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The microstructure of Alloy 625, which has undergone prolonged (ϳ 70,000 hours) service at temperatures close to but less than 600 ЊC, has been characterized by transmission electron microscopy. The precipitation of an intermetallic phase Ni 2 (Cr, Mo) with Pt 2 Mo-type structure has been observed in addition to that of the ␥" phase. Six variants of Ni 2 (Cr, Mo) precipitates have been found to occur in the austenite grains. These particles exhibit a snowflake-like morphology and are uniformly distributed in the matrix. They have been found to dissolve when the alloy is subjected to short heat treatments at 700 ЊC. The occurrence of the Ni 2 (Cr, Mo) phase has been discussed by taking the alloy chemistry into consideration. Apart from the intermetallic phases, the precipitation of a M 6 Ctype carbide phase within the matrix and the formation of near continuous films, comprising discrete M 6 C/M 23 C 6 carbide particles, at the austenite grain boundaries have been noticed in the alloy after prolonged service.

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