Influences Of Heat Treatment On Microstructure And Creep Properties Of Ti-6Al-4V Alloy

In the paper,the regularity of the microstructure evolution for isothermal forging and hot roiling Ti-6Al-4V alloys during the tensile creep is investigated by means of the measurement of creep properties and microstructure observation,and the influence of heat treatment regimes on the microstructure and creep properties of Ti-6Al-4V alloy has been investigated by means of the alloy heat treated by different regimes.Results shows that the microstructure of the isothermal forged and hot rolled Ti-6Al-4V alloys consists ofα+βphases,the a phase in the alloy is presented into the strip-like morphology along the flow directions,and theβphase is reticularly distributed between the strip-like a phase.Compare to the forged alloy,the hot rolled alloy displays the finer grain size.After solution and aging treated inα/βphases area,the microstructure of the alloy consists of the equiaxialαphase with supersaturation and networks basket structure,in which the needle-like and strip-like martensitic structure are dispersely distributed in the alloy.As creep goes on,the concentribution of the denser dislocation and dynamic recrystallization occur in the two-states alloy with theαphase and networks basket structure to form the finer grains,so that obtains the mix structure with the networks basket and finerαphase,this is a main reason of improving creep lifetimes and plasticity. After solution treated at 1000℃,the needle-likeα/βphases in the networks basket structure display the different orientation,both parallel to each other and vertical to each other,or arranged each other at the angle of 60°.Thereinto,theβphase with higher volume fraction and richer-V element in the alloy may enhance the creep resistance,which is a main reason of decreasing the strain rate and prolonging creep lifetimes of the alloy. When the temperature of the solution treated is lower than 990℃,as the solution temperature elevated,the volume fraction of the equiaxialαphase decrease,and the number of the networks basket structure increase,which may result in the reducement of the strain rate and the enhancement of crept lifetimes.In the range of the applied tempertures and stresses,compared to the other states' alloy,the alloy with the networks basket structure alloy possesses a better creep resistance,and the activation energy and apparent stress exponent of the alloy during creep are calculated to be Q_a=249.8(kJ/mol) and n_a=16.8,respectively.During creep,the different alloys displays a different deformation mechanism,the one of the forged alloy is the double orientations slipping of and dislocations in theαphase activated on the cylinder and pyramidal planes, the one of the double state alloy is the dislocations with the wave-like shape activated on the pyramidal planes in theαphase.After solution treated at 1000℃,the deformed mechanism of the alloy is the multiple systems slipping of dislocations activated withinβphase with BCC structure.