Autonomous Repair Mechanism of Creep Damage in Fe-Au and Fe-Au-B-N Alloys

Abstract

The autonomous repair mechanism of creep cavitation during high-temperature deformation has been investigated in Fe-Au and Fe-Au-B-N alloys. Combined electron-microscopy techniques and atom probe tomography reveal how the improved creep properties result from Au precipitation within the creep cavities, preferentially formed on grain boundaries oriented perpendicular to the applied stress. The selective precipitation of Au atoms at the free creep cavity surface results in pore filling, and thereby, autonomous repair of the creep damage. The large difference in atomic size between the Au and Fe strongly hampers the nucleation of precipitates in the matrix. As a result, the matrix acts as a reservoir for the supersaturated solute until damage occurs. Grain boundaries and dislocations are found to act as fast transport routes for solute gold from the matrix to the creep cavities. The mechanism responsible for the self-healing can be characterized by a simple model for cavity growth and cavity filling.

Autonomous Filling of Grain-Boundary Cavities during Creep Loading in Fe-Mo Alloys

“…In Figure 7, orientation maps for the solutionized Fe-Mo alloy are shown (a) before and (b) after creep at a stress of 160 MPa and a temperature of 838 K (565°C). In contrast to earlier measurements on creep-loaded solutionized Fe-Au alloys, [22] no indications for an extensive subgrain formation were observed. However, the average grain size for the Fe-rich bcc matrix phase derived from the EPMA figures was reduced from 33 lm before creep (Figure 7(a)) to 22 lm after creep (Figure 7(b)) (in accordance with the dynamic recrystallization assumed on the basis of the oscillating creep rate).…”

“…Previous TEM studies on similar model alloys (Fe-Cu, Fe-Cu-B-N, Fe-Au, and Fe-Au-B-N) did not reveal a high dislocation density for undeformed specimens that were heated to 823 K (550°C). [16][17][18]22] Precipitates are also present along grain boundaries, as shown in Figure 8(c). Some grain boundaries are completely covered with a thin film of the Fe 2 Mo phase (see Figure 8(d)).…”

“…[18][19][20][21][22] The choice for Au as the active solute was based on the fact that the solute Au atoms are appreciably larger than the matrix Fe atoms (in contrast to solute Cu atoms), resulting in a high nucleation barrier for precipitation within the matrix. As a result, the Au atoms show a strong preference to exclusively segregate at dislocations and creep cavity surfaces.…”

Finite element modelling of creep cavity filling by solute diffusion

“…The height of the creep cavity directly scales with the cavity radius a with h/a = (1 − cos ψ)/ sin (ψ) ≈ 0.77. Throughout the simulations the nominal concentration is chosen to be c 0 = 0.01, with an edge concentration of c 1 = 0.0001, in order to reflect the experimental situation for self healing in Fe-Au alloys [6,7,9]. As a consequence, the supersaturation c is comparable to the nominal concentration c 0 .…”

“…This mechanism was modelled by Karpov and coworkers [5]. Zhang and coworkers subsequently demonstrated that precipitation of substitutional solute leads to the self healing of creep cavities in Fe-Au and Fe-Mo alloys [6][7][8]. The creep defects and repairing precipitates were studied in detail for Fe-Au self-healing creep alloys, using X-ray nanotomography [9].…”