Anelastic attenuation in cubic and hexagonal iron alloys: implications for the core

Grain boundary processes, phase transitions, line defects and point defects may all play a part in controlling the anelastic properties of iron in Earth's core. We have explored the high temperature behaviour of Fe-Ni alloys to investigate the possible role of phase interfaces at phase transitions between cubic phases of iron. The role of point defects has been considered by analogy with previous measurements on hexagonal metals. The mechanical properties of Fe-Ni alloys were studied by torsion pendulum at seismic frequencies from 0.01 to 1 Hz over temperatures from 30 C to 1000 C and atmospheric pressure. The cubic Fe-Ni system undergoes an equilibrium fcc-bcc phase transition for Ni concentration < 5 at.%, but shows a metastable martensitic phase transition from austenite to a distorted bcc martensite when there is more than 5 at.% Ni in the alloy. Mechanical properties show significant variation at the phase transition temperature: significant softening in shear modulus, abrupt change in elastic strain, and massive energy dissipation. Variation in elastic constant as a function of temperature defines the shear modulus, while the volumetric difference between the fcc and bcc structure contributes to a sudden variation in strain at the transition. The movement of interfaces (phase interfaces and martensite variant interfaces) is the key mechanism for anelastic damping at the transition. An intermediate mixture of phases (bcc + fcc or martenste + fcc) was identified over a wide range of temperatures, indicating the importance of phase nucleation process at the transition. For a pure hexagonal iron core there remains the possibility of energy dissipation by an intrinsic atomistic process. While no measurements of anelastic relaxation have been made in hexagonal iron, earlier studies of zirconium and titanium (hexagonal metals).Reorientation of pairs of interstitial atoms or even interstitial-substitutional pairs of atoms provides a mechanism for anelastic relaxation in hexagonal iron, which would be expected to show anisotropy in textured polycrystalline material. Deformation-induced tinning in hexagonal iron provides a further mechanism for energy dissipation through the movement of twin interfaces.