On the evolution of long-range order from short-range order in a Ni2(Cr0.5Mo0.5) alloy

The evolution of long-range order from a disordered state has been studied in a splat-quenched Ni2(Cr0.5Mo0.5) alloy. The evolution of the order in this alloy follows a continuous ordering mechanism without the need of the superimposition of 〈1 ½ 0〉 type static concentration waves that are believed to be necessary for the evolution of ordered Ni2(Cr,Mo) phase (with a Pt2Mo type structure) from a short-range order state. The proposed mechanism is based upon the geometrical evidence of the inherent presence of a subunit cell of Pt2Mo structure in the N2M2 stacking sequence on {4 2 0} planes in the {1 ½ 0} short range order. Addition of Cr to stoichiometric Ni2Mo alloy stabilizes the N2M2 order which in turn stabilizes the Pt2Mo state. The stability of the N2M2 stacking sequence and Pt2Mo order in the short-range ordered state has been discussed in the light of pair interaction energies.