MICROSTRUCTURE CHANGES IN ISOCHRONALLY ANNEALED ALUMINA FIBER-REINFORCED MG-AG-ND-ZR ALLOY

The commercial alloy QE22 (Mg-Ag-Nd-Zr alloy) was reinforced by 22 vol % delta-Al2O3 short fibres applying the squeeze cast technology. Preci pitation effects were studied in this material after a preceding solut ion heat treatment by isochronal annealing up to 300 degrees C by mean s of electrical resistivity, hardness and reversible stress relaxation measurements. The annealing response of the properties was compared t o the annealing response of the unreinforced matrix alloy. The microst ructure changes were studied in detail by transmission electron micros copy. A sharp drop of resistivity between 180 and 280 degrees C was fo und on normalised resistivity annealing curves of both reinforced and unreinforced specimens due to the redistribution of solutes. In compos ites the fibres act as nucleation centres in the precipitation process promoting e.g. precipitation of Al2Nd or Ag compounds. The Al content in the matrix is enhanced due to the decomposition of the preform bin der. The evolution of the particle population inside the grains involv es the formation of new Al2Nd-like cubic particles between 120 and 180 degrees C. Above 180 degrees C these particles are continuously subst ituted by hexagonal beta-phase and/or tetragonal Mg12Nd particles. Thi s process finishes at 300 degrees C by the transformation of all new p articles to semicoherent Mg12Nd precipitates. The precipitation proces s in grain interiors of the unreinforced ahoy is different involving o nly change of the morphological features of tetragonal semicoherent Mg 12Nd particles existing in the alloy already in the initial state afte r solution heat treatment.