The nucleation and propagation of h011]superdislocations in intermetallic TiAl were investigated using molecular dynamics simulations and static energetics calculation,as part of our systematic effort to understand the twining and dislocation behavior of alloys based on c-TiAl.It was found that compared to ordinary dislocations in disordered crystals,superdislocations in ordered TiAl lattice behave differently when sheared in the two opposite senses along[0"11]direction.This difference is due to the lower L10lattice symmetry compared with the face-centered cubic(fcc)lattice that it based on,with different yield stress and strain,and dislocation core dissociation and motion.Superdislocations nucleated in the form of loops dissociated in a planar manner into four Shockley partials separated by three kinds of faults:superlattice intrinsic stacking fault(SISF),anti-phase domain boundary(APB)and complex stacking fault(CSF),with partial separations depending on the sense of shearing and dislocation character.During loop expansion,the dislocation core changes both in width and dissociation manner depending on the character of the segment in the loop.The core contains four partials close to edge orientation,gradually changing to three fold near 60°,and finally into twofold dissociationaround 30°character.Superdislocations may have multiple critical resolved shear stresses(CRSS)for motion depending on dissociation and shearing sense even for the same slip system,with lower critical stress for the motion when SISF is in leading position.
Thermo-mechanical coupled finite element calculations were carried out to simulate the Gleeble compression of the samples of a titanium alloy(Ti60), and the results are analyzed and compared with the actual compression tests conducted on a Gleeble 3800 thermo-mechanical simulator. The changes in temperature, stress and strain distribution in the samples and the source of error on the constitutive relations from Gleeble hot compression test were analyzed in detail. Both simulations and experiments showed that the temperature distribution in the specimen is not uniform during hot compression, resulting in significant deformation inhomogeneity and non-ignorable error in the flow stress strain relation,invalidating the uniform strain assumption commonly assumed when extracting the constitutive relation from Gleeble tests. Based on the finite element simulations with iterative corrections, we propose a scheme to refine the constitutive relations from Gleeble tests.
Molecular dynamics simulation of uniaxial tension along [001] has been performed to study the influence of various surface defects on the initiation of plastic deformation and fracture of γ-TiAl single crystals.The results indicate that brittle fracture occurs in perfect bulk; surfaces and edges will be detrimental to the strength of materials and provide dislocation nucleation site. The defects on surfaces and edges cause further weakening with various effects depending on defect type, size, position and orientation,while the edge dimples are the most influential. For γ-TiAl rods with surface dimples, dislocations nucleate from an edge of the rod when dimples are small, dimple dislocation nucleation occurs only when the dimples are larger than a strain rate dependent critical size. The dislocations nucleated upon [001]tension are super dislocations with Burger vectors <011] or 1/2 < 112] containing four 1/6 < 112 > partials. The effects of surface scratches are orientation and shape sensitive. Scratches parallel to the loading direction have little influence, while sharp ones perpendicular to the loading direction may cause crack and thus should be avoided. This simulation also shows that, any type of surface defect would lower strength,and cause crack in some cases. But some may facilitate dislocation nucleation and improve ductility of TiAl if well controlled.
