Research On Microstructure And Mechanical Properties Of TC4 Titanium Alloy Subjected To The Magnetic Field And Deep Cryogenic Treatment

TC4 titanium alloy is the most widely used titanium product in the world owing to its excellent comprehensive mechanical properties and processability.With the development of industrial production,the high strength of titanium alloys is not only required,but also the plasticity and toughness.Therefore,the production of titanium alloys with high strength,high plasticity and high toughness has become a hot spot in the field of material research.In this paper,TC4 titanium alloy was selected as the research target,and processing time was chosen as the experimental parameter,the deep cryogenic treatment(DCT),static magnetic field and deep cryogenic treatment(MDCT)together with pulsed magnetic field and deep cryogenic treatment(PDCT)on TC4 titanium alloy were carried out.By metallographic examination,XRD,TEM,EBSD,tensile test and micro-hardness,the microstructure and mechanical properties of TC4 titanium alloy subjected to different treatments were explored.In general,the main results obtained in this paper are listed as follows:(1)Firstly,the effects of DCT with different processing time(12h,24 h,36h and 48h)on the microstructure and mechanical properties of TC4 titanium alloy were studied.The results show that the deep cryogenic treatment improved the compactness of the material structure and produced marvelous cold contraction internal stress in the alloy,which induced a large number of dislocations.The grain was refined and the transformation of beta phase to alpha phase promoted.The mechanical properties of TC4 titanium alloy have been improved due to the improvement of its compactness,dislocations interaction and fine grain strengthening.On the basis of mechanical properties,the optimal sample was ascertained as DCT12 with 922.12 MPa tensile strength,13.71% elongation and 435.34 HV micro-hardness,which were increased by 3.09%,10.56% and 34.09% in comparison to those of blank sample without DCT treatment.In the actuation of cold contraction internal stress,the(002)direction of grains was weakened while the(100)direction strengthened.(2)The effects of MDCT with different processing time(12h,24 h,36h and 48h)on the microstructure and mechanical properties of TC4 titanium alloy were explored.In comparison to the DCT samples,the dislocation density was further increased after MDCT,and the transformation of beta phase to alpha phase was more obvious.Although the improvement in tensile strength is little improved,the plasticity enhancement is obvious.Nevertheless the micro-hardness was lower than that of the DCT samples.The optimal sample was MDCT12 with 921.34 MPa tensile strength,13.80% elongation and 402.06 HV micro-hardness,which were raised by 3.01%,11.29% and 23.84% individually in comparison to those of the blank sample.In the MDCT process,the alloy would subject not only to the cold contraction internal stress but the magnetic moment.On this condition,the grains deflected from(100),(101)and(110)to(002)and(103)directions.(3)The effects of PDCT with different processing time(12h,24 h,36h and 48h)on the microstructure and mechanical properties of TC4 titanium alloy were investigated in comparison to the MDCT samples,the influence of PDCT on the elongation of TC4 titanium alloy was better while the micro-hardness was lowered together with little change of tensile strength.The optimal sample is PDCT48 with 935.03 MPa tensile strength,14.47% elongation and 393.64 HV micro-hardness which were enhanced by 4.54%,16.69% and 21.55% compared to the blank sample.The grain orientation in the(100)direction was enhanced while the preferred orientation of the grain was still(002)direction,and the ??? phase transition was weaker than MDCT.(4)The blank sample and the optimal samples as DCT12,MDCT12 and PDCT48 were further detected by TEM and EBSD.The grains of TC4 titanium alloy were recrystallized in three samples.Among them,the amount sequence of recrystallized grains was MDCT12,PDCT48 followed by DCT12.Due to the grain rotation,the texture could be observed in these samples and the strongest one is MDCT12.From the above research results it can be seen that on the basis of properties modification of DCT,the MDCT and PDCT can further enhance the strength and elongation.The mechanism lies in that the magnetic field accelerated the ??? phase transition,improve the dislocation density and promote the grain rotation and recrystallization process.It is regarded as an effective approach in properties modification of materials.