Effects of ultrastrong gravitational field on the crystalline state of a Bi-Sb alloy

An atomic-scale graded structure has been formed previously in a $Bi70Sb30$ (at. %) alloy that is miscible in all proportions, by sedimentation of substitutional solute atoms under an ultrastrong gravitational field up to $1×106 G$ at 220–240 °C [T. Mashimo, T. Ikeda, and I. Minato, J. Appl. Phys. 90, 741 (2001)]. In this study, additional megagravity field experiments were performed on the $Bi70Sb30$ alloy and pure Bi at different temperatures below their melting points, to investigate the change in crystalline state under the ultrastrong gravitational field. For the $Bi70Sb30$ alloy ultracentrifuged at 191–205 °C, no change in composition was observed, and the grain sizes of the crystals decreased from several millimeters to tens of micrometers, while no distinct change in grain size was observed for the pure Bi ultracentrifuged under the same experimental conditions. The $Bi70Sb30$ alloy ultracentrifuged at 220–240 °C consisted of two regions with different morphologies–fine-grained crystals with grain sizes in the range of tens of micrometers in the low gravity region, and large crystals with grain sizes several millimeters long and hundreds of micrometers wide along the direction of gravity in the high gravity region, where sedimentation of atoms was confirmed. The large crystals with hexagonal structures were formed by preferential crystal growth roughly along the c axes, and a large strain that increased as the gravitational field increased existed inside these crystals. Formation of this anomalous crystal state might be correlated with the sedimentation of atoms.