Research On Microstructural Evolution And Property Of Ultrafine-grained Titanium Matrix Composites Processed By Equal-channel Angular Pressing

Titanium matrix composites?TMCs?have good application prospect in aviation and aerospace fields due to their high specific strength and excellent high temperature durability.The researches of TMCs mainly focus on microstructure control and mechanical property optimization in recent years.Grain-refinement strengthening is one of the effective methods to improve mechanical properties of metal materials.Hot working can effectively refine the microstructure of TMCs,but also lengthen or thin the billets.Ultrafine grains can be obtained by equal-channel angular pressing?ECAP?without change the original geometric dimensions of billets.Therefore,in-situ TMCs with ultrafine-grained structure were obtained through ECAP,one of the SPD methods.The effect of ECAP temperature,pass number and reinforcement volume fraction?RVF?on the microstructure and mechanical properties of ECAPed TMCs was investigated.The formation mechanism of ultrafine grains was revealed,and the thermal stability of ECAPed ultrafine-grained TMCs was also studied.The main conclusions are as follows:Increase of ECAP temperature is beneficial to continuous dynamic recrystallization,which facilitates the formation of ultrafine grains.Cell structures?⁵00 nm?formed by dislocation tangling were dominant in matrix at lower ECAP temperature?600/700?°C?in ECAPed TMCs with RVF of 1.2%after the first pass.In contrast,numerous new ultrafine grains?100⁵00 nm?formed due to the annihilation and recombination of dislocations at higher ECAP temperature?800/900?°C?,and La₂O₃ particles inhibited the grain growth.Therefore,grain-refinement strengthening in matrix worked better with higher ECAP temperature.On the other hand,the average aspect ratio of TiB whiskers decreased with increasing ECAP temperature,which was only 2.1 in TMCs ECAPed at 900?°C,lower than the critical aspect ratio of TiB at room temperature.The decrease of aspect ratio weakened the load-bearing capacity of TiB.The factors of matrix and reinforcements worked together,and led to the inapparent difference in ultimate tensile strength?UTS?with different ECAP temperature.The decrease of TiB aspect ratio at higher ECAP temperature also led to debonding between TiB and matrix and caused cracks,which decreased the elongation form 13.25%in forged TMCs to the minimum of 9.69%.Increase of pass number can refine matrix but have no significant influence on reinforcements.Ultrafine grains and lamellae coexisted in the matrix in TMCs with RVF of1.2%ECAPed at 800°C after the first pass.Average grain size?AGS?decreased from 2.11?m in forged TMCs to 0.32?m,and microhardness increased form 278.49 HV to 303.41HV with it.Grain-refinement strengthening was the most obvious after the first pass.The matrix was homogenized and further refined after the second pass.The AGS decreased to0.28?m,and the fraction of high angle grain boundaries?HAGBs?was only 16%.The AGS was 0.27?m,and the fraction of HAGBs increased to 25%after the third pass.The AGS changed little after the second and third pass,so the microhardness was both about308 HV,a little higher than that after the first pass.Multi-phase and multi-scale reinforcements can refine matrix.TiB whiskers interacted with dislocations and facilitated ultrafine-grain formation at their tips and ruptures.Nano-scaled La₂O₃ particles refined matrix by hindering dislocation motion and pinning grain boundaries.Therefore,the UTS of Ti-6Al-4V ECAPed at 900°C after the first pass increased by only 3.9%compared to forged ones,while the UTS of ECAPed TMCs with RVF of 1.2%significantly increased by 15.6%.However,when the RVF increased to 2.4%,the increase of TiB aspect ratio was inconducive to the formation of ultrafine grains.And the deformation inhomogeneity of TMCs became severer during ECAP,resulting in microcracks in matrix.Therefore,the UTS of ECAPed TMCs with RVF of 2.4%was lower than that with RVF of 1.2%,and elongation also decreased remarkably to 4.59%.Ultrafine-grained TMCs obtained by ECAP showed excellent thermal stability at high temperature.The AGS of TMCs with different RVF exposed at 600°C for 50 h was still less than 1?m,which met the definition of ultrafine-grained materials.Grain growth were sensitive to time when TMCs was exposed at 600°C.Obvious recrystallization and grain growth were only observed when TMCs was exposed for more than 25 h,while the AGS increased to 0.76?m.The recovery and recrystallization were sensitive to temperature when TMCs was exposed for 50 h.The fraction of HAGBs increased from 15%at 400°C to more than 22%at 500°C and 600°C.High-temperature oxidation resistance of ultrafine-grained TMCs was excellent.The total width of oxidation zone was only 2.41?m even oxidized at 600°C for 50 h.The oxidation zone can be divided into two parts,compact oxide layers and oxygen high-gradient diffusion zone.Oxide layers combined with matrix strongly and effectively hindered the diffusion of oxygen into the deep of matrix.RVF had no obvious effect on the thickness of oxide layers and oxygen diffusion zone.La₂O₃ and TiB combined with matrix tightly,so there was no abnormal enrichment of oxygen at reinforcement/matrix interface.