The determination of the lattice parameters of the ?’structure formed in an Al–Cu alloy

Abstract

The loss of coherency of θ’plates in an Al-4% Cu alloy has been studied by transmission electron microscopy. Moiré patterns show that the loss of coherency is accompanied by a misfit between the precipitate and matrix lattices parallel to (001) θ’planes. The misfit calculated from the moiré spacing agrees with the misfit calculated by Weatherly and Nicholson (1968) from the dislocation spacing and confirms that the Burgers vectors of the interfacial dislocations are of the type a [100]. The ambiguity in the sign of the misfit has been resolved by measuring the parameters of both phases by x-ray diffraction. The a parameter of θ’is larger than that of the matrix which is consistent with the hypothesis of Weatherly and Nicholson (1968) that coherency is lost by the collapse of vacancy clusters in the θ’structure. The following unit cell parameters were determined for the θ’structure by combining the misfit and the x-ray data: a=4·077 Å, c=5·81 Å. It is suggested that vacancies may be present in the θ’structure to enable it to form coherently with the matrix.