Understanding Relaxation Mechanisms in Metals: Application to Earth’s Inner Core

The majority of metallic materials exhibit viscoelastic or anelastic behavior when subjected to elastic cyclic loading under specific temperature and frequency conditions. This anelastic nature is commonly characterized by the dissipation or loss of mechanical energy, manifested as a hysteresis loop between stress-strain signals. The energy loss is quantified by the loss tangent (tanδ) or the inverse of quality factor (Q^-1). The origin of this dissipation is associated with internal variables, particularly the microstructure, and this phenomenon is referred to as internal friction. The microstructures are inherently complex, and their overall response is governed by multiple factors such as solute type and content, crystallographic texture, dislocation density, residual stresses, and grain boundary characteristics. Consequently, any modification in microstructure directly influences the internal friction behavior. Additionally, the operating temperature and imposed frequency strongly affect the magnitude of  tanδ. This work provides a comprehensive summary of the role of microstructural parameters on the viscoelastic behavior of various metals over a wide range of length and time scales and over an extensive temperature range.

Subsequently, the understanding of internal friction in metallic materials is extended to the earth’s inner core. It is well established that inner core exists under extreme conditions, with very high temperatures (~5700 K) and extremely high pressures (~330 GPa). Under such conditions, reliable estimates of seismic wave dissipation or attenuation are not readily available. At same time, the underlying mechanisms governing seismic wave propagation remain unclear. This study provides a summary and proposes plausible attenuation mechanisms in the earth’s inner core over a range of testing conditions. These are supported by dynamic mechanical analysis (DMA) experiments and atomistic simulations.