Strain-age cracking of a γ′-strengthened nickel-based superalloy additively manufactured by laser powder bed fusion

Artikel i vetenskaplig tidskrift, 2026

Strain-age cracking (SAC) remains a critical barrier in post-processing of γ′-strengthened nickel-based superalloys manufactured by laser powder bed fusion (PBF-LB). In this work, the SAC susceptibility of PBF-LB IN738LC was systematically investigated using V-notch samples under different heating conditions. Residual strain evolution during build plate removal was tracked by strain gauges, and residual stress at different notch depths was measured using synchrotron X-ray diffraction. SAC onset was determined by direct current potential drop (DCPD) and in situ optical imaging, while crack propagation kinetics were quantified under controlled heating rates and isothermal exposures. Post-mortem characterization of SAC morphologies was performed using X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron backscatter diffraction. SAC initiation is governed by γ′ precipitation kinetics, with onset temperatures following the γ′ precipitation behavior. Faster heating delays SAC onset, while isothermal holds trigger cracking after sufficient incubation, with longer times at lower isothermal temperatures. Shallow notches concentrate tensile stresses and develop extensive SAC, whereas deeper notches remain largely crack-free due to stress relaxation, sometimes aided by pre-existing solidification cracks. SAC propagates mainly along grain boundaries, assisted by wedge-shaped oxide intrusions. Fast-heating cross-sections reveal that most stored energy is released prior to SAC initiation, indicating that SAC results from instability of local ductility deficit rather than stress relief. Early-stage nanoscale γ′ formation drives rapid strengthening and localized ductility loss, leading to SAC. These findings provide mechanistic insights and processing guidelines for mitigating SAC in PBF-LB superalloys.