PRECIPITATION HARDENING AND RECRYSTALLIZATION IN CU-4-PERCENT TO 7-PERCENT-NI-3-PERCENT-AL ALLOYS

The effects of microstructure on mechanical properties in three cold-worked Cu-4% to 7% Ni-3% Al alloys have been investigated by changing ageing time at 500-degrees-C. Hardness and strength in the Cu-7% Ni-3% Al and Cu-5.5% Ni-3% Al alloys increase with ageing time and have maximum values at an ageing time of 10(3)-10(4) s at 500-degrees-C, then decrease. During ageing of Cu-7% Ni-3% Al at 500-degrees-C, the coherent Ni3Al phase was first precipitated out and later incoherent NiAl phase was formed. Ni3Al formed during the initial stage of ageing is likely to be a transient phase. The increases in hardness and strength are due to the precipitation of coherent Ni3Al phase. Coherent Ni3Al particles are effective in increasing the strength and retarding the recrystallization process. On the other hand, the hardness and strength in the Cu-4% Ni-3% Al alloy gradually declined with ageing time. Only incoherent NiAl phase was formed during ageing at 500-degrees-C. Decreases in hardness and strength in the Cu-4% Ni-3% Al alloy are attributed to softening during recovery and recrystallization, because incoherent NiAl particles have an insufficient effect to increase the strength.