl composites on TiAl matrix mainly concluded fine grain strengthening,twin grain strengthening,thermal mismatch strengthening and Orowan strengthening.The toughening mechanism is that Ti₂AlC and Al₂O₃reinforced crack deflection,twinned grain boundary passivated crack,and Ti₂AlC layered material toughened TiAl alloy.