Long range ordering in model Ni-Cr-X alloys

Nickel-chromium alloys are used throughout commercial nuclear power systems due to their desirable combination of corrosion resistance and mechanical properties. However, some Ni-Cr alloys can undergo long range ordering (LRO), forming the Ni₂Cr phase when exposed to temperatures < 590 C. degrees. LRO results in lattice contraction, hardening, and a change in slip mode, which, in turn, can cause dimensional changes, internal stress, and appreciable embrittlement. Despite the technological importance of this alloy system, the variables that influence LRO are not well understood and the time-temperature-transformation kinetics poorly defined. In order to assess the risk of LRO in nuclear power systems, the present research uses model Ni-Cr alloys and ageing times up to 10000 hours to define the kinetics of LRO and to assess the effects of cold work, quench rate, and alloying additions. Results show that the hardening caused by ordering is well described by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation with an Avrami exponent, n near 0.65 and an apparent activation energy that depends on the starting condition of the alloy. Furnace cooled samples displayed a Q ∼ 244 kJ/mol, which suggests bulk diffusional growth of the ordered phase, while water quenched samples exhibited a Q ∼ 147 kJ/mol, indicating that excess vacancies accelerate ordering. Cold work (10% or 20%) acts to disrupt any ordering that forms on furnace cooling but has no apparent effect on the apparent activation energy or Avrami exponent. Iron additions decrease the temperature below which the ordered phase is stable but do not appear to affect the rate of ordering. Investigation of other alloying suggest that molybdenum (∼ 2.47 wt.%) may accelerate ordering but other alloying elements studied (Si up to 0.28 wt.%, Mn up to 0.19 wt.%, and Nb up to 2.38 wt.%) have little influence. These findings, combined with a review of LRO in commercial alloys indicate that LRO can develop over a wide range of compositions and suggest that Alloy 690 is not immune to LRO. However, barring some accelerating factor, thermal ageing studies indicate LRO is only a concern for components exposed to high temperatures (≥ 325 C. degrees) for a decade or more