Advanced Technology and Manufacturing Institute (ATAMI)

The thermal stability of Ni-Cr alloys is a potential concern for the long-term performance of nuclear power systems and spent fuel storage containers. Specifically, long time exposure to relatively low homologous temperatures can result in long range ordering (LRO), i.e. formation of the intermetallic Ni2Cr phase. This ordering can increase strength, decrease ductility, and cause dimensional changes or internal stress. Furthermore, ordering promotes heterogeneous planar slip, which can degrade mechanical properties and reduce resistance to environmentally assisted cracking. However, the rate of ordering and the factors that influence this transformation are not well understood. In order to better understand this phenomenon, isothermal heat treatments of model Ni-Cr and Ni-Cr-Fe alloys were conducted at temperatures between 631°F-878°F (333ºC-470ºC) for times up to 10,000 hours. Model alloys were used to help isolate parameters of interest, with the recognition that they may be more prone to LRO than complex industrial alloys. The effects of alloying (0-9 wt.% Fe), cold work (10% and 20% reduction via rolling), and excess vacancies (water quenched vs. furnace cooled) were investigated. The degree of order was assessed by the change in the lattice parameter with ageing time and by microhardness. Selected samples were further characterized by scanning and transmission electron microscopy. Results show the rate of lattice contraction and hardening (i.e. the degree of LRO) is well described by Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetics. The data indicated that the Avrami exponent, n, is ≈ 1 and the apparent activation energy is between 135 kJ/mol (vacancy migration) and 275 kJ/mol (bulk diffusion). Using the water quenched Ni-Cr binary data as a baseline, furnace cooling is shown to shift ordering to shorter times, likely due to the development of some order on-cooling. Cold work appears to delay the onset of lattice contraction and hardening, possibly by promoting disorder in the material. Cold worked material also tends to show higher apparent activation energies (~200 kJ/mol) than either furnace cooled (Q ~155 kJ/mol) or water quenched (Q ~135 kJ/mol) alloys. Iron lowers temperature below which the ordered phase is stable but has little effect on the overall kinetics. This research indicates that the development of LRO is a potential concern for components in nuclear power systems made from nickelchromium alloys that are exposed to temperatures above ~518°F (270°C) for extended times (40-80 years). Extrapolation of the data indicates little concern for spent fuel storage containers, where the low temperatures (≤ 200°C) delay ordering to times > than 10,000 years