Low temperature copper solubilities in Fe–Cu–Ni

An atom probe characterization of the copper concentration in the matrix of a model Fe–1.1at.%Cu–1.4% Ni alloy has been performed after isothermal ageing for various extended times at low temperatures. The matrix copper concentrations from material annealed at 500, 550 and 600°C are approximately 10% lower than the equilibrium values predicted from the Thermocalc™ program with the Kaufman database. Isothermal annealing for 4000 h at either 400 or 300°C was not sufficient to attain the equilibrium copper concentration. The kinetics of copper precipitation was investigated with the Starink-Zahra model derived for nucleation and growth reactions and a good fit with the atom probe data was found for an activation energy of 250 kJ mol−1.