Microstructure Control And Wear Behavior Of In-situ TiC?TiB/Ti6Al4V Matrix Composites

Titanium matrix composites(TMCs)were widely concerned due to their high specific strength,specific stiffness,high temperature mechanical properties and high temperature creep properties in the fields of aerospace,advanced weapon systems and automobile manufacturing.Like most metal materials,friction and wear properties was a key factor affecting its practical performance and service life,especially in high speed,heavy load,high temperature and other harsh working conditions,so higher requirements were put forward for the friction and wear performance of materials.In this paper,Ti6Al4V(TC4)was chose as matrix,TiC and TiB as reinforced phases,TMCs were prepared by in-situ casting,then the microstructure was adjusted by adding rare earth La and different heat treatment processes,to improve the mechanical properties and tribological properties.The friction and wear behaviors of TMCs under different conditions were systematically studied,and the effect and mechanism of titanium matrix composite structure,tribo-layer and tribo-oxide during wear were deeply analyzed,which could provide scientific evidence and theoretical scientific evidence for the application in friction field and wear control.The results could be concluded as follow:For single TiC and single TiB reinforced TMCs,with the increase of reinforced phase,the morphology of TiC phase changed from granular to nearly equiaxed,finally to dendrite,the ratio of length to diameter of TiB increased,and the shape changed from short needle to long needle;For(TiB+TiC)hybrid reinforced TMCs,the growth behavior of the two reinforced phases depended on each other.The existence of TiB inhibited the growth of TiC,distributed uniformly in the matrix and refined the matrix structure.(TiB+TiC)reinforced TMCs had better compressive strength and plasticity,which was mainly due to the synergistic effect of the load-bearing strengthening of the reinforcement,the fine grain strengthening of the matrix structure and the solution strengthening.The existence of TiB and TiC hard reinforced phase improved the hardness and strength of surface and subsurface,and wear resistance was greatly improved,which could prevent the initiation of microcracks during sliding wear.The compact and continuous tribo-layer restricted the direct interaction of the contact surface,thus improving the wear resistance of TMCs.With the increase of reinforcement,the wear mechanism changed from severe adhesive wear,abrasive wear and oxidation wear to slight adhesive wear,abrasive wear and oxidation wear.Due to the effect of friction and shear force,fine-grained TiC particles were easier to fall off from the matrix than equiaxed TiC particles,and the binding force of slender acicular TiB was stronger than that of short acicular TiB,which had a better pinning effect.The wear behavior of 6vol.%(TiB+TiC)/TC4 composite under different friction conditions in dry sliding friction system was studied.The specific wear rate increased with the increase of load,and increased from 0.11m/s to 1.10m/s with sliding speed rose,the wear mechanism was slight to severe wear mechanism.Under high temperature,there was a critical point for the change of wear mechanism.Under the critical temperature,the specific wear rate increased with the increase of temperature and load.The main wear mechanism was severe adhesive wear and abrasive wear,while above the critical temperature,the specific wear rate kept a low value,and the load effect decreased,and it was mainly slight wear.Sliding against different materials,the wear rate against Cr12Mo V and GCr15 was lower than that against Cu under different conditions.During wear,a compact and continuous tribo-layer was formed on the worn surface,which was composed of the wear products of reinforcement,Fe O,Ti O₂ and Fe₂O₃,which prevented the direct contact between the friction pairs,thus reducing the friction coefficient and specific wear rate.The compact and stable tribo-layer reduced the shear strength of the surface of the friction pair,limited the direct interaction of the contact surface,and improved the wear resistance of the TMCsTo further improve the mechanical properties,thermal properties and wear resistance of TMCs,La₂O₃/TMCs composites were developed.With the increase of La₂O₃ content,the proportion of high angle grain boundaries(HAGBs)gradually increased,?grain was obviously refined,and the grain orientation distribution was more random.At the same time,the distribution of La₂O₃ particles,TiB whiskers and TiC particles was more uniform,the size of TiB whiskers and TiC particles decreased,but La₂O₃ particles grew up slightly.After adding La₂O₃,TMCs had more stable,higher thermal conductivity and lower thermal expansion coefficient.The addition of La₂O₃ particles significantly improved the fine grain strengthening,solid solution strengthening and particle improvement of TMCs.In addition,a large number of HAGBs could also effectively prevent dislocation movement and improve the strength of TMCs,and the plasticity of TMCs was improved due to grain refinement.The specific wear rate and friction coefficient of TMCs were decreased,and the sliding time stability of friction coefficient was improved.The improvement of wear resistance was mainly due to two reasons:on the one hand,the hardness and strength of TMCs were significantly improved,thus enhancing the surface wear resistance of TMCs.On the other hand,the dense tribo-layer contained Ti O₂,Fe O and Fe₂O₃ oxides,which played the role of friction reduction and matrix protection.There were less furrows on the wear surface of TMCs with La₂O₃ particles,and the wear mechanism was slight abrasive wear and oxidation wear.The effects of solution and aging:1050?/3h(AC)+500?/4h(AC)(HT1)and two kinds of?triplex heat treatment:1050?/1h(AC)+950?/1h(AC)+500?/4h(AC)(HT2)and1150?/1h(AC)+950?/1h(AC)+500?/4h(AC)(HT3)on the microstructure and properties of TMCs were studied.After different heat treatment,the matrix was refined in different degrees,and the proportion of HAGBs increased.With the increase of solution temperature,TiC particles gradually became equiaxed,the morphology of TiB and La₂O₃ had no obvious change,and the interface with the matrix was good.After different heat treatment,the hardness was slightly improved,and the compressive strength was improved to a certain extent.For 6vol.%(TiB+TiC)/TC4 composite,after HT1,HT2 and HT3 heat treatment,the average friction coefficient was 0.259,0.258 and 0.262 respectively,and the specific wear rate was 2.45x10^-7mm³/N mm,2.48x10^-7mm³/N mm,2.46x10^-7mm³/N mm respectively.For0.4vol.%La₂O₃+6vol.%(TiB+TiC)/TC4 composite,after HT1,HT2 and HT3 heat treatment,the average friction coefficient was 0.262,0.215 and 0.236 respectively,and the specific wear rate was 2.30x10^-7mm³/N mm,2.28x10^-7mm³/N mm,2.30x10^-7mm³/N mm respectively,which were superior to the as cast wear resistance,and the wear mechanism was slight wear.