Research On Deformation Behavior And Microstructure And Mechanical Properties Of (TiB+TiC+Y₂O₃)/?-Ti Composites

With the development of new generation of(high)supersonic and suborbital vehicles,new extreme challenges have been imposed on novel heat resistant titanium alloys.Currently,the most common highest temperature of heat resistant titanium alloys are IMI834 in the United Kingdom,Ti1100 in the United States,BT18y in Russia,and Ti60in China,but they failed to break through 600°C,which was the"thermal barrier"temperature of the high-temperature titanium alloy.Compared to matrix,the in situ discontinuously reinforced titanium matrix composites(DRTMCs),which owns higher high temperature strength,more excellent creep resistance,specific stiffness,impact resistance and fatigue properties,is especially suitable for the extremely harsh working environment of aerospace.Thus,it is the most potential of materials that break through the current"thermal barrier"temperature of high temperature titanium alloy.In this study,the matrix,Y₂O₃/?-Tiand(TiB+TiC+Y₂O₃)/?-Ticomposite were prepared by vacuum self-destructive arc melting to investigate the as-cast microstructure and mechanical properties;to clarify the?and(?+?)deformation behavior and microstructure evolution of the composite;to disscuss the spheroidization behavior during annealing of the composite;in addition,the effect of heat treatment on high temperature mechanical strength was also investigated in present work.With the addition of nano-Y₂O₃ and(TiB+TiC+Y₂O₃)reinforcements,the?grains(and?colonies)size of as-cast Y₂O₃/?-Tiand(TiB+TiC+Y₂O₃)/?-Ticomposite were decreased exponentially,and the microstructure was gradually changed from Widmanstatten microstructure to near basketweave microstructure.On the one hand,the reinforcement promoted heterogeneous nucleation of?and?grains,and refined the size of?grains;on the other hand,the reinforcement also weakened the as-cast texture of?grains,and promoted the formation of non-Burgers?grains thus weakened?-phase texture.In addition,reinforced with Y₂O₃ and(TiB+TiC+Y₂O₃)particles,the room temperature and elevated temperature tensile strength of as-cast Y₂O₃/?-Tiand(TiB+TiC+Y₂O₃)/?-Ticomposite were obvious improved.At room temperature,the yield strength of(TiB+TiC+Y₂O₃)/?-Ticomposite was 900 MPa,which was increased by 41.5%compared to matrix.With the test temperature went by,the ductility of composite was significant improved.When the tested temperature was 650?,the ductility of compostite is equivalent to that of the matrix;Moreover,both the strength and ductility were better than that of matrix when temperature rised to 700?.The?deformation behavior of(TiB+TiC+Y₂O₃)/?-Ticomposite was studied,and the effect of reinforcement on dynamic recrystallization behavior during deformation and the variant selection of?_s during the subsequent???_s transformation were also investigated.During?deformation,the?grains and reinforcements were elongated along the metal flow direction,the microstructure was basketweave structure;and the microstructure of(TiB+TiC+Y₂O₃)/?-Ticomposite was more homogeneous.As for the effect of(TiB+TiC+Y₂O₃)reinforcement,on the one hand,it accelerated the discontinuous dynamic recrystallization behavior(d DRX)of the composite by partile stimulate nucleation mechanism(PSN);on the other hand,it compensated the nucleation energy that required for?_s nucleation and provided additional nucleation sites thus promoted non-Burgers?precipitation during???_s transformation,which together weakened the deformation?texture.After heat treatment,the d DRX region was further refined.The boundaries betwwen above“fine zone”and coarse unrecrystallized regions were distinct.The microstructure of(TiB+TiC+Y₂O₃)/?-Ticomposite was further homogeneous.The degerss of equiaxial was further aggravated.Besides,the silicide was precipitated at?/?boundary.The room temperature and eleavated temperature tensile properties of?deformation and 950?/2 h/AC+700?/4 h/AC heat treatment condition were obtain.Compared with the as-cast condition,the mechanical properties of the three materials were greatly improved;and the yield strength of materials achieved significantly increased after heat treatment.The analysis of high-temperature tensile deformation behavior showed that the grain boundaries slip,?lamellar dislocation slip and slip betwwen?/?phase were involved in deformation.The formation of“fine zone”after heat treatment increased the grain boundary strength,which in turn leaded to a higher equal-strength temperature,thus successfully achieved overwhelmingly excellent yield strength.The(?+?)deformation behavior of(TiB+TiC+Y₂O₃)/?-Ticomposite was clarified,and the deformation mechanism of composite was also discussed.Even though the crystallographic rotations of?lamellar,?phase dislocation slip,?/?phase interfacial slip and grain boundaries slip were accompanied the deformation process,the?colonies still showed strong stability.The effect of reinforcement on(?+?)deformation behavior were mainly:a)refine?grains and?colonies,increase the proportion of grain boundaries,so that more grain boundary slips participates in deformation;b)make the microstructure of(TiB+TiC+Y₂O₃)/?-Ticomposite more basketweave like and the orientation of?grains more diversity,thus leads to the restrain of different dislocation slip system in local areal,resulting in greater lattice rotation and substructure formation;c)the reainforcement itself hinder dislocation move,making it difficult to deformation,thus at last accelerates the spheroidization of?grains.On the whole,the deformation mechanism of matrix was mainly based on the substructure formatted by dislocation,however,the(TiB+TiC+Y₂O₃)/?-Ticomposite was mainly associated with the grain boundary slip mechanism,which achievd a more homogeneous deformation and better deformation ability in composite.In addition,the spheroidization behavior of(TiB+TiC+Y₂O₃)/?-Ticomposite during subsequent annealing heat treatment was carried out,thus the effect of heat treatment on high temperature mechanical strength was also investigated in present work.The results revealed that the spheroidization behavior was sensitive to both annealing temperature and time:in the interval of 960-980?,it had the maximum spheroidization rate;when annealing at 980?,the spheroidization rate was the largest in 15-30 min,the boundary separation was completed in 30 min and the spheroidization was completed in 8h.In the early stage of annealing treatment,the spheroidization behavior was mainly involved in the thermal groove behavior which was associated with boundary splitting mechanism,and this process was affected by storage energy during(?+?)deformation.In the later stage of annealing treatment,the coarsening behavior was mainly involved in termination migration mechanism and Ostwald ripening mechanism,which were affected by annealing temperature and time.The room temperature and elevated temperature tensile test of(?+?)deformation and 1000?/2 h/AC+700?/4 h/AC heat treatment condition were conducted.After(?+?)deformation,the microstructure was immature and it had been yielded during(?+?)deformation,therefore,the strength was not good.However,after heat treatment,the microstructure was transformed into fine duplex microstructure,accordingly,the strength was greatly improved.In particular,the yield strength was equivalent to that of the heat treatment state after the?deformation.The higher strength of composite was due to the fine grain size on the one hand,and the load transfer effect of TiB whiskers on the other.