Electron Microscopy Study On The Deformation Microstructures In Commercial-purity Titanium

Since titanium and its alloys are widely applied in aerospace engineering,chemical industry etc.,their plastic deformation mechanisms have been investigated intensively.For hexagonal close-packed(hcp)titanium(i.e.a-Ti),the independent slip systems are not enough to accommodate the plastic deformation,and hence deformation twinning plays an important role in deformation process.In a-Ti,both of {1012}<1011>tensile twin and {1122}<1123>contraction twin are the most common twinning modes,since their critical resolved shear stress(CRSS)values are almost the same(about 100 MPa)and are much lower than other twinning modes.However,the twinning mechanism and interfacial structure of {1012} twin were well studied,while the mechanism or structure of {1122} twin are less investigated.Using transmission electron microscopy(TEM)and electron back-scattered diffraction(EBSD),we studied the {1122} twin-twin interaction in a cold-rolled hcp titanium,as well as secondary twins formed in {1122}primary twins.The main points are listed as follows:(1)The twin-twin interaction structures of {1122} contraction twin variants in cold-rolled commercial purity titanium have been studied.All the observed twin-twin pairs manifested a quilted-looking structure,which consists of the incoming twins being arrested by the obstacle twins.De-twinning,lattice rotation and curved twin boundary were observed in the obstacle twin due to the twin-twin reaction with the impinging twin.A twin-twin interaction mechanism for the {1122} twin variants was proposed in terms of the dislocation dissociation,which will enrich the understanding for the propagation of twins and twinning-induced hardening in hcp metals and alloys.Besides,formation of the {1122} twin variants was found to deviate the rank of Schmid factor,and the non-Schmid behavior was explained by the high-angle grain boundary nucleation mechanism.(2)The {1122}-{ 1012} secondary twins formed in {1122} primary twins have been studied.Non-Schmid behaviors were observed as for secondary twin variants selection.The most commonly observed secondary variants have a least misorientation(27.4°)between twinning planes of them and primary twins.It has been found that the smaller the misorientation is,the smaller the "compatibility strain" would be introduced,thus easier for a secondary twin to nucleate,i.e.the misorientation between twinning planes of secondary variant and primary twin,namely compatibility strain is the determinant for variant selection.Due to the complicated deformation condition,the highest Schmid factor variant nucleation and twins that conjunct to high-angle boundary facilitated variant nucleation were also observed.(3)The {1122}-{ 1122} secondary twins formed in {1122} primary twins have been studied.The tiny {1122} twin lamella that formed in {1122} primary twin can be secondary twin,or un-twinned matrix that formed by the bifurcate primary twin.To decide which case the tiny lamella is,the primary twin's distortion condition should be taken into consideration.If the primary twin hasn't been further deformed,the crystallographic relationship keeps well,the tiny lamella is most likely formed by the bifurcate primary twin.If the primary twins have suffered heavy stress,etc.lattice rotation,off-center twinning relationship,as well as curved twin lamella caused by non-uniform stress,the secondary twin can be activated to adjust the high gradient stress.All the observed secondary twins have the same twinning planes with their primary twins,namely the misorientation between twinning planes of primary and secondary twins are 0°,which could introduce the least compatibility strain.Twinning shear direction of secondary twin can be same or opposite with the primary one.