Fabrication,Mechanical And Tribological Behaviors Of TiAl-based Composites Reinforced By Network-Structured Ti₂AlN

Due to the low density,admirable creep resistance,high specific strength and specific stiffness,TiAl alloys are considered to be the ideal high-temperature structural material that combines light weight and high strength,which exhibits great potential in the field of aviation and aerospace industry.With the rapid development of technology,the increasingly stringent service environment of the high-temperature structural components puts forward higher requirements for the performance of the high-temperature structural materials,and further enhancing the comprehensive mechanical properties of the TiAl alloys will help accelerate the process of large-scale application.Along with the progressed research on the TiAl alloys,poor wear resistance has been regarded as an important reason for limiting corresponding application range.In the present work,the network-structured Ti₂AlN/TiAl composites were fabricated by nitriding treatment and powder metallurgy in order to achieve strengthening,toughening and high wear resistance of the TiAl alloys.The synthesis process and microstructure of the composite,as well as the mechanical and tribological behavior were systematically studied.The strengthening mechanism and wear mechanism were also revealed.Ti powder and Al powder were used as the raw materials.Firstly,Ti powder was nitrided to obtain the Ti?N?powder with“Ti-N compound”on the surface.Then the mixed Ti?N?powder and Al powder were used to prepare the network-structured Ti₂AlN/TiAl composites with the approximate reinforced phase volume fractions of5%,15%and 20%through hot-pressing sintering process.During the synthesis process,the matrix is formed by the chemical reaction of Ti inside the nitrided Ti powder and Al.With the sintering temperature rising from room temperature to1300?,the synthesized product gradually transforms from the layered TiAl₃,TiAl₂,TiAl and Ti₃Al to the equiaxed?-TiAl,while as the sintering temperature reaches1350?,the equiaxed?-TiAl matrix converts into the lamellar matrix consisted of?-TiAl/?₂-Ti₃Al.The reinforced phase is in situ synthesized on the surface of the nitrided Ti powder and the reaction temperature range is mainly from 1100?to1300?.In this temperature range,the Ti-N compound of the nitrided Ti powder changes into TiN,and the TiN on the surface is combined with the TiAl phase to form the reinforced phase Ti₂AlN.The unique structure of the Ti?N?powder ensures that the in-situ Ti₂AlN is arranged around the TiAl matrix in an orderly manner to form the network structure.As the volume fraction of Ti₂AlN increases from 15%to20%,the quasi-continuous network structure made up of particles is thickened and transforms into a continuous network structure composed of blocks.The three-point flexural tests from 700?to 900?were carried out on the as-sintered 5vol.%,15vol.%,20vol.%Ti₂AlN/TiAl composites and TiAl alloy.The results indicate that the flexural strength of the composites increases first and then drops along with the incrememt of the reinforced phase volume fraction,while the plastic deformation presents the same trend.Among the three composites,15vol.%Ti₂AlN/TiAl presents the best high-temperature bending properties.Compared with the flexural strength of TiAl alloy,that of 15vol.%Ti₂AlN/TiAl at700?,800?and 900?is increased by 28.0%,35.9%and 25.7%.The strengthening and toughening mechanism of the network-structured Ti₂AlN at high temperatures mainly stem from the fine grain strengthening,the load transfer strengthening and the crack deflection toughening.The high-temperature compressive performances of 15vol.%Ti₂AlN/TiAl and TiAl alloys were tested.The results show that the compressive strength of the composite is higher than that of the TiAl alloy as the strain rate is 0.01s^-1 and the deformation temperature is between900?and 1100?.For demonstrating the properties of the network-structured composite more comprehensively,the mechanical properties at room temperature of15vol.%Ti₂AlN/TiAl were studied.In comparison with the mechanical properties of the TiAl alloy at room temperature,the bending strength and elastic modulus of the composite are increased by 10.44%and 27.85%,respectively.Besides,the fracture toughness of the composite is optimized from 6.74MPa·m^1/2 to 8.21MPa·m^1/2.The study on the tribological properties and wear mechanism of15vol.%Ti₂AlN/TiAl manifests that the composite displays the lower friction coefficient and wear rate than those of the TiAl alloy at both room temperature and high temperatures?600?-800??.The wear process of the composite is a competitive relationship between the surface being ploughed and the formation of tribo-films.Along with the rise of ambient temperature,the worn surface characteristic of the composite changes from grooves to tribo-films,and abrasive wear gradually transforms into adhesive wear.During the wear process at room temperature,the reinforced phase Ti₂AlN with high hardness is distributed on the worn surface of the composite in the form of particle protrusion,which effectively carries the ploughing force of Si₃N₄ ball and reduces the contact area between the frictional pairs.At high temperatures,the presence of Ti₂AlN weakens the plastic deformation of the composite and promotes the synthesis of the tribo-films,thus effectively improving the high-temperature abrasion resistance.As the applied load increases from 2.5N to 4N and the rotate speed increases from 400r·min^-1 to600r·min^-1 at high temperatures,the friction coefficient of the composite goes up and the wear rate drops.The higher applied load and higher rotate speed both aggravate adhesive wear.In addition to investigating the effect of external test conditions on the tribological behavior of the composite,the pre-oxidation layer was prepared on the surface of 15vol.%Ti₂AlN/TiAl by pre-oxidation treatment to study the influence of various surface states.The pre-oxidation treatment of 15vol.%Ti₂AlN/TiAl at 900?results in the layered structure of the pre-oxidized layer on the composite surface.After the pre-oxidized process for 2h and 8h at 900?,the pre-oxidation layer consists of TiO₂ layer/?TiO₂+Al₂O₃?mixed layer and TiO₂ layer/Al₂O₃layer/?TiO₂+Al₂O₃?mixed layer in the direction from surface to core,respectively.The tribological tests at room temperature and 600?were carried out on the pre-oxidized composites.The corresponding results show that the pre-oxidized composites exhibit the more stable friction coefficient and lower wear rate in contrast with those of unoxidized composite at both room temperature and 600?.Especially at the wear condition of 600?,the wear rates of the composites pre-oxidized for 2h and 8h are reduced by 94.9%and 95.0%,respectively.The excellent tribological properties of the pre-oxidized composites derive from the tribo-films covered on the worn surface.Compared with the two pre-oxidized composites,the composite pre-oxidized for 8h shows the better abrasion resistance.The pre-oxidation layer provides a sufficient material basis for the tribo-films,which promotes the formation of the tribo-films in the lower temperature range below 600?and markedly enhances the anti-wear performance of the composite.