Al-0.2%Zr合金の時効析出の組織観察

抄録

Structural changes during aging of an Al-0.22wt% Zr alloy at 350, 400 and 450°C for 8-10000 hours were studied by means of optical and electron microscopes. In the initial stage of aging, dendritic or rodshaped precipitates were formed behind moving boundaries. Growth of these precipitates due to a grain boundary reaction was retarded by development of precipitation within grains. The dentritic or rod-shaped precipitates had an ordered cubic crystal structure of L1₂ type, a metastable Al₃Zr which did not change during the aging interval employed in the present study.
Precipitation of the equilibrium phase of Al₃Zr with ordered tetragonal structure of DO₂₃ type was also found at grain boundaries by prolonged aging. A new orientation relationship between the equilibrium Al₃Zr phase and the matrix was established.
Coherent spherical particles were also observed within grains. The particles were round to nucleate homogeneously at 350°C. However, at higher temperature aging, they showed tendency to nucleate preferentially at dislocations or original grain boundaries. Growth of these precipitates by prolonged aging accompanied loss of coherency and occurred preferentially along the <100> and <100> matrix directions, resulting in fobrmation of protrusions. It was found that these precipitate were Al₃Zr with L1₂ type structure. Plate like coherent precipitates, elongating along the <100> matrix directions, were sparsely observed within grains. These precipitates grew large and lost coherency with the matrix by generations of interfacial dislocations around them.