The impact of B and Zr content on processability, microstructure, and mechanical performance of additively manufactured superalloy IN738LC

The minor elements B and Zr have a major impact on the processability and high temperature properties of Ni-base superalloys additively manufactured by the Powder Bed Fusion Laser Beam (PBF-LB) process. To deconvolute the effects of these elements on microcracking and creep life, four variants of the IN738LC superalloy were tested: one with both B and Zr, one with only B, one with only Zr, and one with neither element. PBF-LB microcracking observations were rationalised using a modified Scheil solidification simulation incorporating the solute trapping effects during rapid solidification. Analysis of the microstructures was performed after HIP and heat treatment by SEM-EDX, EBSD, and NanoSIMS. High temperature tensile and creep tests were conducted to ascertain the relative mechanical performance of the four alloys. The alloy variant with both B and Zr resulted in a high crack density of 0.76 mm/mm 2 after PBF-LB processing, whereas variants with only one of the two elements showed significantly reduced crack density of ≈0.02 mm/mm 2 . Creep rupture life of the variant with both B and Zr was 66 h at 760°C and 565 MPa, whereas no other variant produced life of more than 38 h, and the variants without either element ruptured in only 6 h. These results indicate that although B and Zr together promote microcracking, they also provide the best creep resistance only when alloyed in combination. The results are discussed in light of the microstructure analysis and with a view towards satisfying the apparently conflicting requirements of creep resistance and crack-free processability.