Deformation of directionally solidified alloys: Evidence for microstructural hardening of Earth's inner core?

The viscosity of Earth's inner core (IC) plays a key role in its dynamics, being important for understanding IC convection, translation, super-rotation, and development of elastic anisotropy. However, estimates for the viscosity of the IC range from 1013 Pa·s to 1021 Pa·s. One difficulty in estimating the viscosity is that it is not simply a material property, but it depends on the rheology, i.e., the deformation mechanism, which in turns depends on factors such as the temperature, stress, grain size, and microstructure. To examine the effects of microstructure we have carried out constant strain rate torsional deformation experiments on directionally solidified hexagonal close packed (hcp) Zn-rich Sn alloys at high homologous temperature and atmospheric pressure. The directionally solidified hcp Zn-rich Sn alloys have a microstructure that consists of large, columnar, textured crystals composed of dendrites. This microstructure has been proposed for the IC, and hcp Zn at atmospheric pressure serves as an analog for hcp iron under IC conditions, including a likely basal slip (0001) <12′10>, along with some prismatic slip (101’0) <12’10> associated with twinning. The measured torque (or stress, which is a function of the geometry and deformation mechanism, which in turn depends on the grain size) continues to increase past what one would expect for steady state deformation, indicative of hardening. We are yet unclear as to the origin of the hardening, but our hypothesis is that it may involve the relatively few slip systems available in hcp systems, and the large, textured grains of directionally solidified alloys, so that not all strains are easily accommodated by the available slip systems. The semi-brittle behavior of the alloy also supports this hypothesis. An inner core with a textured, columnar microstructure might therefore be harder than estimates of its shear strength might predict.