Nickel-chromium alloys may be susceptible to ordering under certain thermal aging conditions, resulting in the formation of an Ni2Cr phase. The Ni2Cr phase is a superlattice which can result in significant changes of the physical or mechanical properties compared to those of the disordered alloy. Alloy 690, which usually contains a bit less than two nickel atoms per chrom-ium atom, could potentially be susceptible to long-range ordering (LRO) or short-range ordering (SRO). SRO implies that a Cr atom has a high probability of having a Ni nearest neighbor and that small order domains containing at most a few atoms exist. LRO implies that small Ni2Cr clusters exist, and although they may not be visible by transmission electron microscopy (TEM) dark field imaging, they lead to the detection of
additional diffraction spots in selected area electron diffraction (SAED) patterns, contrary to SRO. Below the critical temperature for LRO, the formation of the long-range ordered structures is preceded by a period of short-range ordering.