Electron Microscopy Investigation Of Dynamic Deformation Mechanism Of A TiAl Alloy In Full Lamellar Structure

Intermetallic compound y-TiAl has advantages of high elastic modulus,strength and hardness,especially at elevated temperatures.Owing to these superior mechanical properties,TiAl alloys mostly composed of the intermetallic compound ?-TiAl can be used as promising high temperature structural materials,for examples,low pressure turbine blades.TiAl polysynthetic twinned(PST)crystal in full lamellar structure has shown a potential of further improving the performance of TiAl turbine blades,due to its excellent properties.The development of low pressure turbine blades needs the knowledge of the impact response of materials.Thus it is of urgent need to study the deformation bahaviours of PST crystal.In this study,advanced technics in materials science characterization including transmission electron microscopy(TEM)and aberration-corrected scanning transmission electron microscopy(STEM)were used to study the impact or dynamic deformation bahaviours of PST crystal with Ti-49.5A1 composition.(1)The main deformation behaviours(i.e.deformation twining,twin intersection,shear banding and dislocation slip)were revealed.The non-crystallographic deformation of shear banding indicates that significant difference exists between the dynamic deformation and quasi-static deformation.Twinning along primary(111)and secondary(111)planes took place,and twin intersection of primary and secondary twin lamellae in large number were observed.The primary twin lamellae are as-grown twins or deformation twins,while the secondary twin lamellae are surely deformation twins.The dislocations with the Burgers vectors of 1/3<111>and 1/6<112>were observed on the coherent primary twin boundaries(TBs).Among them,1/6<211]pseudo-twin dislocations were observed on the coherent TBs co-existing with 1/2<101]super-partial dislocations,while 1/6<112]true-twin partial dislocations can exist independently on the coherent TBs.Two types of shear bands were identified.Type ? shear bands are composed of twin lamellae,and their propagation directions are inclined to the internal twin lamellae.Type ? shear bands were formed by shearing {111} planes,and new twinning took place along the sheared {111} planes.The number of type I shear bands formed by the shearing of primary(111)planes is significantly large than others.The interactions of type ? shear bands are in form of new twinning between shear bands,which release the local stress between shear bands.Type ? shear bands are composed of curved(111)twin lamellae and their propagation is nearly parallel to the secondary(111)twin plane.Type ? shear bands were evolved from the pre-existing secondary twin lamellae.The slip of the super-partial dislocations with the Burgers vector of 1/2<101]can induce antiphase domain boundaries,and cross-slip of these dislocations happened.The superlattice dislocations with the Burgers vector of<101]in zigzag shape were observed running through the primary twin lamellae.In addition,dislocation lines in arc shape were also observed running through primary and secondary twin lamellae.(2)The interactions between dislocation and TB in atom scale were investigated in the impacted deformed samples.The step dislocations with he Burgers vector of 1/2<101>resulted from the interactions between corresponding glissile dislocations and coherent TBs.The step dislocations of 1/3<111]and 1/6<211]on coherent TBs reveal the interactions between glissile 1/2<101>dislocations with the TBs.An abnormal stacking fault was found adjacent to the coherent twin boundary.It has the same stacking sequence but different atom species in the[110]direction with an additional displacement of 1/4[110]in two neighboring{111} layers,and is likely induced by the slip of a 1/12[112](i.e.1/4[110]+1/6[211])dislocation.The strain map obtained with geometric phase analysis shows no considerable strain in the abnormal stacking region.When the number of incident twins before and after twin intersection varied,the incoherent twin boundary was formed to release the complicated stress in the twin intersection region.When the number of incident twins before and after twin intersection is constant,the propagation of incident twins is usually fulfilled by the slip of twinning dislocations along {111} planes inside the obstacle twins and then running through the obstacle twins.In addition,secondary twins took place in the twin intersection region.The twins in lenticular shape can be formed by the pole mechanism of dislocations in the impact deformed samples.The size of these twins is small,due to the effect of impact stress.(3)The special deformation behaviours of twin lamellae in the impact deformed samples were revealed.The kink of twin lamellae was observed.When the strain perpendicular to[101]crystallographic direction(i.e.viewing direction)existed,the angles between two {111} planes in the kinked twin lamellae are close to 70.5° in the high-resolution TEM images.The kink of twin lamellae can induce the step dislocations with the Burgers vector of 1/3<111>and 1/6<112>on the coherent TBs,stacking faults and 1/2<011>dislocations inside the lamellae.When the kink displacements of twin lamellae along[101]crystallographic direction(i.e.viewing direction)happened,the angles between two {111} planes in the kinked twin lamellae deviated from 70.5° and varied at different positions in a lamella.In fact,the crystal structure of these kinked twin lamellae did not happen,and the variation of angle between {111} planes in TEM images is a fake.Line contrast of grain boundary(GB)nearly parallel to[01(?)]superlattice dislocations was observed running though primary and secondary twin lamellae.Adjacent GBs can induce the primary and secondary twin lamellae to present S-shape morphology.The deformation band is composed of GBs,S-shape of twin lamellae and secondary TBs distorted severely which are parallel with[011]superlattice dislocations in line contrast.These deformation behaviours above present heterogeneity of dynamic deformation.Three types of non-crystallographic plastic deformation behaviours of twin lamellae were observed.The first one is the overlapping of twin lamellae in same twin plane with a low-angle difference.The second one is distorted twin lamellae in centipede morphology.The third one is migration of primary TBs under stress,whose morphology was changed from straightness to bulge.