Effect of B, Zr, and C on Hot Tearing of a Directionally Solidified Nickel-Based Superalloy

The effect of the minor elements B, Zr, and C on the castability of a Nickel-based gamma'-strengthened superalloy has been investigated. Tube-like specimens were prepared by directional solidification where the rigid ceramic core leads to hoop stresses and grain boundary cracking. It was found that an important improvement in castability can be achieved by adjusting the minor elemental composition. Too low C (a parts per thousand currency sign0.15 pct) and too high B and Zr contents (a parts per thousand yen0.05 pct) lead to material that is very prone to solidification cracking and should be avoided. The results cannot be rationalized on the basis of the current models for solidification cracking. Instead, pronounced hot tearing is observed to occur at high amounts of gamma/gamma'-eutectic and high Zr contents. The critical film stage where dendrites at the end of solidification do not touch and are separated by thin liquid films must be avoided. How Zr promotes the film stage will be discussed in the paper. (C) The Minerals, Metals & Materials Society and ASM International 2016