Precipitate-induced nonlinearities of diffusion along grain boundaries in Al-based alloys

Abstract

The formation of precipitates in metallic alloys is determined by a two-way chemomechanical coupling. While the dependence on solute diffusion is apparent, the opposite effect, namely the impact of nanoprecipitates on the diffusion, is highlighted in the present paper. Using severe plastic deformation and post-deformation annealing of an Al-based alloy, different microstructures and sizes of Al 3 Sc-based nanoprecipitates are produced. The enhanced diffusion along grain boundaries in the so-called C-type kinetic regime is used as a probe to ensure a well-defined distance of the diffusion path from the precipitates and to prohibit their evolution during measurements. Tracer measurements with the radioisotope 57 Co reveal a remarkable nonmonotonic dependence of the diffusion rates on the annealing temperature. It has been fully explained by an ab initio informed phenomenological model that considers the elastic stress around the coherent and noncoherent precipitates.

A Combined Experimental and First-Principles Based Assessment of Finite-Temperature Thermodynamic Properties of Intermetallic Al3Sc

“…The critical particle size is a key parameter for the chemo–mechanical coupling between the strain fields caused by the precipitates and the solute kinetics in its vicinity. As recently demonstrated for the case of grain boundary diffusion in an Al-based alloy [ 101 ], this coupling leads to nonlinearities in the solute diffusivity. The transition between coherent and incoherent precipitate interfaces explained in this case a minimum in the diffusivity.…”

“…The symbols hexagons [ 98 , 102 ], up-triangles [ 103 ], square [ 104 ], crosses [ 105 ], down-triangles [ 106 ], pentagons [ 16 ] represent average experimental values for the coherency loss, whereas the variation in terms of temperature and size is given by error bars. The green symbol corresponds to our recent measurements [ 101 ]. The solid and dashed brown lines correspond to the calculation of critical diameters within a pure geometric consideration for two choices of the Burgers vector.…”

Grain Boundaries and Diffusion Phenomena in Severely Deformed Materials

“…Such a two-way coupling phenomenon was recently investigated using the radioisotope 57 Co as a probe of the impact of forming Al 3 Sc-based nano-precipitates on the diffusion in Al-based alloys. 75) Under ECAP processing and post-deformation annealing, different microstructures and precipitate sizes were obtained.…”

“…This unusual behavior has been fully explained by an ab initio informed diffusion model that considers formation of second phase Al 3 (Sc,Zr) particles at the interfaces, appearance of the elastic stresses around the coherent and non-coherent precipitates, and the evolution of the latter in the course of annealing. 75)…”

Models of recrystallization activated by a diffusion flow of impurities from a thin-film coating with a convection term at the crystal surface: exact solutions

“…It was found experimentally that the motion of the boundary of the recrystallized layer activated by the rest of the impurities obeys the root dependence on the annealing time [16,17,18]. The penetration of impurities from the coating occurs through grain boundary diffusion [19,20,21]. The penetration depth of impurities determines the width of the recrystallized layer in the case of their low solubility in the sample matrix [22,23].…”