Prolonged Aging of an Al-0.22% Zr Alloy

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  • Al-0.22%Zr合金の長時間時効
  • Al-0.22パーセント Zr ゴウキン ノ チョウジカン ジコウ

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Hardness and structural changes during aging of an Al-0.22% Zr alloy at 350, 400 and 450°C for 8∼10 hr were studied. Hardening curves at 350°C aging show a monotonous increase with increasing time and after about 6×103 hr, a pronounced peak appeared. However, at higher temperature aging, no appreciable increase of hardness was observed.<BR>In the initial stages of aging, rod-shaped precipitates were formed behind moving grain boundaries. Growth of these precipitates due to a grain boundary reaction was retarded by development of precipitation within the grains. Plate-like precipitates were sparsely observed within grains. These precipitates and the precipitates due to the grain boundary reaction have no essential role in the age hardening of this alloy.<BR>On the other hand, coherent spherical particles were also observed within grains. It was found that the growth of the spherical precipitate was diffusion controlled. From the growth rate, the activation energy for diffusion of Zr in Al has been estimated to about 50 kcal/mol. It was thought that the coherent particles are the predominant factors for the increment of hardness during aging. Growth of the spherical precipitates by prolonged aging accompanied a loss of coherency and occurred prefferentially along a definite matrix direction, resulting in the formation of protrusions.

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