Study Of Lattice Dynamics And Mechanics Of ?-?₀ Phase Transition Of Ti-(50-x)AlxM(M=V,Cr,Mn)

As an ideal high-temperature structural material,y-TiAl is widely applied in aerospace industries,due to their low density,high specific strength as well as good oxidation resistance.However,the development of TiAl alloys was hindered by poor hot workability and intrinsic brittleness.It is an effective way to improve the plasticity of the alloy by introducing the ? phase with more slip systems.The P phase is ordered to ?0 phase at low temperature,which is damage to the plasticity of the alloy at room temperature.Therefore,it is important to introduce ? phase at high temperature and limit the content of ?0 phase at room temperature to improve the properties of alloys.In this work,the phonon dispersion curves,charge density and elastic constants of Ti-Al-M(M=V,Cr,Mn)were calculated by using first principles and lattice dynamics.The effects of 3d element on phase stability and mechanical properties was studied to explore the way to improve the properties of alloys by component design.The ?0 phase is dynamically unstable at 0 K when the alloying concentrations of V,Cr and Mn are less than 10 at.%,9 at.%and 9 at.%,respectively.The anharmonic effect is helpful to improve the stability of the ?0 phase at finite temperature,and makes the phonon dispersion curves of V,Cr and Mn doping systems exhibit dynamic stability characteristics in the concentration range of(7 at.%,10 at.%),(7 at.%,9 at.%),(6 at.%,9 at.%),and may induce the structural transformation of the ?-?0 and ?0-? phases.Thus enhancing the thermoplasticity of alloys without reducing the room temperature plasticity.The calculated critical temperature of phase stability of ?0 decreases with increase of alloying concentration and atomic number.Electronic structure indicates that there exist a large number of Ti-Al and Ti-Ti,AlAl covalent bonds with strong directionality in y-TiAl.Alloying strengthens the metal bond and weakens the covalent bond,making the directivity of bonds of the system weaker and the anisotropy lower,leading to ?-?0 phase transition.The calculated elastic constants show that the bulk modulus and shear modulus increase first and then decrease with the increase of alloying concentration in the ? phase stable region,but the shear modulus decreases more greatly,which makes the B/G increase significantly and improves the room temperature plasticity of the alloy.However,the bulk modulus decreases and the shear modulus increases near phase transition concentration,resulting in the rapid decrease of B/G.The ?0 phase greatly damages the room temperature plasticity of the alloy.