Microalloying for the controllable delay of precipitate formation in metal alloys

Abstract

A model is presented to predict the effectiveness of dilute solutes in delaying precipitate formation, with application to natural and artificial aging in metal alloys. Control of aging is achieved via the binding, at natural aging temperatures, and release, at artificial aging temperatures, of excess quenched vacancies by the solute atoms. The binding of vacancies to the solute atoms reduces the vacancy concentration in the bulk lattice, and thus reduces the rate of transport processes that control aging. To be useful, this strategy requires sufficiently strong, but not too strong, vacancy-solute binding, and sufficiently slow vacancy-solute diffusion, both quantified through application of the model. First-principles methods are used to compute the controlling materials properties (vacancy-solute binding energies and migration barrier) for a range of solutes in Al and Mg, and Pauli Electronegativity of the solute relative to the host metal is found useful for correlating properties. With the computed inputs, the model is applied to representative solutes (Ga, Sn, Pb) in Al that span weak to strong vacancy binding. Predictions of the model to Sn in Al are then shown to qualitatively agree with experiments, with quantitative agreement possible using a slightly stronger Sn-vacancy binding energy. The general model should be useful for identifying promising dilute solutes for control of precipitation/aging across many alloys in which quenched vacancies are used to control precipitation.

Ab initio modelling of the early stages of precipitation in Al-6000 alloys

“…Aggregates of a few solute atoms that can act as vacancy traps [34] would thus slow vacancy-mediated solute diffusion that is necessary to form larger precipitates, greatly affecting the aging times. This conclusion of dominance of diffusion-controlled aging is also consistent with recent findings that the addition of 100 ppm of Sn to Al-6061 can significantly delay aging, attributed to trapping of the quenched-in vacancies by the Sn atoms [11,35]. Our results thus point toward the need for a systematic study of the energetics of aggregates in the GP-zone regime, and the interactions between those aggregates and vacancies and/or trace elements in the alloy to understand and fine-tune the behavior of Al-6000 alloys in the early stages of precipitation.…”

Precipitation kinetics in metallic alloys: Experiments and modeling

“…In metals, most solutes forming precipitates are substitutional and thus the diffusion mechanisms involve mainly the movement of vacancies. Therefore, the precipitation kinetics is directly controlled by the concentration of vacancies by their diffusion rate within the material and by their interaction with solutes [96][97][98]36,99,100]. An important exception is the precipitation of cementite in steels during martensite tempering, where carbon diffuses interstitially.…”

Structure–Property Correlation of Al–Mg–Si Alloys Micro-alloyed with Sn

“…But the problem arises during the transportation period, during which the formability of the alloy decreases due to an increase in hardness as a result of natural aging. [22][23][24] So, there is a demand for stable formability of this alloy during the transportation stage to the processing stage. Studies 13,22,24 have shown Sn as a potential microalloying element in this regard.…”

“…[22][23][24] So, there is a demand for stable formability of this alloy during the transportation stage to the processing stage. Studies 13,22,24 have shown Sn as a potential microalloying element in this regard.…”