The steady-state creep characteristics of 78Ni-22Cu solid solution alloy have been investigated over the temperature range of 595-1050˚K (0.36-0.64Tm) and at the stress levels from 10 to 392 MPa. Measured activation energy for the creep deformation of the alloy under the given conditions was found to vary with temperature.
Namely, at high temperatures and low stresses, the creep activation energy is that of lattice diffusion (280kJ/mole), with stress exponent n=3, while at intermediate temperatures, the activation energy corresponds to that for pipe diffusion (188kJ/mole), with n=4.5.
The experimental data obeyed the following creep equation and good agreement was obtained between predicted and observed creep rates.
Where, E: Young's modulus, $D_L$: lattice diffusion coefficient, $D_p$: pipe diffusion coefficient, and k: Boltzmann's constant.
The experimental results suggest strongly that the underlying creep mechanism of Cu-Ni alloy is the same at both high and intermediate temperature and best explained by localized climb of edge dislocation, which can be occured by lattice or core diffusion.
And the trend that the activation energy for creep decrease with increasing stress was readily explained by the concept of an effective diffusion coefficient.
The second major purpose of this study was to determine the effect of the miscibility gap on the creep behavior at low temperature (0.36Tm).
Cu-Ni system is classified as group Ⅱ solid solution and this was validated in this study.
However, departures from the normal transient creep behavior generally associated with class Ⅱ alloys have been recorded at low temperature.
This experimental results obtained are compatible with free energy calculations showing the presence of the miscibility gap at low temperatures.