Strain localisation and failure at twin-boundary complexions in nickel-based superalloys

Abstract

Twin boundaries (TBs) in Ni-based superalloys are vulnerable sites for failure in demanding environments, and a current lack of mechanistic understanding hampers the reliable lifetime prediction and performance optimisation of these alloys. Here we report the discovery of an unexpected γ″ precipitation mechanism at TBs that takes the responsibility for alloy failure in demanding environments. Using multiscale microstructural and mechanical characterisations (from millimetre down to atomic level) and DFT calculations, we demonstrate that abnormal γ″ precipitation along TBs accounts for the premature dislocation activities and pronounced strain localisation associated with TBs during mechanical loading, which serves as a precursor for crack initiation. We clarify the physical origin of the TBs-related cracking at the atomic level of γ″-strengthened Ni-based superalloys in a hydrogen containing environment, and provide practical methods to mitigate the adverse effect of TBs on the performance of these alloys.

Origin of strain localization at twin boundary in Inconel 718 superalloy fabricated by laser powder bed fusion

“…High resolution backscattered electron (BSE) image (figure 4a) shows that V-shaped γ″ with one-toone correspondence formed along TBs in IN718 alloy, which is similar to our previous finding in another γ″-strengthened superalloy (Incoloy 945X) [22]. The formation of such symmetric γ″ along TBs is attributed to the extremely low free energy in forming γ″-TB-γ″ structure compared to forming other morphologies, which originates from the interplay between the crystallographic structure of γ″ and TB of Ni matrix [22]. We examined the TBs which have developed DSBs using high resolution SEM to reveal the role of the special γ″ in the initiation of DSBs and strain localization.…”

“…In addition, the two TBs in figure 4c and 4d both have high Schmid factors for {111}<110> slip (0.44 and 0.45, respectively), so they are readily the easy path for dislocation activities. In addition, Zhang et al [22] found out that dislocations are activated at a much lower stress level at these weakened planes compared to that in grain interiors of Incoloy 945X, which results in severe strain localization. Similar phenomenon was also observed in the in-situ HRDIC results in figure 2.…”

First-principles insights into hydrogen trapping in interstitial-vacancy complexes in vanadium carbide

“…1,2 However, the penetration of a large amount of hydrogen and its isotopes in the fusion reactor service environment induces hydrogen embrittlement (HE) and degradation of materials, shortening the lifetime and causing safety issues. [3][4][5][6][7] It is known that the diffusion of H atoms is the leading cause of HE. [8][9][10] The use of hydrogen permeation barrier coatings that reduce the penetration of H is an effective means.…”

Bringing to Life a Newly Discovered Signature of Fatigue Crack Initiation via Multimodal Characterizations

“…Specifically in nickel-based superalloys, prior studies by Villechaise et al and Larrouy et al theorize that areas where localized slip impinges on a neighboring grain boundary, but the slip cannot transmit into the neighboring grain, are favorable crack initiation sites due to the slip shearing the physical grain boundary [6,7]. A recent high-resolution deformation study by Stinville el al on the Inconel-718 nickel-based superalloy furthered these findings by coupling two-dimensional electron backscatter diffraction (2D-EBSD), an microscopic orientation field measurement, and heaviside digital image correlation (H-DIC), a microscopic strain field measurement, to spatially correlate the same slip impingement behavior with plumes of intense intragrain lattice rotation (up to 10 • ), termed microvolumes, as shown in Figure 1 [6][7][8][9][10][11]. These observations, when combined with the crack initiation theory above, provide microvolumes as measurable features that may indirectly identify grains with the most potential for initiating a fatigue crack.…”