Mechanical Behavior And Deformation Mechanism Of Fine-grained Beta-solidified TiAl Alloys Under Superplastic Compression Deformation

γ-Ti Al intermetallics have the potential to replace the traditional heavier nickel base superalloys or steels with the advantages of low density,high specific strength,good resistance against oxidation and corrosion,and good creep properties.These alloys are used in high temperature bearing conditions to achieve the goal of reducing weight and improving structural efficiency.However,this kind of intermetallics alloy is considerably hard to deform due to its intrinsic brittleness and high flow stress.For a long time,the manufacturing ofγ-Ti Al alloys with conventional forming processes has brought about a challenge for technical applications.To solve the problem,two ways were considered.First,composition design,novelβ-solidifiedγ-Ti Al alloys were developed.These alloys have the improved high temperature forming ability by introducingβphase in them.Second,the forming of complex components was achieved through the advantage of superplasticity of these alloys.In the practical isothermal forging processes,the deformation of the billet is mainly occurred under the compressive stress state,and the deformation parameters at different parts of the billet in different stages could be different.This may cause that the deformation of each part of the billet is controlled by different mechanisms,which influences the deformation ability of each part of the billet and the whole deformation efficiency.Therefore,the compressive stress state and the transition between different deformation mechanisms should be considered in the applied study of superplastic isothermal forging in forming complex structures of Ti Al alloys.In the present work,β-solidified Ti Al alloys are research objects.Through the analysis of flow curves and mictrostructure evolution characteristics,the deformation mechanism of these alloys and its transition with deformation parameters were discussed,ant the parameter regions dominated by different deformation mechanisms were determined.On this basis,the microstructures of these alloys with different initial structure states（the content and distribution ofβ/B2 phase）after deformations under different conditions are systematically compared and analyzed,so as to further reveal the deformation mechanisms of these alloys under different parameters,the influence of the content and distribution ofβ/B2 phase on the transition between deformation mechanisms,and the superplastic deformation mechanisms under tensive and compressive stress states.The main research contents and results are as follows:β-solidified Ti Al alloys with the nominal compositions Ti-43.5Al-8Nb-0.2W-0.2B（high Nb containing Ti Al alloy with（α₂+γ）microstructure）and Ti-42.5Al-8Nb-0.2W-0.2B-0.1Y（high Nb containing Ti Al alloy with（β+γ）microstructure）were tested under isothermal compression at 950℃-1050℃ with constant strain rates of 10^-2s^-1 and 10^-5s^-1.It is found that the value of strain rate sensitive factor m increases with the increase of deformation temperature and the decrease of strain rate,and the control mechanism changes from dislocation creep to grain boundary sliding.The microstructures of the high Nb Ti Al alloy with（α₂+γ）microstructure after deformation under the representative parameters of the parameter areas corresponding to different deformation mechanisms were characterized by SEM,TEM and EBSD.The results showed that the dynamic recrystallization ofγphase grains is quite obvious after compression at 1000℃ with the strain rate of 10^-2s^-1.The deformation is dominated by dislocation creep.But,theγphase grains keep equiaxed and the grain boundaries are always clear under compression at 1000℃ with the strain rate of 10^-4s^-1.The deformation is dominated by grain boundary sliding.The total strain rate of the deformation can be expressed by adding the strain rate corresponding to the dislocation creep to the strain rate corresponding to the grain boundary sliding,（?）_T=（?）_slip+（?）_GBS.This indicates that the roles of dislocation creep and grain boundary sliding in deformation ofγphase are independent.The average size ofγphase grains converges to the steady-state recrystallization size(D_DRX-γ),and the relationship between the steady-state recrystallization size(D_DRX-γ)and the Zener-Hollomon parameter（Z）is double logarithm linear.The isothermal compression deformation of Ti-43.5Al-4Nb-1Mo-0.1B alloy（TNM alloy）was carried out under the same parameters,and the microstructures of the TNM alloy and the high Nb Ti Al alloy with（β+γ）microtructure after deformation under the representative parameters were compared and analyzed.The results showed that the initial microstructure of the high Nb Ti Al alloy is a three-dimensional“non-uniform”network structure.The deformation at 1000℃ with the strain rate of 10^-2s^-1 is achieved via the intragranular deformation ofγphase grains andβ/B2 phase grains,and which is dominated by the dislocation creep mechanism.But,the deformation at 1000℃ with the strain rate of 10^-4s^-1 is dominated by the grain boundary sliding between the fine equiaxedγphase grains andβ/B2 phase grains,and which is accommodated by the intragranular deformation of the initial coarseγphase grains.With the development of dynamic recrystallization,the volume of the fine grain region is keep expanding.Compared with the high Nb Ti Al alloy,the initial microstructure of TNM alloy does not have the unique characteristics.With the increase of temperature and the decrease of strain rate,the dislocation creep mechanism also changes to the grain boundary sliding mechanism.However,the deformation of TNM alloy is affected by the mixture of the two mechanisms.The isothermal tension deformation of TNM alloy was carried out in the same parameter range,and the microstructure in the tip of the fractured specimen was analyzed by SEM,TEM and EBSD.The results showed that the“tensin-compression asymmetry”phenomenon exists under the higher strain rate(10^-3s^-1)in the parameter range exhibiting superplasticity(temperture≥1000℃,strain rate≤10^-3s^-1).But,the asymmetry goes down under the lower strain rate(10^-4s^-1).Cavities can be clearly observed in the microstructure of the alloy after tension deformation,but there are no cavities in the microstructure after compression deformation.Cavities are more likely to be produced in the tensile stress state.Cavities nucleate mainly at the bulge of the interface betweenβ/B2 phase andγphase,and the larger cavities tend to form a chain parallel to the tension direction.