Morphological and Crystallographic Characteristics of Incoherent Octahedral FCC Co Precipitates in a Cu Matrix

Abstract

A Cu–3.0 mass%Co alloy was heat-treated by a method named “short-cut aging”. Incoherent fcc Co precipitates of about 200 nm in size formed in an fcc Cu matrix were observed with conventional and ultra-high-voltage transmission electron microscopes. Although the Co precipitates had shapes similar to a regular octahedron composed of {111} of the Cu matrix, small deviations from the regular octahedron were found. Most Co/Cu interfaces deviated from {111} of Cu and the average deviation angle δ was about 7 deg. Dislocations in the Cu matrix intersecting the Co precipitates were observed for those having deviated interfaces. Deviations of the orientation relationship from an exact cube-on-cube relationship were also found. Although the deviation angle θ of the orientation relationship was much smaller than δ, there was a linear relationship between θ and δ. The morphological and crystallographic characteristics of the octahedral Co precipitates are discussed by considering dislocations in the Cu matrix intersecting the Co precipitates.