Order–disorder reaction in Alloy 600

Differential scanning calorimetric tests were carried out in Alloy 600 from room temperature to 800°C on various specimens, in order to investigate the origin of the exothermic and endothermic reactions that occur during thermal analyses. Since it is reasonable that there is a Ni2Cr short-range order in Alloy 600, which is a Cr-rich Ni-base alloy, the differential scanning calorimetry (DSC) results are interpreted on the existence of order–disorder reaction relevantly. The exothermic reactions were observed at lower temperatures in the cold worked specimen, compared to the quenched one. However, the endothermic reactions were observed at 490–580°C in all specimens. Especially, a convex endothermic reaction was observed at about 500–600°C for a specimen aged at 474°C for 1100 and 10 000 h. The extent of energy released by the exothermic reaction occurring at 430–520°C in a specimen quenched after a 874°C–1-h treatment was 8.4 J g−1, which is nearly the same as the energy of ordering for the Ni–25%Cr alloy. In addition, the activation energy for an exothermic reaction in a specimen quenched after a 1095°C–1/2-h treatment was Q=190 kJ mol−1 (2 eV atom−1), which suggests that this exothermic reaction is related to a substitutional diffusion process. It was concluded that the exothermic and endothermic reactions in quenched Alloy 600 are due to the ordering and disordering reactions, respectively. It seems that the existence of ordering in Alloy 600 should be considered in the physical metallurgy of Alloy 600.