The Precipitation of Primary Carbides in Alloy 718

MITCHELL,A. J; A.J. SCHMALZ,; C. SCHVEZOV; S. L. COCKROFT

Superalloys 718, 625, 706 and Derivatives

TMS

Lugar: Warrendale, Pensylvania 15086; Año: 1994; p. 65 - 78

The precipitation of primary MC carbides in alloy 718 has been studied using techniques of directional solidification with and without quenching. The precipitation reaction; {Nb} + {C} = NbC (s) is found to follow normal thermodynamic relationships in alloys which have an amount of nitrogen well in excess of the saturation solubility of TiN at the carbide precipitation temperature. For alloys containing only small amounts of nitrogen (N< 14ppm), made by electron beam remelting, the carbide precipitation is delayed to a lower temperature due to a lack of TiN nucleating sites. As a result we find that in these alloys both the as-cast structure and the response to homogenization are significantly changed from those of the conventional alloy compositions. The implications of this finding in relation to potential alloy composition modifications in order to improve carbide distribution are discussed.