Microstructure And Properties Of TiBw Reinforced TC18 Matrix Composites

The aim of this paper is to study the microstructure and properties of TiBw reinforced titanium matrix composites.Based on the design of quasi-continuous network structure,a range of titanium matrix composites which were reinforced by low levels of TiBw(?2.0vol.%)were successfully fabricated by in-situ technique,low energy ball milling and hot-pressing sintering processes.The effects of heat treatment and hot extrusion deformation on microstructure and mechanical properties of TiBw/TC18 composites were studied.Optical microscopy(OM),scanning electron microscopy(SEM),electron backscattering diffraction(EBSD)and transmission electron microscopy(TEM)were used to observe the microstructure of the composites in different states and with different reinforcement content,and the microstructure characteristics and changes of the composites in different states were studied.The fracture mechanism and properties of materials under different conditions were analyzed.The microstructure observation shows that the TiBw in the as-sintered composites presents a quasi-continuous network distribution in the matrix.The size of the ?-grain decreases from 815?m in the alloy to 70?m in the composites.Tensile test results show that the tensile strength and ductility of as-sintered composites decrease with the increase of TiBw content.The tensile strength and elongation of 0.5vol.% TiBw /TC18 are1176 MPa and 15.8%,which are superior to TC18 alloy.The fracture of as-sintered composites is similar to the cup-cone fracture,and only has the fiber zone and shear lip zone,which belongs to the typical micro-porous aggregation toughness fracture.The matrix of low content TiBw composites shows obvious plastic deformation traces during the deformation process,and the fracture of TiBw at grain boundary is the main cause of crack initiation.When the TiBw content reaches 2.0vol.%,the crack aggregation and propagation are easier.The ?-grain size was refined from 70?m in as-sintered state to about 40?m in asextruded state by hot extrusion,and TiBw were aligned.The strength of as-extruded composites is obviously lower than that of as-sintered composites due to the lack of ?-precipitation strengthening,but the ductility increases markedly.The tensile strength of1.0vol.% TiBw/TC18 is 926 MPa,the elongation is 29.4%,which has the best strong plastic match.After solution and aging treatment,the ?-grain size of as-sintered composites is the same as that of as-sintered composites,and the ? colony disappears.The strength decreases with the increasing of aging temperature and increases with the increasing of solution temperature.The optimal treatment temperature is 830? for solution and 600?for aging.The tensile strength and elongation of 0.5vol.% TiBw /TC18 are 1356 MPa and7.2% respectively after heat treatment,which shows the best performance.The ?-grain size of as-extruded composites after heat treatment is obviously smaller than that of as-sintered composites.After double annealing,the tensile strength of 1.0vol.%TiBw /TC18 is 1195 MPa and the elongation remains 23.5%,which results in an excellent strong plastic matching.After solution aging treatment,the strength of 1.0vol.% TiBw/TC18 reaches 1384 MPa,and the elongation is 12%.The fracture of the composites after double annealing treatment also belongs to the cup-cone fracture.Cracks can be initiated at the grain boundary triple points,or at the interface between GB? or ?p and ?t.The crack propagation is accompanied by the occurrence of ductile fracture.