Revealing the local lattice strains and strengthening mechanisms of Ti alloys

In this work, effects of solute atoms (X) on lattice parameters, bulk modulus, enthalpy of formation, lattice distortion energy, electron work function (EWF) and bonding morphology/strength of HCP Ti95X are comprehensively studied by first-principles calculations. Here, X includes the alpha-stabilizer Al, and the beta-stabilizer Cr, Mo, V and Nb, which are commonly combined in the high-strength Ti7333 and Ti5553 alloys. Attributing to various atomic size and number of valence electrons of these solute atoms, the mechanical (lattice distortion) and the chemical (solute atom) contributions to the local lattice strains are clearly distinguished in terms of lattice distortion energy and bonding charge density. It is found that the equilibrium volume of Ti95X decreases linearly with the increased HCP volume of each solute atom. The less change of volume yields minimum lattice distortion energy. Moreover, a higher value of Delta rho caused by the electron redistributions of solute atoms than the matrix indicates an improved bonding strength via the coupling effects of lattice distortion and valence electrons. The bonding strength of Ti95X increases in the order of Ti-Al < Ti-Cr < Ti-V < Ti-Nb < Ti-Mo. According to the available measured yield strength of Ti-Al alloys, the proposed a power-law-scaled relationship in terms of EWF and grain size results in the predicted yield strength agree well with those experimental data. This work provides an atomic and electronic basis for the solid solution strengthening and grain refinement hardening mechanisms, paving a path accelerating the development of advanced high-strength Ti alloys.