Segregation-Assisted Plasticity in Ni-Based Superalloys

Abstract

Correlative high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy are used to study deformation-induced planar faults in the single-crystal superalloy MD2 crept at 800°C and 650 MPa. Segregation of Cr and Co at microtwins, anti-phase boundaries (APB), and complex/superlattice extrinsic and intrinsic stacking faults (CESF/SESF and CISF/SISF) is confirmed and quantified. The extent of this is found to depend upon the fault type, being most pronounced for the APB. The CESF/SESF is studied in detail due to its role as a precursor of the microtwins causing the majority of plasticity under these conditions. Quantitative modeling is carried out to rationalize the findings; the experimental results are consistent with a greater predicted velocity for the lengthening of the CESF/ SESF-compared with the other types of fault-and hence confirm its role in the diffusion-assisted plasticity needed for the microtwinning mechanism to be operative.

The evolving landscape for alloy design

“…This provides a new pathway for control of fault energies and higher length scale mechanical properties sensitive to this property. Similar phenomena have also been observed in commercial nickel-based superalloys, 59,60 suggesting new strategies for alloying in both classes of materials. This approach to tailoring of interface properties promises improved fracture properties, control of grain growth, and stabilization of nanocrystalline structures.…”

“…This has enabled, for example, rigorous studies of segregation to grain boundaries and stacking faults. [57][58][59][60][61][62][63][64][65][66][67][68] Refractory element segregation has been observed at superlattice intrinsic stacking faults in a new class of cobalt-based alloys containing L1 2 precipitates, 57,58,69,70 and is revealed by both local electrode atom probe tomography and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) (see Figure 3a). This provides a new pathway for control of fault energies and higher length scale mechanical properties sensitive to this property.…”

Understanding creep of a single-crystalline Co-Al-W-Ta superalloy by studying the deformation mechanism, segregation tendency and stacking fault energy

“…Stacking faults are formed due to a thermally activated reaction of a/2<110>{111} dislocations, which results in a leading partial dislocation of a/3<112> type creating a superlattice intrinsic stacking fault (SISF) and a trailing partial dislocation of a/6<112> type that remains at the / interface. Additionally, it is known that segregation of solutes to planar faults influences the defect energies and their formation process significantly [26,[29][30][31][32]. Furthermore, local diffusive reordering processes occur and defect arrangements can be changed driven by an energy reduction [30,[33][34][35].…”

On the Rapid Assessment of Mechanical Behavior of a Prototype Nickel-Based Superalloy using Small-Scale Testing

“…[40] Recent studies have identified diffusion-activated plasticity as an additional operating mechanism which could explain the decreasing strength in this temperature regime. [41,42] A further significant weakening effect at higher temperatures is a decrease in c 0 volume fraction, / p . Calculations using Thermo-Calc and the TTNi8 Ni-alloy database predict a decrease from 58 pct at 600°C to nearly 0 pct at 1150°C (see Figure 13(d)).…”