| By introducing reinforcement and regulating its distribution,titanium matrix composites(TMCs)exhibit excellent heat resistance,high strength,and high elastic modulus.To further improve the ductility and toughness of TMCs,a kind of laminated TMCs with multi-scale structure was designed in the present study.Raw materials were pure Ti powders,TA15 powders(Ti-6.5Al-2Zr-1Mo-1V),and TiB2 particles.Through low energy ball milling,alternate powders stacking and reaction hot pressing,a series of laminated TiB/Ti-TiB/TA15 composites have been successfully fabricated.By subsequent hot rolling and annealing treatment,the microstructure of the as-sintered composites was further regulated,and a kind of laminated TMCs with multi-scale microstructure was obtained.The as-fabricated composites exhibited excellent ductility.Microstructure observation and fracture analysis were based on optical microscope,scanning electron microscope,and electron backscattered diffraction.Transmission electron microscope was also adopted for microanalysis.Tensile and bending properties at ambient temperature were tested to evaluate the strength and ductility of the as-fabricated composites.Combining microstructure evolution and numerical simulation,the strengthening and toughening mechanisms of the laminated composite with multi-scale microstructure were revealed.Using Ti layer and TiB/TA15 layer,TiB/Ti layer and TiB/TA15 layer as layer units respectively,two kinds of as-sintered laminated Ti-TiB/TA15 and TiB/Ti-TiB/TA15 composites were fabricated,and both of them exhibited a laminated structure on a macro level.In the TiB/TA15 layer,in-situ synthesized TiB whiskers constructed a network microstructure,namely a laminated-network structure was formed,and the TA15 matrix was(?+?)basketweave structure.The pure Ti layer in the Ti-TiB/TA15 composite showed columnar?with a length of approximately200?m and a width of 30?m.While the matrix of TiB/Ti in the TiB/Ti-TiB/TA15composite was near-equiaxed because of TiB whiskers,and the average grain size was measured to be nearly 30?m.The composites exhibited a strong layer interface bonding strength of 870MPa,which should be attributed to the metallurgical combination of the layer interface and elements diffusion at the layer interface during the sintering process.The result of tensile and bending tests at ambient temperature showed that the as-sintered laminated composites had good tensile and bending ductility.In comparison with TiB/TA15 composite,the tensile fracture elongation,uniform deformation ability and bending fracture work of the Ti-TiB/TA15 composite were increased by 1.7 times,1.3 times and 1.7 times,respectively.The tensile fracture elongation of the TiB/Ti-TiB/TA15 composite was increased by 1.9 times,and the strength was decreased by only 150MPa.During stretching,the network center region in the TiB/TA15 layer deformed first,and the network structure can delay the macroscopic necking by dividing the deformation of the TA15 matrix,and thus guaranteed the uniform deformation ability of the composites.The fracture of TiB whiskers in the TiB/TA15 layer led to debonding.The debonding then resulted in micro-cracks that finally formed a through crack in the TiB/TA15 layer.The Ti based layer can coordinate deformation by generating stable micro-cracks.Through bending fractography and crack analysis,it was believed that elements diffusion,crack blunting by the Ti based layer,crack deflection at layer interface,and back stress strengthening jointly contributed to the strengthening and toughening of the laminated composite.The effect of laminated structural parameters on mechanical behaviors was investigated.By analyzing composites with layer thickness ratios of1:1,1:2 and 1:3(Ti layer:TiB/TA15 layer),it was concluded that the thicker the TiB/TA15 layer,the higher the strength,but the poorer the deformation ability.Composites with 0vol.%and 0.5vol.%TiB in the TiB/Ti layer and 3.5vol.%and 5vol.%TiB in the TiB/TA15 layer were prepared,and the results showed that the introduction of a small amount of reinforcement in the pure Ti layer can enhance the strength and ductility simultaneously.As for the layer stacking sequence,the specimen with one TiB/TA15 layer between two pure Ti layers exhibited excellent bending ductility.While for the specimen with one Ti layer between two TiB/TA15 layers,brittle fracture occurred.Strain rate also affected the mechanical properties and deformation mechanisms of the laminated composites.Under quasi-static tension,the yield strength and ultimate strength both increased with the rising strain rate.The value of strain rate sensitivity exponent of the laminated composites agreed with the rule of mixture.Under dynamic load,both twining and slip occurred in the matrix of each layer,and the composite exhibited higher compressive strength than the quasi-static one.The as-sintered laminated composite was hot rolled at 950? with a small thickness reduction of 50%.The layer thickness was dropped after rolling and the network structure in the TiB/TA15 layer was squashed.For the TA15 matrix,dynamic recrystallization occurred during hot deformation,and elongated?as well as?/?lamellae were formed.The tensile strength of the hot rolled Ti-TiB/TA15 composite was increased by 40MPa compared with the as sintered one,and the elongation was improved from 7.9%to 8.9%.For the hot rolled TiB/Ti-TiB/TA15 composite,the elongation was more than 20%,and the tensile strength can be up to 980MPa.It was analyzed that multi-scale?grains with various sizes and shapes formed in the hot rolled TiB/Ti-TiB/TA15 composite contributed to the strength and ductility enhancement.The columnar?grains in the TA15 matrix achieved work hardening by producing low angle grain boundaries.The nano-scale dynamic recrystallized?grains in the TA15 matrix played the role of fine grain strengthening and orientation strengthening.The equiaxed?grains in the Ti matrix coordinated the non-uniform deformation in the composite.To construct the multi-scale grain structure in a more controlled way,the rolling reduction was further increased to 90%.By subsequent annealing,grain growth can be promoted,multi-scale grain structure was constructed,and the ductility was dramatically enhanced.With rising annealing temperature(700??800?),the elongation of the composite was increased first.But when the temperature reached900?,the ductility decreased due to the formation of?colonies.As for the holding time,the prolongation of holding time promoted the microstructure homogenization,destroyed the multi-scale microstructure and led to the decrease of ductility.The hot rolled TiB/Ti-TiB/TA15 annealed at 800? for 10min exhibited the highest elongation,which was up to 20%.The ultimate tensile strength can reach to near 1000MPa.The multi-scale grain structure showed excellent back stress strengthening effect,which was related to the generation of geometrically necessary dislocations.For the matrix grains,prismatic slip dominated during tension,while some equiaxed grains near the layer interface also coordinated the interlaminar deformation by activating pyramidal slip systems,and thus the ductility was thus improved. |
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