Atomic Simulation Of Effect Of Nb Element And Twin Thickness Gradient On Mechanical Properties Of Lamellar TiAl Alloy

TiAl alloys have attracted much attention due to their excellent properties such as low density and high specific strength.They are considered to be the most promising high-temperature lightweight alloys,and have broad application prospects in aerospace,automobiles,and ships.The lamellar microstructure composed of thinα₂ lamellae and relatively wideγ-phase correlated twinning lamellae in TiAl alloys is beneficial for engineering applications.Among them,the research ofγ-TiAl alloy has received widespread attention,butγ-TiAl alloy has poor room temperature plasticity,and has relatively low tensile strength,high temperature strength and plasticity above 1000°C,which limits the development and application in the industrial field.A large number of theoretical and experimental studies have shown that the mechanical properties ofγ-TiAl alloys are significantly affected by the microstructure,and the deficiencies of high temperature strength,room temperature brittleness and plasticity can be compensated to a certain extent by reasonable regulation and optimization of the microstructure.To this end,the molecular dynamics method was used in this paper to study the effect of adding Nb element and gradient nanotwin lamella structure on the mechanical properties ofγ-TiAl alloys,and to analyze the effects of these two microstructures on the uniaxial tensile mechanical properties of lamellar TiAl alloys.Its mechanical behavior is described,and its deformation and fracture mechanism are analyzed in combination with the evolution law of microstructure.The main research work is as follows:1)The uniaxial tensile simulation of lamellar TiAl-Nb alloy samples was carried out,and the strength of differentγ/γinterface samples and the strengthening mechanism caused by Nb atom doping were analyzed.It is found that there are obvious differences in the strength of TiAl alloys with differentγ/γinterfaces,the true-twin sample has the highest strength,and the pseudo-twin sample has the lowest strength,with a significant layered boundary structure effect.The main strengthening mechanism caused by Nb atoms as alloying elements is solid solution strengthening.The volume of Nb atoms as solute atoms is different from that of solvent atoms,which causes lattice distortion around Nb atoms,resulting in local stress fields,hindering dislocation movement,and causing strengthening effects.2)The interaction between dislocations and the interface was analysed during tensile simulations of lamellar TiAl-Nb alloys.In the true-twin specimen,dislocations meet the interface and produce a disordered atomic region at the interface,which does not act as a dislocation source to emit dislocations to the other layer,whereas in the pseudo-twin and rotating interface specimens,dislocations meet the interface and produce a disordered atomic region that can act as a dislocation source to emit dislocations to the other layer;at the fracture stage,as the dislocation density of the true twin specimen exceeds the critical value,the dislocation concentration region becomes weaker and cannot withstand The shear fractional stress increases.As a result,the dislocations interact with a large number of secondary slip dislocations to form a shear zone,releasing excess shear strain energy.3)The tensile behaviour of TiAl alloy gradient nano-twin layer structures were analysed in terms of the evolution of microscopic defects and fracture processes in a gradient model with different sizes of twin layer thicknesses.The specimens with gradient nano-twin layer structure have higher yield stresses than those with uniform nano-twin layer structure.The excellent mechanical properties are mainly due to the gradient dislocations and the accumulation of dislocations and interactions between dislocations caused by the gradient structure;during plastic deformation,the plastic deformation occurs first in the larger grain areas and then gradually extends to the smaller grain areas.Stress redistribution exists between grains of different sizes,thus inhibiting localisation of deformation;different sizes of grains in the gradient grain structure activate different mechanisms of plastic deformation,thus further enhancing the material properties.Plastic deformation of grains with small twin boundary spacing is dominated by grain boundary migration,while plastic deformation of grains with larger twin boundary spacing is dominated by dislocation activity.