Research On The Fabrication And Strengthening Mechanism Of TiBw Reinforced Columnar Structure TMCs

Titanium alloys are light,high-strength and have a wide range of applications in many fields,such as aerospace and navigation.Titanium matrix composites?TMCs?have the potential to increase the wear resistance,high temperature resistance,and specific strength of titanium alloys by adding reinforcements into titanium alloys.It has great potential in improving the comprehensive mechanical properties of titanium alloys and broadening their application prospects.In this paper,based on the effect of the space distribution of the reinforcement in the matrix on the strengthening and toughening mechanism of TMCs,the two processes?reaction hot pressing-extrusion and powder extrusion?were used to prepare columnar structure TMCs bar with excellent properties,and the preparation process was optimized,and the strengthening mechanism and fracture mechanism were studied.Firstly,the columnar structure reinforced TMCs was prepared by reaction hot pressing-extrusion process.Using TA15 spherical titanium powder with a diameter of¹10?m and TiB₂ with an average particle size of 3?m as raw materials.Low-energy ball milling followed by hot-pressing sintering process and different raw material ratios were used to prepare TiBw/TA15 with 3D mesh distribution with different TiB whisker content to study the effect of TiB whisker content on the microstructure and properties of TMCs.Then,the 2.5vol.%TiBw/TA15 composites with excellent room-temperature properties were selected and extruded to prepare TMCs bars with reinforcements in columnar distribution.The hot extrusion deformation resulted in the refinement of the matrix,the distribution of TiBw along the extrusion direction and the better connectivity of the matrix,which further improved the strength and toughness of the composites.The tensile strength and elongation at room temperature in the extrusion direction were increased from 1098MPa,6.2%to 1227MPa,10.2%.In order to shorten the preparation process and reduce the cost to meet the high cost effectiveness requirements of modern industry,the columnar structure reinforced TMCs bars were prepared using powder extrusion process,and extrusion process parameters?extrusion temperature and extrusion ratio?were studied and the process parameters were optimized.The room-temperature mechanical properties and microstructure analysis of TMCs prepared at different temperatures??+?phase zone--950°C,close to?phase transition point--1050°C,?phase zone--1150°C?indicated that the extrusion temperature had no obvious effect on the whisker distribution,but it made the matrix structure significantly different and then affected the mechanical properties of TMCs bar.When the extrusion temperature was near the phase transition point?1050°C?,there was obvious grain boundary embrittlement in TMCs bar,so extrusion temperature should be avoided near the phase transition point;when the extrusion temperature was 1150°C,the composites appeared excellent overall performance,room temperature strength and plasticity could reach 1140MPa,7.8%,respectively.The room temperature tensile properties and microstructure analysis of TMCs prepared with different extrusion ratios?6:1,11:1,17:1?showed that the extrusion ratio had a significant effect on the whisker distribution and the matrix structure was typical dynamic recrystallization structure.With the increase of the extrusion ratio,the texture along the extrusion direction in the composites was significantly enhanced,the strength was gradually increased,and the plasticity was gradually reduced.When the extrusion ratio reached 17:1,the deformation of the matrix and the whisker was inconsistent.Therefore,for TiBw reinforced TMCs,the extrusion ratio should not be too large.The room-temperature fracture mechanism results showed that during the stretching process,the whiskers broke before the matrix and then produced micro-cracks,but the cracks would not be propagated rapidly because they were blunted by the matrix,so that the columnar distribution of reinforcement brought the toughness of TMCs into full play.The high-temperature fracture mechanism results showed that TiBw distributed on the boundary of primary?grain could effectively strengthen the grain boundary forming high-temperature'grain boundary strengthening'effect,and then increased the high-temperature strength of the composite.The use temperature could be increased by about 100¹50°C compared to the matrix TA15 alloy.However,when the stretching temperature exceeded 700°C,due to softening of the matrix the whiskers were pulled out from the matrix,and the TiBw could no longer strengthen the matrix.Observing the different positions of tensile specimen could qualitatively analyze the room temperature and high temperature fracture process and fracture mechanism of TMCs.