Study On The High Temperature Mechanical Properties Of Ti-45Al-7Nb Alloy Prepared By Powder Metallurgy

Ti Al-based alloys have excellent high temperature properties.It is an ideal material for the manufacture of aircraft engines,stationary gas turbines and certain parts of automobiles.However,the inherent brittleness and processing difficulties of Ti Al-based alloys seriously affect their wide application in various fields.Casting is the traditional process route for the preparation of Ti Al-based alloys.However,there are many defects in the Ti Al-based alloys prepared by casting.During the cooling crystallization process,the alloy forms a coarse flake-like structure,resulting in composition segregation.At the same time,shrinkage and porosity will reduce the mechanical properties of the alloy.Powder metallurgy technology is a near-net forming technology for the efficient preparation of Ti Al-based alloys.It can avoid the shortcomings of macroscopic segregation and coarse organization of the alloy.Powder metallurgy technology can prepare alloy materials with good grain fineness and good performance.In this study,three elemental powders of Ti,Al and Nb were used as raw materials.Ti-45Al-7Nb mixed powder was prepared by high-energy mechanical ball milling.Then,an isoaxial fine grain Ti-45Al-7Nb(at.%)alloy was prepared by vacuum hot pressing sintering method.The high temperature deformation mechanism of the material was analyzed by high temperature tensile experiments.The high temperature compression deformation behavior of alloys was studied by high temperature compression experiments.The optimal thermal working window was determined by means of the power dissipation pattern.From the above work,this article drew the following conclusions:(1)In this paper,Ti-45Al-7Nb mixed powder prepared by high-energy mechanical ball milling method was used.The powder was mechanically alloyed while refining.The mixed powder of the ball mill for 40 h reached the nanometer-micron level and formed?-Ti Al and?₂-Ti₃Al phases.After vacuum hot pressing sintering and subsequent hot extrusion of the envelope,a nearly fully dense Ti-45Al-7Nb alloy with a relative density of99.1%could be obtained.The alloy structure consisted of?-Ti Al and?₂-Ti₃Al bi-phase isometric ultrafine grains.the grains of the?-Ti Al phase were between 1.0?3.0?m,and the grains of the?₂-Ti₃Al phase were between 0.5?1.0?m.(2)Isoaxial ultrafine grain Ti-45Al-7Nb alloy obtained by mechanical alloying and hot pressing sintering method.The room temperature and high temperature plasticity of the alloy were improved.The higher the temperature and the smaller the tensile rate,the better the plasticity of the alloy.The alloy elongation reached 192.8%at 1000°C/5×10^-5 s^-1.(3)The high temperature tensile and compression set process of Ti-45Al-7Nb alloy was a thermal activation process.Under high-temperature conditions from 900°C to1000°C,the high-temperature tensile constitutive equations and the compressive constitutive equations were:??=e^30.33[sinh(0.0129?)]^1.1042exp(-398360/RT)??=e^39.95[sinh(0.00832?)]^1.8098exp(-488180/RT)(4)Isoaxial fine grains had good microstructural uniformity.In the process of high temperature deformation,isoaxial fine grains were conducive to promoting grain slip and alleviating stress concentration,so that the deformation was more uniform and coordinated,which was conducive to improving elongation.The high-temperature plastic deformation mechanisms were dislocation and twin crystallization,and the main softening mechanisms were dynamic recrystallization.Twin crystallization caused?/?grains to split,excitation of the slip of the?phase,and twin crystallization could improve the thermal deformation behavior of the alloy to a certain extent.