The effects of cold reduction ratios on mechanical properties and precipitation phenomena were studied in Cu-4~7%Ni-3%Al alloys. After solution treatment the specimens were cold worked (i.e, 67% to 97% in thickness reduction)before various aging treatments at 250-550℃.
Utilizing transmission electron microscopy two different types of precipitate were identified: one was an incoherent NiAl Precipitated at lower temperature(i.e, 250-350℃)and the other was a coherent $Ni_3Al$ precipitated at higher temperatures (i.e, 450-550℃). The hardening due to $Ni_3Al$ precipitate was much more significant compared to that due to NiAl precipitates. The hardening due to $Ni_3Al$ was strongly dependent upon the Ni-content and upon prior cold reduction ratio. When the Ni-content was high(i.e,7%)the lower cold work ratio, the higher hardening due to NiAl precipitate. When the Ni-content was low (i.e,4%) the precipitation of $Ni_3Al$ was not clearly observed resulting very little hardness increase after 2hrs annealing at 450℃.
The finely distributed $Ni_3Al$ retards the recovery and recrystallization behavior, while hardening effect of $Ni_3Al$ was reduced if recrystallization was started.
Specimens without prior cold work showed $Ni_3Al$ precipitate size of about 250Å in diameter at maximum hardness while specimens cold worked to 67% showed $Ni_3Al$ precipitate size of 50Å at maximum hardness.
