EFFECT OF COHERENCY STRAINS ON PHASE-SEPARATION IN THE ALN-AL2OC PSEUDOBINARY SYSTEM

AlN and Al2OC, both of 2H structure, form a homogeneous solid solution at elevated temperatures. Annealing at lower temperatures leads to the formation of two solid solutions, one AlN-rich and the other Al2OC-rich. Samples of near equimolar composition form disk-shaped precipitates upon annealing at temperatures below approximately 1900-degrees-C. In the present work, samples of several compositions were fabricated by hot-pressing mixtures of AlN, Al4C3 and Al2O3. Lattice parameters of the resulting solid solutions were measured by X-ray diffraction. Samples of an equimolar composition were annealed at 1500-degrees-C for various periods of time up to a maximum of 512 h. Disk-shaped precipitates with their axes along the [0001] direction were initially coherent. In the coherent stage, the a lattice parameters of the precipitate and the matrix were identical because of coherent fitting or lattice planes. The shape of the precipitates and their orientation were in accord with predictions based on theoretical calculations of coherency strain energy. Using the experimentally measured values of the aspect ratios, the precipitate/matrix interfacial energy for coherent precipitates was estimated to be on the order of a few mJ/m2 . Long-term annealing led to the loss of coherency as manifested by the formation of interface dislocations.