Effect Mechanism Of Oxygen Content On Microstructure And Mechanical Properties Of TZM Alloy

Titanium-Zirconium-Molybdenum（TZM）alloy is an attractive material with high strength,high hardness,good thermal conductivity,wear resistance,and other properties.It is extensively utilized in the metallurgical industry,high-temperature heating elements,and fusion reactors.Several nations created a series of molybdenum alloys over time,including TZM alloy,one of the most widely used molybdenum alloys,as well as molybdenum-titanium,molybdenum-zirconium,molybdenum-lanthanum.The strengthening methods of TZM alloys are generally divided into solid solution strengthening and dispersion strengthening.Most of TZM alloys are made by the powder metallurgy process.One of the disadvantages of this method is that it cannot effectively control impurity elements,particularly oxygen control.Studies have shown that the plasticity-brittle transition temperature,recrystallization temperature,and ductility of molybdenum alloys are all related to the oxygen content in the alloy.A series of Titanium-Zirconium-Molybdenum（TZM）alloys with continuous oxygen content gradients were fabricated by powder metallurgy and the correlation between microstructure and properties was investigated.By controlling the sintering process,TZM alloys with an oxygen content of 2000 ppm,1000 ppm,and 300 ppm（±100）were obtained,and subsequently the TZM alloys with oxygen content of 3000 ppm,2000ppm,1000 ppm,and 300 ppm were rolled and heat treatment,and the effect of oxygen on the development of the microstructure and the mechanical properties of the alloy was studied.And all the sintered billets of TZM alloys with different oxygen contents were prepared as the research objects,and the Gleeble-1500 thermal simulator was used to do the thermal simulation compression experiments with the real strain of 0.7 and the compression rates of 0.1 s^-1,1 s^-1,10 s^-1 and the compression temperatures of 1100°C,1300°C,and 1500°C,respectively,to study the influence of oxygen content,compression temperature and compression rate on the microstructure and mechanical properties of TZM alloys with different oxygen contents during high temperature compression.The evolution of microstructure and mechanical properties of TZM alloys with different oxygen contents during high temperature compression was investigated,and the effects of oxygen content,compression temperature and compression rate on the microstructure and mechanical properties of TZM alloys with different oxygen contents were compared.Finally,the microstructure evolution during the thermal deformation of TZM alloys was observed by EBSD technique and the corresponding thermal deformation mechanism and DRX pattern were investigated.The results show that the changing the oxygen content has a significant impact on the mechanical properties and microstructure of the alloy.With the increase in oxygen content,the secondary phase particles increase significantly,titanium and zirconium precipitate from the molybdenum matrix to form oxides of titanium and zirconium（Ti O2,Zr O2）,and the alloy strengthening mode changes from solid solution strengthening to secondary phase dispersion strengthening.Owing to the inhibition of grain boundary expansion by the secondary phase,the grain size of the alloy decreases as its oxygen content rises,which causes an increase in hardness of around 10 HV0.1 for every 1000ppm increase in oxygen content.According to the Hall-Pitch relationship,the yield strength of the alloy increased from 336.29 MPa（300 ppm）to 557.86 MPa（2000 ppm）with smaller grain size.KAM is used to characterize the dislocation density of the alloy,and the results show that the value of KAM is negatively correlated with the toughness of the alloy.The KAM value of the alloy is the lowest at 1000 ppm and its greatest elongation reaching 16.77%.Dislocation cells formed in the long-striped grains with an oxygen content of 300 ppm,greatly enhancing the strength of the material and maintain good plasticity.TZM alloy with 300 ppm oxygen has the highest tensile strength of 865.6MPa and has an elongation of 15.04%,which is the leading performance in the industry.The experimental study of hot compression shows that the true stress continues to increase with increasing strain for TZM alloys with different oxygen contents at 1100°C,1 s^-1and low oxygen content（oxygen content less than 1000 ppm）;in the rest of the deformation conditions,with increasing strain,the typical deformation behaviors of three TZM alloys are observed in the true stress-strain curves,which are 1)work-hardening behavior;2)steady-state behavior;3)softening behavior.And the rheological stress decreases with increasing temperature and decreasing strain rate.At high temperatures and low strain rates,dynamic recrystallization causes softening to be more pronounced,i.e.,the rheological stress peaks at lower true strains and then decreases,and the peak stress arrives earlier as the oxygen content increases.Eventually,as the oxygen content increases,the recrystallization temperature of TZM alloys can be reduced.The softening of TZM alloys occurs during thermal deformation by a combination of dynamic reversion and dynamic recrystallization,and as the oxygen content increases,the softening mechanism changes from a predominance of dynamic reversion to a predominance of dynamic recrystallization.