The Effect of Boron and Zirconium on the Microcracking Susceptibility of IN-738LC Derivatives in Laser Powder Bed Fusion

The effect of boron (<0.01 to 0.03 wt%) and zirconium (<0.01 to 0.07 wt%) on the microcracking susceptibility of the γ’-strengthened Ni-base superalloy IN-738LC during laser powder bed fusion (LPBF) was studied using custom designed powder grades. It was found that both elements have a strong effect on the microcracking susceptibility, the microcracks are located at high angle grain boundaries based on EBSD measurements and crack density increases with the content of both elements. High crack density in the material with high boron and zirconium content corresponds to a large fraction of intergranular decohesion facets exhibiting a dendritic morphology on the fracture surface, typical for solidification cracking. Investigation of the fracture surface chemistry by X-ray photoelectron spectroscopy (XPS) indicates that considerable amounts of B and Zr are present in oxide state. Auger electron spectroscopy (AES) confirms that both elements are segregated to the intergranular decohesion facets on the fracture surface. Thin layers of B- and Zr-containing oxide on the microcrack surfaces were indicated by atom probe tomography (APT) as well. Hence, it is suggested that the cracking susceptibility of the studied alloying system is caused by formation of B- and Zr-containing oxide at high-angle grain boundaries during solidification.