Preparation And Strengthening Behavior Of (TiC_p+TiB_w)/Ti Composites By In-situ Reaction

Due to the high strength and stiffness,good wear resistance,similar thermal expansion coefficient with titanium matrix and good interfacial bonding,TiC and TiB are considered as ideal reinforcements for titanium substrate.At present,the preparation of high strength(TCp+TiBw)/Ti composites by in-situ reaction based on Ti-B4C system is one of the important methods in the research field of high-strength titanium materials,and has achieved a good strength.However,the strengthening and toughening mechanisms of(TiCp+TiBw)/Ti composites,especially the hybrid effect of TiCp and TiBw has not been reported in detail.In this paper,(TiCp+TiBw)/Ti composites,TiCp/Ti composites and TiBw/Ti composites were prepared by in-situ reaction Scanning electron microscopy,X-ray diffractometry,electron probe microanalyzers and transmission electro microscopy were used to examine the microstructure and phase composition of the composites.Vicker's hardness tester and universal testing machine were used to analyze hardness and tensile properties at room temprature.In-situ tensile test under scanning electron microscopy was ultilized to investigate the deformation behavior of TiB and TiC.Classical strengthening theory was used to calculate the contribution of different strengthening factors for the strength of composite materials.Based on the above experimental results,the strengthening and toughening mechanism of(TiCp+TBw)/Ti composites was discussed.The main conclusions are as follows:(1)TiC and TiB reforcement phases form in the titanium matrix through in-situ reaction of Ti-B4C-C system,and the ratio of TC and TiB can be adjusted by changing the amount of C addition.Microstructure of the(TiCp+TiBw)/Ti composite consists of equiaxed titanium matrix,particle-like TiC phase,and whisker-like TiB phase.With the increase of TiCp content,especially when its volume content is higher than 4.1%,particles tend to agglomerate together.When TiBw content increase,the diameter and length of the whisker increases slightly.(2)With the increase of TiCp and TiBW content,both the tensile strength of TiCp/Ti and TiBw/Ti composites strengthened shows an increasing trend while the elongation decreases continuously.In contrast,the contribution of TiCp to the strength of composites is much smaller than that of TiBw,but its damage to the ductility is also relatively small.Tensile strength of(TiCp+TiBw)/Ti composites decreases continuously and the elongation increases to some extent with the increase of TiCp/TiBW molar ratio.When the molar ratio of TiCp/TiBw is 1:4,the strength of the composite reaches a maximum of 1183 MPa,and can maintain an elongation of 6.25%,suggesting that the prepared composites can be used as structural materials.(3)According to the classical reinforcement theory calculation,the order of the contribution of various factors for the strength(TiCp+TiBw)/Ti composites is:TiB fiber>hybrid reinforcement>grain refinement>O/N solution>TiC particles in general.The theoretical calculation results agree well with the experimental results,indicating that the classical solid solution strengthening,Hall-petch and shear-lag models can well predict the strength of(TiCp+TiBw)/Ti composites.The contribution of the hybrid effect is closely related to the molar ratio of TiCp to TiBw.With the increase of molar ratios,the contribution of the hybrid effect gradually decreases.(4)When the content of TiCp exceeds 4.1 vol%,both TiCp/Ti and(TiCp+TiB w)/Ti composites exhibit an "pseudo yielding" phenomenon on the tensile stress strain curve.The main reason is that when the content of TiCp exceeds the critical concentration(3.84vol%)of uniform dispersion in titanium matrix,the connecting areas of the agglomerated particles are preferentially cracked due to the compressive strain accumulated during the extrusion,resulting in the decrease of bearing capacity of the composites.Because the stress concentration coefficient of the particles is larger than that of the fiber,TiC particles are easy to break during the tensile test but the cracks do not extend into the titanium matrix,resulting in obvious influence on the deformation of titanium matrix.However,TiB whiskers do not change significantly until the matrix fractured,and the strong interface combination between TiBw and Ti matrix significantly affects the plastic deformation of the matrix.As a result,TiBw leads to more serious damage for the ductility of composite than TiCp.