A First-principles Study On The Stability Of β And γ Phases In TiAl-Cr Alloys And Their Mechanics Properties

As the first step of the β-γ TiAl alloy designing,a first principles plane-wave psendopotentials method based on Density Functional Theory(DFT)has been used to investigate the site preference of Cr in β and y-TiAl.The effect of Cr on structural stability of P and y TiAl intermetallic systems and the basic mechanics properties of P-TiAl-xCr and y-TiAl-xCr alloys under the site preference of Cr are also studied.A reasonable explain has been given on the basis of the analysis of the density of states(DOS)and charge density.The results show that:(1)Cr is a strong β stabilizer(B2 structure),the stability of P-TiAl-12.5Cr alloy is the strongest.Due to the limited solid solubility of Cr element in the y-TiAl(L10 structure)phase,the stability of y-Ti50Al50-xCrx alloys is decreased as the increase of Cr addition.Cr prefer to occupy Al sites in B2-TiAl crystal,and substitute for Ti atoms in Ll0-TiAl crystal.(2)It is found that the concentration of Cr atoms to stabilize P phase in stoichiometric 3-Ti50A150-xCrx and Ti-rich P-Ti56Al44-xCrx alloys are 10%～15%and 9%～20%,respectively.Cr addition can enforce remarkably the strength and hardness of stoichiometric β-Ti50Al50-xCrx alloys,and their strengths rise with the increase of Cr-alloying concentration,but its strengthening effect on Ti-rich β-Ti56Al44-xCrx alloys is not obvious.The increasing Cr-alloying concentration can improve significantly the ductility and toughness of Ti-rich P-Ti56Al44-xCrx alloys,whereas no distinct toughening effect induced by Cr-alloying can be observed in the case of stoichiometric β-Ti50Al50-xCrx alloys.(3)With no addition of Cr element,the strength and hardness of Ti-rich y-Ti56Al44 and Al-rich γ,-Ti44Al56 alloys are weaker than stoichiometric γ,-Ti5oAl50 alloys;the ductility of the three alloys is in the order of Ti-rich γ-Ti56Al44>stoichiometric γ-Ti50Al50>Al-rich γ-Ti44Al56.With addition of Cr element,the ductility of stoichiometric y-Ti50-xAl50Crx(x=2.25%,especially)alloys is improved significantly,while the strength is decreased.The strength of Ti-rich γ-Ti56-xAl44Crx alloys is decreased remarkably,whereas no distinct toughening effect induced by Cr-alloying can be observed.The strength of Al-rich γ-Ti44-xAl56Crx alloys is improved significantly with the ductility unchanged,and remain good as the increaseing Cr-alloying concentration.