KINETICS OF THE DECOMPOSITION OF AUSFERRITIC STRUCTURE IN COPPER-NICKEL ADI

To determine the kinetics of ausferrite decomposition in copper-nickel ADI, a kinetic model of Johnson-Mehl-Avrami (JMA), designed for the investigations under non-isothermal conditions, was used. Applying the Netzsch Thermokinetics 3 programme and the method of linear regression, the experimental data obtained by differential scanning calorimetry (DSC) were fitted. The obtained activation energy values were higher than the activation energy of carbon diffusion in austenite and were comprised in the range of 150-240 kJ/mole, depending on the temperature of isothermal transformation, thus showing good agreement with the literature data. In the examined alloys, the influence of copper and nickel on the kinetics is not significant, but with increasing nickel content at low concentrations of copper, the activation energy of the thermal decomposition of ausferrite increases, which indicates an increase in its stability. In terms of the alloying elements impact on the thermal stability of ausferrite, the composition of copper-nickel ADI equal to 0.5 wt. % Cu and 1.5 wt. % Ni seems to be optimal. The obtained results also lead to the conclusion that the simultaneous increase of Cu and Ni content in the copper-nickel cast iron is not intentional.

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