Investigation of the internal failure mechanism of laser-additively manufactured nickel-based superalloy

Abstract

Nickel-based superalloys exhibit excellent high strength and thermal fatigue resistance at 650 ℃, making them widely used for manufacturing elevated-temperature components such as turbine blades for aero-engines. Laser-powder bed fusion (L-PBF) is a rapidly developing metal additive manufacturing technology that is increasingly important for producing nickel-based superalloy products. The design and service life of aero-engine turbine blades typically require more than 107 load cycles. Therefore, it is crucial to investigate the very-high-cycle fatigue characteristics of L-PBF nickel-based superalloys at increased temperatures. Internal failure is a common elevated-temperature fatigue mode of L-PBF nickel-based superalloys that is currently not well understood. To address this issue, first, axial fatigue tests with stress ratios of −1 and 0.1 are conducted at 650 ℃. Specifically, partial typical internal failure fractures at a stress ratio of 0.1 are selected as the focus of this study. Second, scanning electron microscopy and ultra-depth field microscopy are employed to observe the 2D and 3D morphology of the fatigue fracture surfaces and analyze the crack nucleation areas and growth paths. The results show that irrespective of the presence of defects, the emergence and aggregation of numerous facets occur in the “facetted cracking area (FCA),” a typical internal failure characteristic of L-PBF nickel-based superalloys. Measurements indicate that the size of these facets leading to cracking is comparable to that of large grains and correlates with variations in grain orientation. Therefore, internal failures are categorized into two modes of cracking: “defect-assisted faceted cracking” and “non-defect-assisted faceted cracking.” Third, the FCA exhibiting typical internal failure fractures is sectioned and subjected to electron backscatter diffraction analysis to observe surface and subsurface crystallographic features related to crack nucleation and growth behavior. The analysis reveals that microcracks mainly originate from large grains with softer orientations, propagate through slips, expand along the direction of maximum shear stress, and ultimately form a perforated fracture pattern. Fourth, subsurface microcrack features beneath the FCA are examined via focused ion beam milling and imaging. Transmission electron microscopy is then employed to observe slip bands and dislocation structures near the microcracks. The results confirm that the fatigue deformation mechanism of facets at 650 ℃ is mainly controlled by a combination of anti-phase boundary shearing, precipitate bypassing, and stacking fault shearing. This mechanism is especially evident under stress concentration effects induced by cracks or defects. Finally, according to the definition of the crack tip stress intensity factor, a crack nucleation life prediction method that accounts for the characteristics of faceted cracks is proposed. The predicted results align well with the experimental findings.

Keywords

• additive manufacturing
• nickel-based superalloy
• very-high-cycle fatigue
• internal failure
• dislocation structures