Atomistic Simulations Of Deformation Behaviors At Interfaces Of TiAl Alloys And Some Metals

The plastic deformation behaviors of metal materials under external load is an important part of material science research.By introducing a large number of interfaces in the materials without changing its chemical composition,it contributes to improving mechanical properties.When the structural units decrease in size towards the nanometer scale,its strength and hardness tend to increase greatly to some degree.Both the grain boundary and the phase boundary have an important influence on the mechanical properties of the materials.Therefore,it is of great significance to study the deformation behaviors at different interfaces of the metal materials.With the further deepening of computational materials,molecular dynamic simulations can provide details on the atomic scale for the optimal design of materials as a combination of convenience and high efficiency,and reproduce the dynamic evolution processes that are difficult to be observed in some experiments,which contribute to the good understanding of its deformation mechanisms.TiAl alloys with the ?-TiAl/?2-Ti3Al lamellar structure exhibit excellent high temperature performance.However,the lamellae are strongly anisotropic,in particular the ?/?2 interface,which leads to the strong dependence of the interface structure and mechanical properties,e.g.,fatigue and fracture,on lamella orientation,thickness,volume fraction,etc.In the present work,molecular dynamics with the embedded-atom potential is employed to investigate the energy of both the coherent and semicoherent?/?2 interfaces.The interface coherency is found to depend on the thickness ratio of the? lamellae over the ?2 lamellae,resulting in a critical lamella thickness,below/above which the interface is coherent/semicoherent;loading perpendicular to the lamella interface indicates that the yield strength of coherent interface is higher than that of the semicoherent interface and the crack nucleation behavior varies with the thickness ratio of the ? lamellae over the ?2 lamellae;plastic deformation occurs first in the y region,forming Shockley partial dislocations and crosses the ?/?2 interface via slip transfer,activating stacking faults on the pyramidal plane in the ?2 region;the ?/?2 interface provides nucleation sites for subsequent dislocations and cracks.The present results thus contribute to the evaluation of the structural stability and the improvement of mechanical performance of TiAl alloys.Secondly,the stress-driven migration of the 90° grain boundary in hcp metals(Ti,Mg)is comparatively studied,which involves the influences of different loading conditions(uniaxial compression,tension and shear deformations)and temperatures at the same time.The results show that the grain boundary migration is driven by the combined action of dislocation slip and deformation twinning under uniaxial loading,involving the relative sliding and climbing of the two sets of interface dislocations.It is a new way to form twins by the local transformation of the lattice.However,interestingly,there is no corresponding migration movement under shear conditions.Compared to the compression loading,different deformation mechanisms are observed during the continuous process of grain boundary migration for Ti and Mg.This is of vital significance for finding new deformation modes that are different from the traditional slip and twinning.Finally,the deformation mechanisms in the vicinity of the grain boundaries of bcc metal Ta are studied by a combination of computational simulations and in-situ tensile tests,especially the nucleation and growth processes of deformation twins.The results show that the nucleation of deformation twins is initiated via sequential nucleation of twinning partial dislocations on adjacent {112} planes from grain boundaries,but the thickening mechanism may be changed with the further deformation.Under tensile loading,twin lamellae are observed growing inside crystal grains by extruding out of and then expanding with bulges on coherent {112} twin boundaries.The entire growth process operates inside crystal grains thus the thickening mechanism of deformation twins effectively eliminate the influence from the free surface of metals.The twin growth takes place without continuous supply of dislocations and can be described by a self-thickening mechanism through dislocation reactions.This growth process of deformation twins and the self-thickening mechanism can provide insights to deformation twinning in bulk BCC metals.