Thermodynamic model for lattice point defect-mediated semi-coherent precipitation in alloys

Abstract

The formation of precipitates with an atomic volume different from their parent phase eventually leads to a loss of the lattice continuity at the matrix–precipitate interface. Here, we show the creation or removal of lattice sites mediated by lattice point defects is an accommodation mechanism of the coherency loss and even a precipitation driving force. We introduce a thermodynamic approach that rationalizes the selection of phases resulting from chemical and crystallographic constraints in relation to point defect properties. The resulting semi-coherent phase diagram and the precipitation kinetic model depend on the equilibrium phase diagram, the eigenstrain of the precipitating phase, and the chemical potential of point defects. From a joint experimental and modeling study, we uncover the prominent role of excess point defects in unforeseen phase transformations of the Fe–Ni metallic system under irradiation. By addressing the fundamental role of lattice point defects in the accommodation mechanisms of precipitation, we provide a step torwards the understanding of semi-coherent phase transformations occurring in solid materials upon synthesis and in use.

Efficient and transferable machine learning potentials for the simulation of crystal defects in bcc Fe and W

“…The formation free energy of mono-vacancies is an important quantity for parametrization of multi-scale models like kinetic Monte Carlo, cluster dynamics, dislocation dynamics, etc. Its magnitude controls the density of vacancies, which acts as a driving force of self-diffusion [119], vacancy flux coupling [155][156][157][158], Ostwald ripening [159], climb mechanisms [160][161][162], phase transformations [160], etc. Until now, in bcc metals there are no studies that investigate the anharmonic contribution to the formation free energy of vacancies in Fe and W. The vast majority of multiscale models, cited above, use the values of vibrational entropy suggested by experience or the values computed using ab initio techniques within the harmonic approximation.…”

Impact of the local microstructure fluctuations on radiation-induced segregation in dilute Fe-Ni and Ni-Ti model alloys: A combined modeling and experimental analysis

“…The authors of Ref. [16] report that radiation-induced precipitation of metastable face-centered cubic phases (containing around 25 at.% Ni and 50 at.% Ni) may occur at dislocation loops in Fe-3.3Ni model alloys [17]. Most of the measured segregation profiles presented in Fig.…”

“…In these samples, in low-fluence regions, dislocation loops are systematically enriched in Ni. At higher fluences, dislocation loops were replaced by fcc precipitates [16,17]. Here, we provide a systematic characterization of the Ni segregation at dislocation loops in the same model alloy, but exposed at higher radiation flux.…”

“…An early-dose RIS of solutes at dislocation loops, a late-dose observation of Mn-Ni-Si-rich precipitates in RPV ferritic steels [13,14,15], as well as Ni-rich austenitic phases in an ion-irradiated Fe-3%Ni model alloy [16] suggest that the precipitation starts from RIS at dislocation loops. Note that, in these systems, precipitates seating on dislocation loops were not found, as if the removal of the dislocation loop was a necessary step of the phase transformation [17].…”

Atomistic simulations of diffusive phase transformations with non-conservative point defects

“…The response of the system to a non-equilibrium PD concentration is many-fold. It mainly depends on the competition between diffusion, annihilation/creation reactions at SAS, and agglomeration reactions involving PDs [3] and/or alloying elements [17][18][19][20]. The agglomeration of non-equilibrium PDs in clusters is recognized to be responsible for damaging electronic and optical properties of semi-conductors [16,21], or the dimensional and mechanical properties of nuclear plant components [3].…”