Study On Microstructure Evolution And Mechanical Properties Of TC4 Titanium Alloy Treated By Induction Quenchin

Titanium alloys are emerging structural and functional materials with good corrosion resistance,high specific strength,low density,and good resistance to high and low temperature.Which has been widely used in engine blades,aircraft landing gear and various aviation equipment parts.Different morphology,grain size and intragranular structure in titanium alloy microstructure can determine the difference of its mechanical properties,therefore,effective microstructure control of titanium alloy is an important means to improve mechanical properties.In this paper,TC4 titanium alloy was used as the research object,the induction heating combined with OM,SEM,and TEM was performed to study the microstructure and mechanical properties with different annealing and aging treatment after induction heating quenching.The main conclusions are as follows:（1）The microstructure of TC4 titanium alloy with gradient distribution was obtained by high-frequency induction heating,and the number of equiaxedαphase（α_p）from inside to outside gradually decreased.With the increase of heating time,the thickness of gradient layer increases.The microhardness of the alloy cross section gradually decreases from the surface to the core,and increases with the increase of heating time.（2）In the study of the effect of aging temperature on microstructure and mechanical properties of TC4 titanium alloy,with the increase of aging temperature,the secondaryαphase（α_s）decomposed fromα′martensite becomes small and becomes short rod-like dispersion distribution,which gradually coarsens and becomes longer,and presents a certain regular distribution.Under the same aging conditions,the tensile strength and yield strength of the alloy increase with the increase of induction heating time,but the elongation decreases.Under the same heating conditions,the overall microhardness from the surface to the core decreases with the increase of aging temperature,and the tensile properties（tensile strength,yield strength and elongation）of the alloy also decrease with the increase of aging temperature,the alloy presents an optimal strength-ductility synergy after aging at 400°C.（3）In the study of the effect of annealing temperature on microstructure and mechanical properties of TC4 titanium alloy,under the same induction heating time and aging conditions,the secondaryαphase gradually coarsens with the increase of annealing temperature,and a small amount of fine and unevenly distributed agingαphase（α₂）precipitates inβphase after annealing at 900°C.The tensile properties of the alloy gradually decrease with the increase of annealing temperature,and the elongation of the alloy gradually increases in the range of 750°C～850°C,while the elongation decreases after annealing at 900°C.（4）At different aging temperatures,the decrease in strength of TC4 titanium alloy with the increase of temperature is mainly due to the coarsening of the secondaryαphase size decomposed by aging at higher temperatures,resulting in the decrease of the volume fraction of the phase interface that hinders the dislocation movement.At lower aging temperature,because the secondaryαphase is dispersed and very small,dislocations can be well coordinated movement,and the strain difference of small secondaryαphase near the phase boundary is small,so the plasticity is the best at lower aging temperature.（5）At different annealing temperatures,there is Hall-Petch relationship between the strength value and the reciprocal square root of the thickness of secondaryαphase in TC4 titanium alloy.With the increase of annealing temperature,the secondaryαphase coarsens,the number of phase interfaces decreases,and dislocations are easier to slide in the crystal,so the strength and plasticity decrease.When annealed at 900°C,the size of the agingαphase is much smaller than that of the secondaryαphase.The stacking of dislocations at the interface ofα₂/βphase causes stress concentration,and the crack preferentially initiates and extends at this position,so the plasticity decreases.