Abstract
Precipitation and hardening of the chill cast alloys during aging were studied by means of hardness measurements and metallography. Decomposition of the supersaturated solid solution during aging occurs by forming a spherical intermediate cubie Al3Zr phase with Ll2 type crystal structure. The spherical particles are slowly coarsened and then preferentially grow along the definite directions in the same manner as in the aging process of subperitectic Al-Zr alloys. A small addition of Si remarkably accelerates precipitation of the alloy. An addition of Si increases the maximum hardness. The time required for 90Å of precipitate radius vs aging temperature was determined. The resulting Arrhenius plots yield activation energies for diffusion of Zr in Al ranging from 51 to 52 kcal/mol over the composition range.
