Viscoelastic softening of copper and zinc at high homologous temperatures under seismic-frequency cyclic loading

There is an absence of high pressure experimental data relevant to seismology on metals and alloys close to the liquidus, which hampers our understanding of the composition and structure of the Earth's inner core. Here, we address this by performing high-pressure cyclic deformation experiments on hexagonal close-packed zinc and face-centred cubic copper up to melting, as analogues for iron. Samples, together with an elastic reference, were heated to the liquidus at 3–4 GPa while simultaneously being subjected to cyclic deformation at strains of 10−4–10−5 at frequencies of 0.1–0.3 Hz. X-radiographs were used to monitor changes in sample length throughout the experiments, from which Eω (the finite-frequency Young's modulus) was derived. We report a phenomenon in which both samples exhibit significant softening under approximately constant stress amplitudes at homologous temperatures above ∼0.85. Softening is consistent with previous experimental reports of rapid recrystallization in metals. The alignment of Eω with the minimum elastic Young's modulus suggests that small sinusoidal stresses may have the ability to induce a preferred orientation in metals near the liquidus.