Lattice Imaging of θ′ Precipitate-Matrix Interfaces in Al-4 mass% Cu Alloy

The two-beam and many-beam lattice imaging techniques of transimission electron microscopy have been utilized to study the interfacial structures between the semi-coherent θ′ precipitates and the Al-rich matrix in an Al-4 mass% Cu alloy. The (200) lattice fringes are continuous from the matrix toward the θ′ precipitate along the broad faces of the θ′ precipitates, indicating complete coherency along this cube direction. For an edge-on θ′ precipitate, the (002) lattice images show 0.202 nm spacings in the matrix and 0.29 nm spacings in the θ′ precipitate. This proves the tetragonality of the θ′ precipitate, and it is along this cube direction that θ′ is incoherent with the matrix.The laser optical diffraction patterns indicate that there exists a residual strain field at the θ′-matrix interface which is possibly associated with a composition gradient near the interface. This result is also confirmed by the many-beam lattice images. The STEM/EDS analysis shows that the Cu concentration in the θ′ precipitate (∼10–20%) is lower than that from X-ray results. This is due to the thinness of the θ′ precipitate (∼15 nm) which is approaching the resolution limit of this technique that the matrix is contributing to the results. The matrix composition, however, is in agreement with X-ray data.