Microstructure Evolution And Silicide Precipitation Behavior Of TiB Reinforced Near ? High Temperature Titanium Matrix Composites During Hot Deformation

TiB reinforced titanium matrix composites have been diffusely used in aerospace field because of their high specific strength and high creep resistance.The introduction of TiB reinforcement phase improves the poor thermal strength of the matrix alloy,but it worsens the formability of the material.Hot deformation can fully stimulate the potential of titanium matrix composites and optimize its poor plasticity,so it achieves a good strong-plastic matching.It is of great significance for the engineering application of structural materials to study the evolution of microstructure,the precipitation behavior of silicide and the synergistic strengthening mechanism among fine grain strengthening,load strengthening and precipitation strengthening during hot deformation.In this paper,the microstructure evolution and silicide precipitation behavior during hot deformation were studied by means of as-cast heat treatment,hot compression and multi-directional forging,the mechanisms of dynamic recrystallization and synergistic strengthening were expounded,and the corresponding relationship between microstructure and mechanical properties was established.The microzone properties of titanium matrix composites and their matrix were tested,and it was found that the average hardness of titanium matrix composites is 6.18 GPa,which is higher than titanium alloy matrix by 14.4%.By comparing the hardness with different thickness of lamellar?phase and the hardness with different distances to the TiB respectively,it is found that the TiB reinforced high temperature titanium matrix composites present higher hardness than the titanium alloy matrix.The reason is that the matrix microstructure is refined by TiB and the deformation of matrix near the TiB is easy to be blocked.The as-cast titanium matrix composites were different heat treated,and the evolution of microstructure and silicide precipitation were observed.The diffusion rate of silicon element increases with the increase of heat treatment temperature in?/?two-phase region,the content of silicide increases and the size increases.The dissolution rate of silicide increases and the content of silicide decreases with the increase of temperature in?single-phase region.When the holding time exceeds 30 min,the precipitation behavior of silicide is no longer affected by time.Compared with water quenching and air cooling,the slow cooling rate of furnace cooling leads to the growth of primary?phase,and the silicide precipitated at the original?phase boundary is surrounded by coarsening?phase,so it is observed that part of silicide is distributed in?phase.Due to the continuous precipitation of silicide in the heating process,the content of silicide in titanium matrix composites heated with the furnace is much higher than that of rapid heating.The effects of hot compression deformation temperature,strain rate and strain on microstructure evolution and silicide precipitation behavior were studied.The recrystallization degree of the primary?phase is low during the hot compression in the low temperature of the?/?two-phase region,and the fine silicide is dispersed at the interface of the?phase,and the recrystallization of titanium matrix composites becomes intense with the increase of the temperature.The?phase changes into fine acicular martensite during the water quenching process in the?single phase region.The dynamic recrystallization of the?phase and the precipitation behavior of silicide are discussed in detail.A decrease in the content,dynamic recrystallization of the?phase and the vertical distribution of TiB along the compression axis lead to stress stability.The continuous dynamic recrystallization of the primary?phase weakens the matrix's textural strength due to the continuous difference in the orientation.Simultaneously,the TiB whisker is perpendicular to the compression direction with an increase in strain.The key mechanisms of recrystallization are:strain-induced grain boundary migration and particle stimulated nucleation by TiB.The silicide of type Ti₆Si₃ is mainly distributed on the interface of TiB and?phase during hot compression,and the interface between Ti₆Si₃ silicide and TiB whisker is incoherent.TiB whisker's hardness is much higher than that of the matrix,accelerating silicide's precipitation behavior by hindering the movement of dislocations and providing nucleation particles.The microstructure characteristics and silicide precipitation behavior of multi-directional forged titanium matrix composites and the strengthening efficiency of fine grain strengthening,precipitation strengthening and load-bearing strengthening were quantified.The microstructure of the resultant composite after multi-directional forging was composed of a spheroidized?-phase,uniformly-distributed TiB and dual-scale silicides.The precipitation of silicides was attributed to the acceleration of elemental diffusion due to dislocations during forging.The strength of the composite after multi-directional forging at room temperature increased by about 151.9 MPa due to the fine-grain strengthening of the?-phase and the precipitation strengthening of the silicides,whereas the load-bearing efficiency of the TiB whiskers was reduced with the decrease in the aspect ratio.The strength of the multi-directional-forged titanium matrix composite at 700?was reduced.However,its plasticity was significantly improved,and the elongation reached 116.8%.The main reasons for the decrease in the strength were the easy initiation of cracks the grain boundaries and the weakening of the precipitation strengthening of the silicides.