The microstructure of several 80 Ni-20 Cr alloys was studied after creep at 1500 and 1800 F with respect to the influence of atmosphere. In particular the propagation of intercrystalline cracks was investigated. By comparison with tests conducted in argon it can be shown that the atmosphere has a strengthening effect in all cases. The mechanism of this effect changes with the variation of minor constituents in the metals (such as silicon and Mn/S ratio). Crack propagation is strongly affected by the melting process, which influences the purity of the grain boundaries.

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