Ordering reactions in an Ni-25Mo-8Cr alloy

The transformations from short- to long-range order in Haynes(R) Alloy 242, a nominal Ni-25Mo-8Cr (in wt%) alloy, during isothermal aging at temperatures between 550 and 750 degrees C are reported using microhardness measurements, and optical and transmission electron microscopy. Aging below the critical transformation temperature (similar to 775 degrees C) led to considerable hardening; this hardening was associated with the formation of a very high volume fraction of Ni-2(Mo,Cr) domains/precipitates. The transformation from short- to long-range order was observed to proceed by a mechanism of continuous ordering at temperatures less than or equal to 700 degrees C, whereas at temperatures (750 degrees C) close to the critical temperature a first order nucleation and growth mechanism appeared to be operative. The sequence of transformations are correlated with the associated diffraction effects and discussed in terms of theoretical formulations and experimental observations of other studies.