Influence of Hf on the heat treatment response of additively manufactured Ni-base superalloy CM247LC

The influence of Hf on the heat treatment response of a Laser Powder Bed Fusion (L-PBF) fabricated Ni-base superalloy was investigated. A Hf-free version of CM247LC, CM247LC NHf, and CM247LC (similar to 1.4 wt% Hf) were subjected to identical thermal treatments and characterized. After solutionizing at 1260 degrees C for 2 h, both alloys displayed recrystallization and grain growth. The CM247LC NHf had numerous annealing twins suggesting that Hf inhibits twin formation. The CM247LC NHf alloy displayed a lower peak hardness than CM247LC (5000 MPa and 5200 MPa, respectively) and this was attributed to a similar to 6% point reduction (Calphad calculated) in gamma' phase fraction due to the removal of Hf. Despite the lower hardness values, the CM247LC NHf displayed a higher yield strength than CM247LC at 760 degrees C (969 MPa and 926 MPa, respectively). Elongation at fracture at 760 degrees C was similar (similar to 1%) for both CM247LC and CM247LC NHf. The removal of Hf was shown to be effective at mitigating the strain age cracking susceptibility of CM247LC, likely due to the decrease in y' phase fraction and lattice mismatch.