Extension of solid solubility in magnesium by rapid solidification

Abstract

Lattice parameters of the extended solid solution of 22 elements in magnesium made by rapid solidification have been monitored by X-ray diffraction using the Guiner technique. Accordingly, solid solubility in magnesium has been extended by factors ranging from 1.5 for MgAg to about 1000 for MgBa compared with the corresponding equilibrium solid solubility. Major decreases in the axial ratio c/a of magnesium have been found for its extended solid solution. Except for samples of the binary systems MgGd, MgY and MgAl, all samples showed a two-zone microstructure often associated with two distinct lattice parameters. Following the Mullins-Sekerka criterion for absolute stability, the degree of maximum extension of solid solubility observed was found to correlate qualitatively with kinetic parameters rather than with parameters underlying equilibrium solid solubility. Maximum solid solubility extension observed in the systems MgSr, MgBa and MgNi, however, was associated with the occurrence of a two-phase field (solid solubility extension gap) at lower concentrations and is interpreted as a result of solute trapping forming the upper boundary of the extension gap.