Shear banding mediated fracture mechanisms in additively manufactured IN738 superalloys under low-strain-rate loading

Abstract

Shear banding coupled with grain refinement plays a critical role in fracture behavior under dynamic loading and (very) high-cycle fatigue but is rarely observed during low-strain-rate loading. In this study, we report for the first experimental evidence of shear banding mediated fracture mechanism in an electron beam powder bed fusion (EBPBF) fabricated IN738 superalloy upon low-strain-rate (1 × 10⁻³ s⁻¹) uniaxial tensile loading. The optimized EBPBF process mitigates solidification defects and produces well-aligned columnar grains with a <001> fiber texture along the building direction, achieving superior mechanical properties compared to cast alloys through the synergistic effect of multiple strengthening mechanisms. Notably, the relatively uniform distribution of nano-sized carbides in the EBPBF-fabricated alloys prevents strain-incompatibility cracking caused by coarse carbides in cast alloys and facilitates shear banding mediated transgranular fracture. The shear band, formed due to concentrated plastic deformation along the crack path, is associated with complete grain nanocrystallization and γ′ precipitate fragmentation through intensive dislocations and twinning activities. The formation of shear banding potentially dissipates crack propagation energy and enhances the crack growth resistance. These findings provide new insights into fracture mechanisms and underscore the potential of additive manufacturing for designing damage-tolerant superalloys.