The effects of cold work on the ordering reaction in Alloy 600

Recently it is reported that there is ordering reaction in Alloy 600 and that the exothermic reaction near 500 .deg. C in the water quenching specimen originated from formation of short range order (SRO). Marucco and Nath have investigated and concluded that most commercial Ni.Cr based alloys like Alloy 600 have an ordering reaction based on Ni2Cr at temperatures below 550 .deg. C. Ordering is known to have the following consequences: (i) dimensional instability due to lattice parameter changes; (ii) electrical and thermal characteristics; (iii) changes in mechanical properties due to pile-ups of dislocations; and (iv) phenomenon of negative creep or material contraction under load. Completing the picture further, Lang et al. found that order and precipitation phenomena in Ni.Cr based alloys is strongly dependent on thermo-mechanical treatments producing different metallurgical states. Slow cooling from solution annealing temperatures produces a rather advanced precipitation stage and a degree of SRO corresponding to the thermodynamic equilibrium at 500 .deg. C while a more disordered state is retained. The kinetics of SRO formation is strongly determined by the nucleation of ordered zones, and carbon in the solution appears to suppress the formation of SRO nuclei by trapping excess vacancies during water quenching and hence, retarding the establishment of SRO. Cold working destroys SRO but also produces various lattice defects, which enhance SRO formation and carbide precipitation during re-heating. Marucco has established the existence of an order. disorder transformation in Ni.Cr base alloys from the study of Ni2Cr, Ni3Cr, and Ni.Cr.Fe. She concluded that the degree of order primarily depends on Cr content and that the ordering kinetics become slower when the Ni:Cr atomic ratio departs from that of stochiometric Ni2Cr. The presence of Fe has a strong delaying effect on ordering kinetics, even in small quantities and other alloying elements can also influence the transformation. It has been ascertained that the limited lattice contraction accompanying SRO markedly increases owing to long range order (LRO), while the resistivity decreases sharply and the LRO kinetics are markedly delayed by Fe addition. The equilibrium degree of LRO depends on temperature and Cr concentration. The SRO.LRO transition can be confirmed by the appearance of appreciable variations in the physical property like lattice contraction. In this study, the specific heat variation with temperature, amount of energy released due to the exothermic reaction, and activation energy for the exothermic reaction were systematically investigated to understand how the cold work affect ordering reaction in Alloy 600