Ordering behaviors and age-hardening in Cu_0.5Au_0.5-xNi_x alloys

The effect of partial substitutions of nickel for gold in an equiatomic CuAu alloy on the CuAu I ordering and age-hardening behaviors were investigated by electrical resistivity measurements, x-ray diffraction, transmission electron microscopy, and hardness testing. Age-hardening in Cu_0.5Au_0.5-xNi_x alloys with nickel content of up to about 12 at.% was considered to be attributed to the coherency strain between ordered nuclei and the surrounding matrix. Upon progressive ordering, the coherency strain was removed by the occurence of twinning, and this brought about a slight decline in hardness at the initial stage of overaging. The growth of antiphase domains in twin platelets gave rise to a further decrease in hardness at the later stage of overaging. The growth rate of anitphase domains in twin platelets apparently decreased with increases in the nickel content. Because of this fact, a softening rate of a Cu_0.5Au_0.5-xNi_x alloy in the overaging stage decreased as the nickel content increased.