An electron microscope investigation of the interfacial structure of semi-coherent precipitates

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Abstract
A transmission electron microscope study of four semi-coherent precipitates (θ’ in Al–Cu, S in Al–Cu–Mg, β’ in Al–Mg–Si and γ’ in Ni–Cr–Ti–Al) has revealed a misfit dislocation structure at their semi-coherent interfaces. The general visibility criteria for interface dislocations are considered and a Burgers vector analysis of the dislocation arrays on each of the precipitates has been carried out. The dislocation structure of γ’ interfaces consists of hexagonal networks of a/2[110] dislocations, the S and β’ needles have arrays of dislocation loops along their length having Burgers vectors of the type a/2[101] for needles with axes parallel to [100], while the misfit across the flat interface of θ’ discs is accommodated by orthogonal sets of dislocation loops having Burgers vectors a[100] and a[010] for an (001) disc. The mechanism of loss of coherency has been studied in each case. The γ', S and β’ precipitates all lose coherency by the attraction of matrix dislocations to the particle/matrix interface followed by climb and interaction of the dislocations at the interface. θ’ loses coherency by the nucleation of dislocation loops within the precipitate, probably from collapsed point defect clusters, followed by climb of loops to the particle/matrix interface. The rate of loss of coherency of γ', S and β’ precipitates is controlled by the supply of matrix dislocations and, in the absence of these, the particles can grow to large sizes while retaining (metastable) coherency. Creep deformation under these conditions can lead to a rapid loss of coherency due to the introduction of a plentiful supply of matrix dislocations.

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