Study On Strengthening And Toughening Mechanism And Failure Behavior Of Discontinuous Reinforced TiBw/Ti-6Al-4V Composites

The comprehensive properties of discontinuous reinforced titanium matrix composites are better than that of continuous reinforced titanium matrix composites.The quasi-continuous structure enhances the plasticity,and the discontinuous distribution of reinforcement makes the strength of composites close to the upper limit of H-S theoretical strength.The strengthening and toughening mechanism and failure behavior need to be clarified.The characterization of performance is still in the qualitative description stage.In this paper,the 2.6vol.%TiBw/TC4 composite with fixed whisker orientation was prepared by low energy ball milling,pre-sintering and hot extrusion.The reinforcement mechanism and failure behavior of Ti-6Al-4V titanium alloy and composite during deformation were investigated by observing different regions of tensile specimens at different temperatures.Furthermore,based on micromechanics,a prediction model of tensile strength under temperature change was established to quantitatively characterize the mechanical response of composites during deformation and to verify it.After low-energy ball milling(100r/min/6h),the powder was compacted and sealed well,and then sintered at 1150? for 0.5h.Thermal short-term hot extrusion(extrusion ratio: 10.6)was then carried out at 1150?.Microstructure analysis of hot extruded products showed that the introduction of reinforcement would refine the grain size,and dynamic recrystallization would occur in the beta phase region during hot extrusion.Composites consist of grain boundary ?,spheroidized ?,Ti B and Widmanstatten.The properties of different microphase zones are different.The anisotropy can be caused by the different deformation degree of grains in different directions.The deformation ability of microphase zone parallel to extrusion direction is weaker than that perpendicular to extrusion direction.By comparing the microstructure and mechanical properties of the tensile specimens at 25?-700?,it is found that the fracture at low temperature of the composites is a integrated fracture consisting of brittle fracture of the reinforcement and ductile fracture of the matrix,and only ductile fracture of matrix at high temperature.Shear force at interface caused by inconsistent deformation between matrix and reinforcement.Therefore,it will cause the accumulation of normal stresses in the reinforced phase to realize load-bearing strengthening.The failure of reinforcement is manifested by multiple fracture and debonding.The composites have better comprehensive properties than Ti-6Al-4V titanium alloy.The strength of the composites can be increased with the addition of reinforcing phases,while the plasticity decreases slightly.With the increase of temperature,the yield strength of the matrix decreases in proportion to the increased ratio of yield strength of the composites.The increase ratio of tensile strength is stable at about 15% below 400?,and the strengthening effect decreases when it is higher than 400? due to debonding.The addition of reinforcement can inhibit the processing hardening and flow softening ability of matrix.After continuing to deform after extrusion,the texture of < 0001 > a//ED is retained.From the point of meso-mechanics,the mechanism of the transition from elastic deformation to plastic deformation of composites is analyzed,and the fact that the matrix around the fracture of reinforcement gives priority to plastic deformation is clarified.After analyzing the trend of dislocation strengthening and fine grain strengthening varying with temperature and fitting,the prediction models of tensile strength of room temperature whisker non-fracture composites based on whisker load-bearing strengthening and multi-fracture composites based on maximum aspect ratio were given respectively.The predicted values of the tensile strength model of the composites with multiple whisker fracture at room temperature are close to the experimental mean values,and the relative error is less than 0.03.On this basis,the debonding length is introduced,and the polynomial equation of the debonding length varying with temperature is fitted from the point of view of energy loss.Then a prediction model of the tensile strength of composite materials with debonding under temperature change is established,and the prediction error is less than 0.1.This work can provide a reference for predicting the tensile strength of Ti B reinforced titanium matrix composites at different temperatures.