Analyses and interpretations of X-ray diffraction effects in patterns of aged alloys

The limitations of the various X-ray diffraction methods which have been used to study the structure of aged alloys are discussed. A method which employs a stationary single crystal and characteristic radiation is described. The method is applied to the structures in aged Al-Ag and Al-Mg-Si alloys. Evidence for one- and two-dimensional diffraction is reported for both alloys. Limitation of particle dimensions is proposed as a general explanation of the diffraction effects; this is the most suitable explanation for patterns of Al-Mg-Si alloys. Reciprocal-lattice points for thick particles of the precipitate structure evolve from rods for thin platelets, and evolve earlier from planar areas in the reciprocal lattice corresponding to particles that have only one resolvable dimension. A change in structure to that of the precipitate has occurred at the earliest detectable stage. Indefinite periodicities in the matrix structure cannot account for the data for either this alloy or for a previously studied Al-Mg alloy. The interpretation based on particle size is also adequate for other alloy systems, such as Al-Cu and Al-Ag, in which the diffraction effects of the precipitate are more intimately related to those of the matrix. The existence of predominantly one-dimensional particles prior to the platelike particles in the sequence of growth is identified. This is a new concept in the theory of the precipitation process.