A Study of Effects on Phosphorus, Sulfur, Boron and Carbon on Laves and Carbide Formation in Alloy 718

Alloy 718 base chemistries were modified with additions and/or deletions of P, S, B, Si and C to determine the effect of various minor or residual elements on eutectic solidification products in alloy 718-type compositions. The chemistries were such that single additions and interactions could be evaluated. The combinations studied were very clean base alloy, +P, +B, +Si, +BC, +SC, +BSPCSi and commercial 718. Differential thermal analysis(DTA), metallography and energy dispersive x-ray analysis were used to evaluate the solidification path and products. DTA provided significant information on the solidification behavior of these alloys. It was found that B, Si and P were significant Laves formers when present alone. In the presence of C, the solidification moved toward proeutectic carbide formation with the elimination of Laves phase. Sulfur had little effect on the solidification path. It was found that the clean alloy showed little tendency toward Laves formation with the residual elements. This suggests that P, B, and Si determine the Laves forming behavior of Nb. Applications of these results to alloy design, microstructure and solidification cracking are discussed.