Hot tearing in polycrystalline Ni-based IN738LC superalloy: Influence of Zr content

Abstract

a b s t r a c tZirconium is always present in Ni base superalloys as it enhances their creep properties. In the present study, the influence of very small Zr additions, 100-400 ppm, i.e. 0.01-0.04 wt.%, on hot tearing of IN738LC superalloy is experimentally investigated using dedicated turbine blade castings. Although the Zr content remains very small, it has a strong effect on hot tearing tendency. Microstructure of hot tear in as-cast samples reveal that grain size and secondary dendrite arm spacing have no significant effect on hot tearing. On the other hand eutectic phase volume fraction and its dispersion or spreading along grain boundaries drastically affect the hot tearing propensity and strongly increase with increasing amounts of Zr. Hence grain coalescence becomes impossible at grain boundaries covered with eutectic phase films. With increasing Zr content, gain coalescence between two distinct grains with no interdendritic phase requires more undercooling. Coalescence is retarded and occurs deeper in the mush zone, i.e. at lower temperatures resulting in a higher sensitivity to hot tearing. Finally, it is shown that a reduction of Zr content to 0.02 wt.% is required to fully suppress hot tearing in polycrystalline IN738LC blades.

Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys

“…Our observations imply that a bulk-criterion may not allow to appropriately depict near-atomic-scale processes. It is also known that Zr promotes hot cracking in nickel-based superalloys produced by casting [42][43][44][45] and AM processes [5]. Although Zr is added in the bulk composition of the investigated alloy (0.02 -0.025 at.%), APT revealed no segregation of Zr at the grain boundary.…”

Hot cracking mechanism affecting a non-weldable Ni-based superalloy produced by selective electron Beam Melting

“…The literature dealing with welding and weldability of Ni-based superalloys turns out to be relevant towards the effect of minor elements such as B on the hot cracking susceptibility, see e.g. [4,6,19,47].…”

Additive manufacturing of Ni-based superalloys: The outstanding issues

“…Several studies have recently emerged on the effects of Zr-segregation on solidification cracking in IN738LC. It is suggested that Zr-films become trapped between dendrite arms towards the grain boundaries, embrittling the grain boundary region; when stressed, cracks form similar to those of solidification cracking morphology (49,55,56).…”