Low Young’s modulus achieved by anelastic relaxation arising from reversible atomic shuffling in bcc Ti–Nb alloys

Metastable body-centered cubic (bcc) Ti alloys are attractive biomedical implant materials because they exhibit a low elastic modulus which helps mitigate bone degradation. The low elastic modulus in these alloys is believed to be correlated with a low-stability bcc structure. However, the physical nature of this correlation remains unclear. In this study, we show that the low elastic modulus of bcc Ti–Nb alloys originates from the anelastic relaxation driven by reversible atomic shuffling events that act as precursors to the ω (hexagonal) and α ′ ′ (orthorhombic) martensitic transformations. A combination of molecular-dynamics simulations and measurements of Young’s modulus and internal friction revealed that the reversible atomic shuffling events have a low average activation energy of 0.20 eV, leading to significant anelastic relaxation even at room temperature ( ∼ 300 K), thereby lowering the elastic modulus. The reversible atomic shuffling events occur in sub-nanometer-scale, low-stability bcc regions that are depleted in the bcc-stabilizing element Nb. These regions originate from quenched-in statistical compositional fluctuations, which exist even when the constitutive elements are randomly distributed. Thus, controlling the reversible atomic shuffling events by manipulating the compositional fluctuations and the resultant local chemical composition is an effective strategy for lowering the elastic modulus of biomedical bcc Ti alloys.