Modeling Electrical Resistivity of Naturally Aged Al–Mg–Si Alloys

Abbas Bahrami,Maryam Yazdan Mehr

doi:10.3390/met9030310

Abbas Bahrami, Maryam Yazdan Mehr

Open Access

https://doi.org/10.3390/met9030310

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Abstract

Isothermal ageing of Al–Mg–Si alloys, stored at room temperature for more than 5 months, is associated with an unexpected significant increase in the overall electrical resistivity. This unexpected anomalous increase is not observed in alloys with shorter storage (natural ageing) times. This phenomenon is explained with a scenario, based on the evolution of the size distribution of Guinier–Preston (GP) zones during natural ageing and during subsequent artificial ageing. The proposed scenario can explain the contribution of natural ageing atomic clusters to this anomalous increase in the electrical resistivity. A physically based combined precipitation–electrical resistivity model, with the former being based on simultaneous nucleation-growth-coarsening reactions and the latter based on the Bragg scattering of electrons from atomic clusters, has been used to explain the electrical resistivity evolution. It is shown that the proposed model is capable of reproducing the experimental data in both short natural ageing (less than 5 months) and long natural ageing (more than 5 months) regimes.

Highlights

The heat treatable Al–Mg–Si alloys are widely used in structural, automotive, and aerospace industries, since they show an optimum combination of weldability, formability, corrosion resistance, and mechanical properties [1]
A→ B: In this stage, both hardness and resistivity slightly increased. This slight increase in both hardness and resistivity was due to the nucleation of GP-II zones [9,10,11]
A few minutes after nucleation of GP-II zones, their sizes were in the range of the wavelength of conduction electrons at the Fermi level

Summary

Introduction

The heat treatable Al–Mg–Si alloys are widely used in structural, automotive, and aerospace industries, since they show an optimum combination of weldability, formability, corrosion resistance, and mechanical properties [1]. The mechanical properties of Al–Mg–Si alloys are very sensitive to the precipitation hardening and precipitation sequence. Guinier–Preston (GP)-II zones, on the other hand, are atomic clusters with a structure similar to β00 but with, to some extent, different positions for Mg atoms along the Al direction [5]. These precipitates have needle-shaped morphologies with the composition of Mg2+x Si2+y Al7-x-y [7]. Β0 and β (Mg2 Si) are precipitates, which are formed after peak-age, when the coarsening reaction is prevailing. Recent theories [9,10,11]

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Conclusion