The effect of Thermomechanical History on the Stability of Alloy 718

Abstract

Previous studies showed that Ni3Cb precipitation behavior appears to follow a predictable pattern with increased strain in the precipitation range for this phase. Additional investigation was initiated in order to further the understanding of Ni₃Cb precipitation under similar strain conditions and to develop some information on the effects of such conditions on the stability of gamma prime and the formation of sigma or other phases often considered detrimental to performance. Samples from a production heat were treated above the Ni3Cb precipitation range and subsequently strained between 0% RA and 35% RA at 1500°F. Light and electron microscopy and X-ray diffraction studies were performed on samples in the as-strained condition; after a 1750°F gamma prime solution treatment; after a 1750°F treatment followed by a double aging, gamma prime precipitation treatment and after a fully aged plus 1250°F-500 hour exposure. It was found that the quantity of Ni3Cb which precipitated during the 1750°F treatment step increases markedly with the degree of strain, that gamma prime stability is not significantly affected by strain or the quantity of Ni₃Cb precipitate, and that sigma and similar phases do not form during prolonged exposure at the upper end of the useful temperature range for this alloy, even at the highest strain level investigated.