First-principles Study Of Displacive Phase Transitions And Plastic Deformation Mechanisms In βTi-V Alloys

Beta titanium alloys have high specific strength and toughness,low elastic modulus,good fatigue resistence,excellent corrosion resistance and good formability,which make them an attractive choice for advanced engineering applicaitions in demanding conditions.One of the most important properties in these alloys is that theβ-phase stability,which is a function of its composition,influences the displacive phase transitions（includingβ→α’/α’’martensitic transition andβ→ωphase transition）and those productions and deformation mechanisms（including full dislocation slip and twinning）.However,the systematic study for displacive phase transition and plastic deformation in beta titanium alloy has not yet been reported.Since V is one of the most common beta phase stabilize alloying elements in titanium alloys,therefore,Ti-V alloy as a model to study the related properties of beta titanium alloy has a very high universality.First principles method combination with Virtual Crystal approximation（VCA）has been proved that it is very accurate calculation method to calculate the ground state properties and generalized stacking fault energy（GSFE）of disordered alloy.Through the first-principles method in combination with Debye model,Peierls-Nabarro model and generalized planar fault theory,the present work systematically investigate the effect of alloying element V content on martensitic transition,ω-phase transition,dislocation slip and{112}<111>twinning inβTi-V alloys.The main content and major findings as following:1)With the virtual crystal approximation（VCA）method,the effect of alloying element V content on the ground state propertie and elastic properties of theβ,martensite（includeα’andα’’）andωphases of Ti-（0³0%）V alloys were systematically studied.The results show that the lattice parameters of theβ,α’andωphases decrease with increasing V content,while the lattie parameter a ofα’’phase increase and b,c and the atom shuffle y decrease.The energy comparison results of the three phases show that both the energy difference of theωphase and the martensite phase relative to theβphase increase monotonously with increasing V content,but are always less than zero.Meanwhile,the mechanical stability ofβphase increases monotonously with increasing V content,and the mechanical stability ofα’phase decreases monotonously.The critical alloy concentration of the two phases satisfy the mechanical stability criterion is 25%and 15%,respectively.The elastic constants of theα’’andωphase satisfy the stability criterion.The critical compositionalα’/α’’phase boundary of Ti-V alloy is 10%.The bulk modulus B of the three phases are substantially equal,while the elastic properties G and E of theωphase are always the largest,followed by the martensitic phase,and the elastic properties G and E of theβphase are the smallest.2)The effect of alloying element V content on theβ→α’/α’’martensitic transformation andβ→ωphase transformation inβTi-（0³0%）V alloy were systematically studied.The metastable diffusionless phase diagram of Ti-V alloy can be achieved by the Debye model.It is found that both the transition temperature of theβ→α’martensite transformation andβ→ωphase transition monotonically decrease with increasing V content,while the transition temperature of theα’→ωtransition monotonically increases.Theβ→α’/α’’martensitic transformation and theβ→ωphase transition induce a decrease in volume inβTi-（0³0%）V alloy,and the elastic strain energy induced by the two phase transitions are very small.There is no energy barrier during theβ→α’/α’’martensitic transition and theβ→ωphase transition at 0 K.When the temperature increases,there is still no energy barrier during theβ→α’/α’’martensitic transition,however,an energy barrier occurs during theβ→ωphase transition.The energy barrier during theβ→ωphase transition increases with increasing V content and temperature.3)According to the generalized planar fault energy theory,the effect of structural stability on the dislocation slip and{112}<111>twinning deformation inβ-Ti-（20⁵0%）V alloys are systematically studied.The results show that all the generalized stacking fault energy of the{110}<111>,{112}<111>and{123}<111>slip systems increase monotonically with increasing V content,the order of the stacking fault energy of the three slip systems isγ^sf_{112}<γ^sf_{110}<γ^sf_{123},while the order of the unstable stacking fault energy isγ^us_{110}<γ^us_{112}<γ^us_{123}.Theγ^sf/min{γ^us}ratio value of Ti-V alloy increases monotonically with increasing V content and all of the ratio values are less than 1.The dislocation width of the three slip systems decrease monotonously with increasing V content,and the order of the dislocation width is x_{110}>x_{112}>x_{123}.The critical Peierls stress of the full dislocation slip of the three slip systems increase with increasing V content,and the order of the critical Peierls stress are s_{110}<s_{112}<s_{123}.The number of twin nucleus for the{112}<111>twinning deformation in Ti-V alloy is three,and its twin boundary is isosceles twin boundary.With increasing V content,the generalized planar fault energy of{112}<111>twinning deformation increase monotonously,and the twin migration energyγ_TBM also monotonically increases.The energy barrierΔγ^twinwin of the twinning and the energy barrierΔγ^slip of full-dislocation slip increase monotonically with increasing V content,whileΔγ^twin is always smaller thanΔγ^slip.The critical twinnig stress increases with increasing V content,however,it is smaller than the critical Peierls stress of full-dislocation slip.In this paper,the effect of alloying element V on the displacive phase transition and plastic deformation mechanism inβTi-V alloys were systematically studied by using the first principle method,which help confirming the critical concentration range for the phase stability of different phases in Ti-V alloys,revealing the reason for the change of the mechanical properties in Ti-V alloys,explaining the formation reasons of the displacive phase transition production in experimentally,clarifying the competition among the dislocation glide in different slip systems and between the dislocation slip and deformation twinning in Ti-V alloys.