Study Of Lattice Dynamics And Mechanics Of γ-β₀ Phase Transition Of Ti-（50-x）AlxM（M=Zr,Nb,Mo）

As an ideal high-temperature structural material,TiAl alloy is widely applied in aerospace industries,due to their low density,high temperature resistance and high specific strength.However,the development of TiAl alloys was hindered by poor hot workability,intrinsic brittleness and fracture toughness.In recent years,it has become an effective way to improve the thermoplasticity of the alloy by introducing the β phase with more slip systems.The β phase is ordered to β0 phase at low temperature,which is damage to the plasticity of the alloy at room temperature.Therefore,it is important to interpret the microscopic mechanism of elemental alloying affecting the β phase to improve the properties of alloys.In this work,the effects of Zr,Nb and Mo doping on the phase stability and mechanical properties of TiAl alloy were calculated by using first principles and lattice dynamics,so as to explore effective ways to improve the properties of TiAl alloy.The results of formation energy and lattice structure show that 4d element occupying Al potential can promote the alloy to change from square structure to cubic structure and reduce the alloy anisotropy.The phonon calculation shows that the stability of β0 phase increases with the increase of alloying concentration.When the alloying concentration reaches Zr:13 at%,Nb:10.5 at%,Mo:10.5 at%,The β0 phase has dynamically stability at 0K.The anharmonic effect is helpful to improve the stability of the β0 phase at finite temperature,and makes the phonon dispersion curves of Zr,Nb and Mo doping systems exhibit dynamic stability characteristics in the concentration range of（9.5 at%,12.5 at%）,（8 at%,10 at%）,（9 at%,10 at%）.Within 1800 K,the critical concentration of teat-cubic phase transition of Ti-Al-M（M=Zr,Nb,Mo）can be reduced by 1.5-3.5 at%with the increase of temperature.Reasonable control of alloying concentration and temperature can better induce the structural transformation of the γ-β0 and β0-β phases,enhance the thermoplasticity of the alloy and reduce the damage to the plasticity of the alloy at room temperature.The calculated elastic constants show that the β0 phase has mechanical stability when the alloying concentration is Zr:12 at%,Nb:10 at%,and Mo:10 at%.Chemical disorder can stabilize the system mechanics at a lower alloying concentration,Zr:4 at%,Nb:3 at%,Mo:3 at%,respectively,which reduces the critical concentration of tetragonal-cubic phase transition.7-8 at%.The Cauchy pressure show that the alloy has the best ductility at the alloying concentration of Zr:6 at%,Nb:4 at%,Mo:6 at%,and its ductility will gradually decrease until brittleness occurs with the increaseof alloying concentration.Electronic structure indicates that there exist a large number of Ti-Ti and Al-Al 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 γ-β₀ phase transition.