Relative stability of amorphous phase versus solid solution in the Ni-Ti system revealed by molecular dynamics simulations

We employ molecular dynamics simulations with n- body potentials to calculate the energies of solid solutions and metallic glasses in the Ni-Ti system to reveal relative stability of two phases against the change of solute concentration. For Ni1-xTix alloys, the calculations show that the energies of the solid solutions are lower than the amorphous phases when x <= 0.24 and vice versa for x > 0.24, in accord with thermodynamics analysis and experiments. To reveal the underlying physics responsible for this, the evolution of structures in the solid solutions and the amorphous phases are studied via the coordination number (CN) and common neighbour analyses. The results show that, with an increase in Ti concentration, the total average coordination number is close to 12 for all solid solutions owing to the face- centred cubic (fcc) crystalline structure remaining, while it changes from less than 12 to greater than 12 for the amorphous phases, suggesting that more bonds or more fractions of large CNs (> 12) exist in the amorphous phases with a high Ti content. The relative stability of the amorphous phases versus the solid solutions revealed by the order of energy is thus correlated with the structural change of the amorphous phases.