Precipitation of coherent Ni2(Cr, W) superlattice in an Ni–Cr–W superalloy

• The DF image of SRO cluster has been found for the first time experimentally.
• The transformation of SRO to LRO gives rise to the Pt2Mo-type Ni2(Cr, W).
• Variants of Ni2(Cr, W) occur equiprobably.
• The interfaces between Ni2(Cr, W) and matrix are coherent at the atomic scale.

It is demonstrated that a nanometer-sized Ni2(Cr, W) superlattice with a Pt2Mo-type structure can precipitate in an Ni–Cr–W alloy by means of a simple aging treatment at 550 °C. The dark-field image of short-range order domains has been found for the first time experimentally. The mechanism of short-range order to long-range order transformation has been revealed based on transmission electron microscopy result and static concentration waves theory and found to be continuous ordering. The randomness of the transformation of static concentration waves leads to equiprobable occurrence of the different variants. The transformation of short-range order to long-range order gives rise to the Pt2Mo-type Ni2(Cr, W) superlattice. The interfaces between Ni2(Cr, W) and Ni-based matrix and the different variants of Ni2(Cr, W) have been investigated by high resolution transmission electron microscopy. The results reveal that the interfaces between Ni2(Cr, W) and surrounding matrix are coherent at the atomic scale.

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