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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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In addition to the precipitation of the strengthening intermetallic phases, namely, the metastable y" and y' phases, carbide phases also precipitate during age hardening heat treatments of Alloy 718 and Alloy 625. A detailed investigation has been carried out on the precipitation of primary and secondary carbide phases in these two alloys with reference to their identity, distribution and morphology. It has been found that the predominant carbide phase in Alloy 718 is a niobium rich MC phase, while in the case of Alloy 625, mainly M,,C, and, to some extent, M,C type carbides precipitate. Carbide precipitation during ageing has been seen to be confined almost exclusively to the austenite grain boundaries in both the alloys. NbC formation at the grain boundaries in Alloy 718 is associated with the appearance of distinct intermetallic precipitate ( y" and y' ) free zones near these boundaries whereas in Alloy 625, no well defined intermetallic precipitate free zones (PFZ) have been observed. The precipitation of different types of grain boundary carbides is discussed in terms of the differences in the chemistry of these alloys. The presence or absence of PFZ at grain boundaries is discussed in the light of solute depletion /vacancy depletion mechanisms.The influence of grain boundary carbide precipitation on the mechanical behaviour, especially the fracture mode, of these alloys has also been examined. It has been found that the progressive coverage of these boundaries with carbide particles results in a gradual transition in the fracture mode from transgranular to intergranular. Microcracks have often been found to nucleate at the matrix-carbide interface at the points of impingement of deformation bands.

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