The physical metallurgy of cast and wrought alloy 718 is discussed in terms of the importance of Nb on the phase reactions found in alloy 718. The role or influence of Nb is first noted during the solidification of alloy 7 18 as the large atoms Nb, MO, and Ti segregate to the interdendritic regions where the Nb rich Laves phase forms. The Laves phase must be solutioned at high temperatures and the Nb must be distributed throughout the dendrite areas. The normal phases found in alloy 718 are the MC, TiN, delta, Laves, ?/' and ?/. The y and ?/ are the main strengthening phases and their precipitation behavior is determined primarily by the amount of Nb, the temperature and time of exposure. Any incomplete homogenization of the cast or wrought material will produce non-uniform precipitation of the delta, 1/' and f phases during working or aging heat treatments. The longtime stability of alloy 718 is related to the stability of the 1/' phase which transforms to delta and ?/ with increasing time and temperatures. Eventually the f phase will solution and the delta phase becomes the terminal phase in the 718 system. An aCr phase is found in the grain boundaries in long time exposures in the 1100°-13500F temperature range. The Mzs C6 carbide phase normally found in Ni base alloys is not found in alloy 7 18.

The effects of Ta on phases and mechanical properties in conventional 718 type compositions have not been fully explored. While Ta and Nb have similar atomic sizes, the solubility of Ta in nickel is much greater than that of Nb. This difference in solubility would affect the initial segregation on solidification and subsequent phase reactions. To study the role of Ta in phase reactions and on alloy stability in a homogeneous material, a comparative study of conventional P/M 718 and P/M Ta 7 18 was undertaken. The results of this study showed that the heat treatment used for conventional 7 18 did not produce an significant strengthening phases in Ta 718 and a modified heat treatment was to reci necessa~ p p itate ?/' and 1/ strengthening phases. The ?/' phase in Ta 718 is still present at 1750 F. The r to delta transition in Ta 718 is more sluggish and occurs at higher temperatures than in conventional 718 materials. Data from tensile tests at 1400'F indicates that Ta 718 has a higher temperature capability than conventional 718.

A T-T-T study was carried out on the phase reactions in high quality wrought alloy 718 billet. Samples were exposed from 12OO'F to 17OO'F for times of 0

A detailed investigation of cast IN 718 was conducted on phases formed during solidification, phases precipitated on cooling and phase stability. Clearly cast 718 is a composite structure. Extensive as-cast segregation is evident which shows a low Cb content, about 2%, in the dendritic matrix, and a high Cb content, about 10% in the interdendritic region. Homogenization studies show that Laves solutioning does not uniformly redistribute the Cb. This work also indicates that the standard GE homogenization heat treatment of 2OOO'F/l hour dissolves most of the delta and a portion of the Laves. Depending on the Cb content in the alloy, total Laves solutioning requires at least 3 hours at 21OO"F, but a more uniform Cb distribution requires 100 hours or more beyond the standard heating cycle. The results of this investigation may serve as a guide in microstructural interpretation and can offer a viable explanation for the variability of mechanical properties on bars extracted from castings.

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