TEM Investigation Of The Interaction Between ?/? Interface And Deformation Band In Fully Lamellar Ti-6Al-4V Alloy

Titanium and titanium alloy are widely used in aerospace due to its high special strength,remarkable corrosion resistance and well biological compatibility.Many researchers focused on the deformation mechanism of titanium and titanium alloy,which is helpful to deeply understand the relationship between microstructure and properties.Although many breakthroughs are accomplished with the development of characterized and analytical technology,the interaction mechanisms of defects with second phases are still unclear.Especially the interaction between second phase and twin in Ti alloy has not been revealed,which plays an important role in the design and manufacture of high strength and toughness Ti alloys.So it is necessary to do further research about it.In this paper,we are focused on the micro-deformation mechanism in titanium and titanium alloy with the help of spherical-aberration-corrected transmission electron microscopies,such as the nucleation mechanism of deformation band(DB)in lamellar Ti-6A1-4V alloy,and the interaction mechanism of deformation bands with?/? interface in lamellar Ti-6Al-4V alloy.In this way,we can obtain the response mode of second phase to deformation band,and then broaden the understanding of the role of ?/? interface in deformation process in lamellar Ti-6Al-4V alloy.All of these will provide a theoretic basis for the manufacturing of stronger and tougher Ti alloy.The main results are as follows:The orientation relationship(OR)between ? and ? phases in lamellar Ti-6A1-4V alloy obeys the Burgers OR.In addition,the atomic interface between ? and ? phase is step-like.The terrace plane is(0110)??(121)? and the ledge plane is(1100)??(101)?,which can be explained by the TLK(Terrace-Ledge-Kink)modal.After fatigue deformation in fully lamellar Ti-6Al-4V,numerous deformation twins are found in the vicinity of fatigue crack,which is induced by the dissociation of piled-up a2 and a3 dislocations at the ?/? interface.More importantly,the deformation twins can continuously propagate through multiple ? phase lamellae.The continuous propagation is accomplished by the double-kink mechanism.This result broadens the understanding of the interaction between deformation twins and ? phase,and it also indicates that the nucleation and propagation of deformation twins in the vicinity of crack can effectively release the stress concentration near the crack tip,which provides a theoretic basis for the design and manufacture of lamellar Ti-6Al-4V with higher crack resistance.After tensile deformation,a large number of deformation twins come out near the fracture in lamellar Ti-6Al-4V.Apart from {1012} twins,some {1121} twins are also found according to the selected area electron diffraction(SAED).The {1121} twin can also continuously propagate through the ? phase,but it will result in sequential {1012}twins at the interacted area between {1121} twin and ? phase.Additionally,according to the crystallographic analysis and characterization of transmission electron microscopy,the twinning dislocation of {1121} twin cannot pass through the ? phase,whereas transmission of(1121)deformation twin is completed with the increasing and transmitting of twinning shear strain field and no deformation is observed in ? phase.The results show that the interaction mechanism between {1121} twin and ? phase is distinctly different from {1012} twin,which enriches people's cognition of the interaction between twin and ? phase.All of these provide theoretic supports for the design and manufacture of tougher and stronger Ti alloy with the help of twin.After tensile deformation in fully lamellar Ti-6Al-4V alloy,apart from deformation twins,numerous other DBs come out,which mainly consist of slip band and kink band.The slip band is along(0001)plane and result in the shear fracture of ?phase along(0001)??(101)? plane.The fracture of ? phase will make the crystallographic plane exposed with higher interface energy,which will promote the decomposition of ? phase.In addition,another DB(DB ?)is also found in lamellar Ti-6A1-4V alloy.According to the analyses of selected area electron diffraction pattern and Kukichi pattern,the DB ? can be determined to be kink band which is induced by the pile-up of<a>type dislocations in the(1010)plane.Meanwhile,after the transmission of kink band,the ? phase can also be decomposed.The reason for the decomposition of ? phase is the martensite transformation of ? under severe plastic deformation.In this way,the decomposition of ? will promote the slip concentration in the DB and then induce the softening and failure in lamellar Ti-6Al-4V.The research of the interaction between DB and ? phase can deepen the understating of the interaction between defects and ? phase other than slipping and twinning,which further provides theoretic basis for the understanding of failure and strength mechanism in Ti alloys.