The Study Of Nanocrystallization And Properties On The Surface Of TC4 Titanium Alloy After Ultrasonic Impact Treatment And High Temperature Annealing

Ultrasonic impact treatment(UIT)is one of the surface mechanical strengthening methods.The drastic plastic deformation on material surface will occur when the ultrasonic vibration outputted from transducer is directly in contact with the metal surface.This method can not only change the microstructure,but also alter the residual stress distribution on the surface of the material,which has a positive effect on strengthening material surface.In this paper,the surface of TC4 titanium alloy was mechanically strengthened by UIT,and nanocrystals were prepared on the surface of alloy.Then,the annealing treatment was carried out for the sample treated after 6 passes under different temperature(0.5h).The X-ray diffractometer(XRD),optical microscope(OM),scannig electron microscope(SEM),transmission electron microscope(TEM),laser scanning confocal microscope(CLSM),differential scanning calorimetry(DSC),residual stress analyzer,microhardness tester and electrochemical workstation were used to characterize the microstructure and performance changes of sample surface after UIT and annealing treatment.Findings were as follows:(1)Phase transformation(the β phase shifted to α phase)induced by stress was caused on the material surface and the grains refined gradually during UIT process.When the impact pass was increased to 6,the grains on surface of sample were basically refined into nanocrystals distributed uniformly,and the orientation of that was chaotic.What’s more,the average grain size was 60 nm;(2)The microhardness and residual stress value on the surface of the samples increased sharply at first,then tended to increase at slow speeds.Meanwhile,the corrosion potential tended to increase,while the corrosion current density decreased gradually with passes,which indicated that the corrosion rate slowed down gradually and the corrosion resistance of samples was improved to a certain extent;(3)The recrystallizing temperature studied was 550 ℃ at the very start.When the temperature achieved 637.4 ℃,the recrystallization transformation was the most intense,and the heat absorption reached the maximum.The recrystallization of deformed layer occurred from 550 to 650℃;(4)Recrystallization nucleation occured about 600 ℃.During this period,crystalline grains were very fine and the microhardness was increased slightly.When annealed at 650 ℃,the distortionless crystal nucleus grew into equiaxed grains equally distributed.When the annealing temperature was above 700 ℃;the grains grew big remarkably.All in all,it had a good thermal stability under 650℃.