718 Alloy

Alloy name: 718
Diagram No.: 1194
Type of diagram: TTT
Chemical composition in weight %: 0.05% C, 19% Cr, 53% Ni, 5% Cb, 3% Mo, 1% Ti, 0.5% Al, 0.005% B, 18% Fe
Alloy group: Nickel-based alloys
Note: Isothermal transformation diagram for start of formation of grain boundary CbC film in Alloy 718 [2].

TiN is apparently of little concern in Alloy 718 since it forms in such small amounts during solidification. It appears under the microscope as a pink phase, massive and angular in shape. Barker reports that, it is stable up to the melting point.

CbC, however: forms in larger quantities and is apparently not as stable as TiN. It forms on solidification as massive angular particles. These carbides, which appear gray under the microscope, have a tendency to remove columbium and thus reduce strength. It can form in large amounts at grain boundaries.

CbC dissolves on annealing above 1900F (1038C) and will precipitate on aging from 1300 to 1800F (704 to 982C) asa grain boundary film. Eiselstein did not find that its presence affected mechanical properties; however, by the very fact that it can be a grain boundary film, it should be avoided. Eiselstein does show that formation of CbC film can be pronounced aging after annealing at 2100F (1149C) where CbC is dissolved. On annealing at 1700F (927C), the film should not be precipilated on aging because none is dissolved. The effect of annealing at temperatures above 1700F (927C) on CbC film formation is illustrated in Figure 7 where it is shown that with only a prior anneal of 1900F (1038C), an aging time of about ten hours al 1400F (760C) is required for a CbC film to form; and with an anneal of 2100F (1149C), only a few minutes are required for the film to form. No CbC film will form upon aging with a 1750F (954C) solution anneal.

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