Rafting, TWIP steel and Al-Cu

August 4, 2011

[1] Creep deformation and rafting in nickel-based superalloys simulated by the phase-field method using classical flow and creep theories

Y Tsukada et al

A phase-field model has been developed to simulate the evolution of both (γ + γ′) microstructure and inelastic strain between γ′ phases (i.e. γ channel) during high-temperature creep in nickel-based superalloys. Inelastic strain is defined as the sum of time-independent and time-dependent components. Previously reported mechanical properties of single-phase γ alloys are considered in the calculation of inelastic strain evolution. A two-dimensional phase-field simulation is performed, and the results of microstructure evolution and the creep rate vs. time curve are fitted to the experimental data of the high-temperature creep of CMSX-4. The slope of the creep rate vs. time curve during the initial stage of transient creep, the plasticity preference in different types of γ channels, and the rafting phenomenon are reproduced well by the simulation. Furthermore, it is demonstrated that the creep rate increases locally at γ/γ′ interfaces when the rafted structure is formed.

[2] Dislocation and twin substructure evolution during strain hardening of an Fe–22 wt.% Mn–0.6 wt.% C TWIP steel observed by electron channeling contrast imaging

Gutierrez-Urrutia and Raabe

We study the kinetics of the substructure evolution and its correspondence to the strain hardening evolution of an Fe–22 wt.% Mn–0.6 wt.% C TWIP steel during tensile deformation by means of electron channeling contrast imaging (ECCI) combined with electron backscatter diffraction (EBSD). The contribution of twin and dislocation substructures to strain hardening is evaluated in terms of a dislocation mean free path approach involving several microstructure parameters, such as the characteristic average twin spacing and the dislocation substructure size. The analysis reveals that at the early stages of deformation (strain below 0.1 true strain) the dislocation substructure provides a high strain hardening rate with hardening coefficients of about G/40 (G is the shear modulus). At intermediate strains (below 0.3 true strain), the dislocation mean free path refinement due to deformation twinning results in a high strain rate with a hardening coefficient of about G/30. Finally, at high strains (above 0.4 true strain), the limited further refinement of the dislocation and twin substructures reduces the capability for trapping more dislocations inside the microstructure and, hence, the strain hardening decreases. Grains forming dislocation cells develop a self-organized and dynamically refined dislocation cell structure which follows the similitude principle but with a smaller similitude constant than that found in medium to high stacking fault energy alloys. We attribute this difference to the influence of the stacking fault energy on the mechanism of cell formation.

[3] Precipitates in Al–Cu alloys revisited: Atom-probe tomographic experiments and first-principles calculations of compositional evolution and interfacial segregation

A Biswas et al

Atom-probe tomography, transmission electron microscopy, X-ray diffraction and first-principles calculations are employed to study: (i) compositional evolution of GPII zones and θ′ precipitates; and (ii) solute segregation at α-Al/θ′ interfaces in Al–1.7 at.% Cu (Al–4 wt.% Cu) alloys. GPII zones are observed after aging at 438 K for 8 h, whereas higher aging temperatures, 463 K for 8 h and 533 K for 4 h, reveal only θ′ precipitates. Most GPII zones and θ′ precipitates are demonstrated to be Cu-deficient at the lower two aging temperatures; only the 533 K treatment resulted in θ′ stoichiometries consistent with the expected Al2Cu equilibrium composition. For alloys containing not, vert, similar200 at. ppm Si we find evidence of Si partitioning to GPII zones and θ′ precipitates. Significant Si segregation is observed at the coherent α-Al/θ′ interface for aging at 533 K, resulting in an interfacial Si concentration more than 11 times greater than in the α-Al matrix. Importantly, the Si interfacial concentration undergoes a transition from a non-equilibrium delocalized profile to an equilibrium localized profile as the aging temperature is increased from 463 to 533 K. Consistent with these measurements, first-principles calculations predict a strong thermodynamic driving force favoring Si partitioning to Cu sites in θ′. Silicon segregation at, and partitioning to, θ′ precipitates results in a decrease in interfacial free energy, and concomitantly an increase in the nucleation current. Our results suggest that Si catalyzes the early stages of precipitation in these alloys, consistent with the higher precipitate number densities observed in commercial Al–Cu–Si alloys.

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