Internal friction in complex ferroelastic twin patterns

We report the first attempt to study the internal friction (IF) of complex ferroelastic twin patterns by atomistic molecular dynamics simulations. At different stress amplitudes, linear and non-linear IF regimes are observed. They are separated by a pinning/depinning threshold. At external stresses below the pinning/depinning threshold, a low linear IF background is nearly independent of temperature and configuration of the twin patterns. With increasing stresses, twin boundary motions generate non-linear anelasticity where the stress dependent IF increases to a maximum and then decays. The IF is directly related to the motion of needle twins. The effects of vacancy pinning and thermal activation are studied. The IF maximum is reduced and shifted to higher stresses by extrinsic pinning effects. The IF maximum decreases further with increasing temperature from 8 × 10 ⁻⁶ T _c to 8 × 10 ⁻⁴ T _c where T_c is the ferroelastic transition temperature. The non-linear IF depends by power law on the strain with exponents near 2 below the IF maximum and -1 at strains higher than the maximum. Vacancies destroy the power law and break the scale invariance of the domain boundary movements. The overall picture obtained in this study is consistent with basic experimental observations.