Four Cu-Ni alloys (pure Ni, 78Ni-Cu, 55Ni-Cu, 25Ni-Cu) were crept to investigate the effect of thermodynamic factor (1+dln. $_\gamma$/dln X) on the creep behavior at intermediate temperature.
Two temperatures, 720K and 600K were chosen such that creep was predicted to be controlled by pipe diffusion rather than lattice diffusion.
Results showed that though constant elastic modulus compensated stress (σ/E) was applied 55Ni-Cu alloy had the minimum creep rate among the prepared alloys at 720K. While 78Ni-Cu alloy had the minimum value at 600K.
To explain the above results, proper creep equation is proposed by
$\dot{\epsilon}=A(Eb/kT)(D + f_pD_p) (\sigma/E)^3$
where
$D_p=(D^{\ast}_{p,Ni}X_{Ni}+D^{\ast}_{p,Cu}X_{Cu})(T.F)$
Then not only creep rate, but also the measured activation energy Q and stress exponent n obeyed the creep equation with good correspondency.
And cyclic creep rate is lower than static creep rate at high temperature and low stress, while the former is higher than the latter at low temperature and high stress. The reason of this is though to be the effect of excess vacancy.
Another interesting result is that where T.F is minimized, the cyclic effect tend to be minimized.
