Direct observations of defects in quenched gold

Abstract

An electron microscope transmission study has been made of thin foils of gold produced by electro-polishing from quenched gold specimens. Thin gold sheet was quenched from a vertical tube furnace at 910°c to 960°c into iced brine. Subsequent ageing treatment was carried out, for example for an hour at 100°c. Contrast effects were observed of geometrical shapes with sides parallel to [110] projections in the plane of the foil. Analysis of the shapes of the contrast in two different orientations shows that they arise from tetrahedra of stacking faults on (111) planes with 1/6[110] type stair-rod dislocations along the edges of the tetrahedron. A reaction is pointed out whereby the defect may arise from an equilateral triangle, with [110] edges, of stacking fault surrounded by a Frank sessile dislocation on a (111) plane. On the assumption that the triangles were formed by the disc-like condensation of vacancies, the size of the defects, about 360 Å, and the density, about 5 × 10¹⁴ cm^−s, correspond to an initial vacancy concentration of 6 × 10⁻⁵. The interaction of dislocations with the defects confirms the hypothesis of stacking fault tetrahedra. Finally the formation of the defects and their influence upon the physical properties of the crystal are discussed.