First-principles Study On The Elastic Properties And The Diffusion Of Gap Atoms In The Lamellar TiAl Alloys

Due to the good specific strength,high temperature creep resistance and the thermal conductivity,Ti Al-based alloys have a great potential for application in the aerospace field as medium-high-temperature and lightweight structural materials.However,the low plasticity at room temperature and the poor ability of process are restricted to their application.In addition,the ever-increasing thrust ratio of aero-engine requires higher requirements on the oxidation resistance of the materials.Then,How to improve the plasticity and high-temperature oxidation resistance of the alloy will be practical for the TiAl-based alloys,so the research is meaningful.Alloying,as an important means to improve the properties of materials,has been widely used in TiAl-based alloys,which has improved the properties of the alloy in many aspects.In this paper,the first-principles method was used to study the effects of the common alloying elements on the elastic properties and the diffusion of gap atoms on theγandα₂phases of TiAl-based alloys,thereby obtaining some experimentally difficult to obtain performance parameters for TiAl.The basic alloy composition design and optimization provide theoretical support.The crystal structure,electronic structure,elastic properties,thermodynamic properties,and lattice diffusion properties of hydrogen and oxygen interstitial atoms ofγphase andα₂ phase with the alloying elements are calculated.The conclusions are in the following:（1）The crystal structure,elastic properties and thermal properties ofγphase andα₂phase under pressure were calculated.The results have shown that:.The main compression directions ofγphase andα₂ phase under pressure are different,they are c axis and a axis respectively.Under pressure zero,γphase has higher strength and worse plasticity,and the strength and plasticity of two phases will rise by the pressure.Under the pressure of about 20GPa,the strength,plasticity,thermal expansion coefficient and heat capacity of the two phases tend to be consistent.（2）The lattice constants,electronic structures,elasticity and thermal properties of R elements（R=Zr,Hf,Sn,V,Nb,Ta,Cr,Mo,W,Mn,Co,Ni,and Ga）solutes in theγphase andα₂ phase were calculated..The results show that:in theγphase,the elements that tend to displace the Ti atoms are Zr,Hf,V,Nb,Ta,Cr,Mo,and W,while those that tend to replace the Al atoms are Sn and Ga,and the Mn,Co and Ni occupying trend is not obvious.The W,Mo,Cr,V,Sn,Ta,Nb,and Ga is significant,which can increase the strength and hardness of theγphase.The reason is that the alloying leads to an overall shift in the density of states and the structural stability of the crystal increases.The covalent bond between the atoms increases.The Co,Mn,and Ni contribute to improving the ductility of the alloy due to alloying-induced broadening of the main bond-forming peaks of the density of states,slowing down of the Ti（d）-Al（p）bond,and intermolecular formation.The Covalent bond will be weaken.In theα₂ phase,in addition to alloying elements Sn and Ga tend to displace Al atoms,other elements tend to displace Ti atoms;Ta,Ga,Hf,V,Nb,and Zr elements have obvious solid-solution strengthening effects,and the strength of the alloy can be increasing by increased bond strength in the crystal.Co,Cr,Ni,Mn,Sn,W,and Mo elements are beneficial to improve the plasticity of the alloy,and the effects are more significant with Co,Cr,and Ni elements.（3）The diffusion behaviors of hydrogen in theγandα₂ phases and their alloys were calculated.The results show that the most stable gap positions of the hydrogen atoms in the two phases are all Ti-rich octahedron gaps;the hydrogen atoms in theγphase are more prone to in-layer diffusion,with a diffusion barrier of approximately 0.48 eV;the interlayer diffusion inα₂ phases is even greater.Easily,the energy barrier is about 0.72 eV,and lattice diffusion of hydrogen atoms in theγ-phase crystal is easier.V,Cr,Fe,Co,Ni and Cu elements can reduce the initial diffusion barrier of hydrogen atoms in theγphase,while Mo and W elements have little effect;Nb,Ta,and Mn elements increase the effect of hydrogen diffusion.It is more difficult because the alloying atoms cause a change in the bonding type and strength between the three atoms of the composition plane of the stable gap octahedron.Nb,Ta,and Mn can increase the diffusion of hydrogen atoms in theγphase and the diffusion barriers in theα₂ phase.The diffusion barriers in theγphase are 0.55 e V,0.51 eV,and 0.99eV,respectively.The diffusion barriers in theα₂ phase are 0.94 eV,1.04 eV,and 0.88 e V,respectively.（4）The diffusion behavior of oxygen atoms inγandα₂ phases and their alloys were calculated.Oxygen atoms are Ti-rich octahedral interstitials in the purestγ-phase andα₂-phase at the most stable positions.The diffusion barriers of oxygen atoms in theγ-phase is1.26 eV,andα₂ phases are respectively about 2.83eV and 2.79eV.Oxygen atoms in Ti₁₅A₁₆R（R=V,Nb,Ta,Cr,Mo,and W）all tend to occupy the Ti-rich octahedral gap farthest from the R atoms;Cr and V element can lead the diffusion coefficients increases.However,Nb,Ta,and W play an inhibitory role in the diffusion of oxygen atoms,and their diffusion barriers increase to 1.29 eV,1.30 e V,and 1.31 eV,respectively.The diffusion barrier of oxygen atoms in theα₂ phase with Nb element will increased to 3.53eV,and the diffusion coefficient at high temperature also decreases significantly.Nb element is favorable to the oxidation resistance ofα₂ phase.