Insight into solid-solution strengthened bulk and stacking faults properties in Ti alloys: a comprehensive first-principles study

In the present work, the effect of solute atoms on the lattice parameters, atomic volume, stacking fault energies (γ_SF), bulk modulus, and bonding structures of HCP Ti is studied comprehensively by first-principles calculations. Here, the alloying effects on the growth fault (I1), deformation fault (I2) and extrinsic fault (EF) are considered, with the solute atoms (X = Al, Cr, Mo, Nb, and V) commonly utilized in the high-strength Ti-7333 and Ti-5553 alloys selected. It is found that the stacking fault energies of pure Ti increase in the order of γ_I1 < γ_I2 < γ_EF, which is proportional to their corresponding numbers of fault layers. The variation tendencies of γ_SF of the binary Ti–X alloys are in the order of Al > V > Cr > Mo > Nb for I1 and I2 and V > Al > Cr > Nb > Mo for EF, respectively. The bonding charge density is utilized to characterize the electronic redistributions caused by the fault layers and the lattice distortions. It is presented that the rod-type bonds of the non-fault layers change into the tetrahedral-shaped bonds of fault layers, displaying the local HCP–FCC-type phase transformation. With the addition of various solute atoms with different atomic size and valance electrons, the bond strengths of Ti–Al and Ti–Nb are weaker than those of Ti–Cr and Ti–Mo as there are fewer densities of bonding electrons. This work gains some insights into the atomic and electronic basis for the solid-solution strengthened bulk and stacking faults of HCP Ti, providing fundamental information to the development of advanced high-strength Ti alloys.