A Kinetic Study of Order-Disorder Transition in Ni–Cr Based Alloys

Alloy 690 is a nickel-based alloy (60% Ni, 30% Cr, 10% Fe) used in nuclear Pressurized Water Reactors (PWR) for different components and welds (steam generator tubes etc.). They are subjected to thermal ageing up to 60 years which could lead to an order-disorder transition (Ni2Cr ordered phase formation) by a diffusion-assisted mechanism. This transformation might modify mechanical properties and is suspected to influence the stress corrosion resistance of the affected components. To study ordering kinetics, hardness, thermoelectric power (TEP) alongside transmission electron microscope (TEM) observations were conducted on Ni-33%Cr alloys with different iron contents (0–3 wt%) after various ageing thermal treatments. The ordering activation energies have been determined: they are found to be independent of the iron content. A correlation between macroscopic properties and TEM diffraction results is proposed. Finally, the distribution of iron between matrix and ordered domains was studied.