Journal Article

Experimental Observation of a New Attenuation Mechanism in hcp‐Metals That May Operate in the Earth's Inner Core

Michalik, Stefan, Whitaker, Matthew L., Walker, Andrew M., Parsons, Andrew J., Lord, Oliver T., Armstrong, Lora S., Lloyd, Geoffrey E., Hunt, Simon A., Wheeler, John, Schardong, Lewis, Stackhouse, Stephen, Fenech, Danielle M.

2024

Overview

Seismic observations show the Earth's inner core has significant and unexplained variation in seismic attenuation with position, depth and direction. Interpreting these observations is difficult without knowledge of the visco‐ or anelastic dissipation processes active in iron under inner core conditions. Here, a previously unconsidered attenuation mechanism is observed in zinc, a low pressure analog of hcp‐iron, during small strain sinusoidal deformation experiments. The experiments were performed in a deformation‐DIA combined with X‐radiography, at seismic frequencies (∼0.003–0.1 Hz), high pressure and temperatures up to ∼80% of melting temperature. Significant dissipation (0.077 ≤ Q−1(ω) ≤ 0.488) is observed along with frequency dependent softening of zinc's Young's modulus and an extremely small activation energy for creep (⩽7 kJ mol−1). In addition, during sinusoidal deformation the original microstructure is replaced by one with a reduced dislocation density and small, uniform, grain size. This combination of behavior collectively reflects a mode of deformation called “internal stress superplasticity”; this deformation mechanism is unique to anisotropic materials and activated by cyclic loading generating large internal stresses. Here we observe a new form of internal stress superplasticity, which we name as “elastic strain mismatch superplasticity.” In it the large stresses are caused by the compressional anisotropy. If this mechanism is also active in hcp‐iron and the Earth's inner‐core it will be a contributor to inner‐core observed seismic attenuation and constrain the maximum inner‐core grain‐size to ≲10 km. Plain Language Summary The Earth's inner‐core is the most remote and inaccessible part of our planet. Knowledge of the inner‐core's structure comes from interpretation of the information held in seismic waves that have passed through the inner‐core. These waves show measurable variation in wave speed and damping with depth. To investigate the wave damping in the inner‐core we performed experiments that mimicked the passage of seismic waves through zinc. Zinc was used as a low‐pressure analog because it has the same crystallographic structure as the iron in the inner‐core. In these experiments, we observed new behavior in the zinc samples that can only be explained by the behavior of different directions within the zinc crystal lattice. These we named “elastic strain mismatch superplasticity” and if the same phenomena occurs in the Earth's inner‐core it could explain the seismic observations. Key Points Zinc, a low pressure analog for hcp‐iron, deforms by internal stress superplasticity during small amplitude sinusoidal‐strain deformation Internal stress superplasticity due to mechanical oscillations has not been previously reported Internal stress superplasticity is another attenuation mechanism that could be active in the Earth's inner‐core

• Publisher: John Wiley & Sons, Inc,American Geophysical Union,Wiley
• Subject: anelasicity / Anisotropy / Cyclic loading / Deformation / dissipation / D‐DIA / Earth / Earth core / Experiments / Geochemistry & Geophysics / High pressure / inner‐core / Iron / Microstructure / P-waves / Pressure / Radiography / Seismic activity / Seismic attenuation / Seismic waves / Strain / Wave damping / X‐radiography / Zinc