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The effect of calcium on the aging behavior of 6061 alu- minium
alloy was investigated by adding calcium of various contents in
the alloy. Both aging temperature and aging time were changed
in the aging treatment. The results show that when the addition
of calcium is within 0.1wt%, the time to peak aging is delayed.
When the calcium addition reaches 0.2 wt%, the peak hardness
reduces substantially and the peak aging time is shortened
again. It is thus suggested that the effect of calcium addition
on delaying aging is effective until a critical calcium content
in-between 0.1wt% and 0.2 wt%. The aging behavior will
disappear when the calcium ad- dition reaches 0.3wt%. The
metallugraphy and X-ray diffraction of the as-cast samples show
that calcium and silicon formed CaAlSi. or CaAlSi during
solidification.TEM/EDX analysis confirms the compound as
CaAlSi.. According to the TEM result, the delay of aging
behavior rised from two possible reasons. First,since silicon
and calcium form CaAlSi., the con- centration of silicon in the
matrix reduced, giving rise to a lower solute flux. Secondly,
the increase of dislocation density leads to more vacancy
annihilations. It redeces the effect of vacancy-assisted solute
diffusion and hence causes the delay in aging behavior.
Moreover, silicon clusters nu- cleated on the dislocations will
relax coherence strain en- ergy around the precipitates and
slow down the transforma- tion rate from G.P. zones to .beta.
prime. This may be a minor factor of the delay in aging
behavior. The disappearance of aging behavior should be due to
the formation of mass CaAlSi.which deplets the precipitation
hardening element, Si. In other words, it suppresses the
precipitation of G.P. zones and .beta. prime.
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