Mechanism of the Bauschinger effect in Al-Ge-Si alloys

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2017-01, Vol.684, p.353-372
• Main Authors: Gan, Wei, Bong, Hyuk Jong, Lim, Hojun, Boger, R.K., Barlat, F., Wagoner, R.H.
• Format: Article
• Language: English
• Subjects: Aging (metallurgy); Al-Ge-Si; Aluminum base alloys; Bauschinger effect; Bypasses; Compression tests; Computer simulation; Dilution; Dislocation pinning; Grain boundaries; Heat treating; Heat treatment; Microstructure; Orowan bypass; Precipitate; Precipitates; Precipitation hardening alloys; Shearing; Solution strengthening; Super-Dislocation model; Superdislocations; Tensile tests; Tension tests
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2016.12.020

Wrought Al-Ge-Si alloys were designed and produced to ensure dislocation bypass strengthening (“hard pin” precipitates) without significant precipitate cutting/shearing (“soft pin” precipitates). These unusual alloys were processed from the melt, solution heat treated and aged. Aging curves at temperatures of 120, 160, 200 and 240°C were established and the corresponding precipitate spacings, sizes, and morphologies were measured using TEM. The role of non-shearable precipitates in determining the magnitude of Bauschinger was revealed using large-strain compression/tension tests. The effect of precipitates on the Bauschinger response was stronger than that of grain boundaries, even for these dilute alloys. The Bauschinger effect increases dramatically from the under-aged to the peak aged condition and remains constant or decreases slowly through over-aging. This is consistent with reported behavior for Al-Cu alloys (maximum effect at peak aging) and for other Al alloys (increasing through over-aging) such as Al-Cu-Li, Al 6111, Al 2524, and Al 6013. The Al-Ge-Si alloy response was simulated with three microstructural models, including a novel SD (SuperDislocation) model, to reveal the origins of the Bauschinger effect in dilute precipitation-hardened / bypass alloys. The dominant mechanism is related to the elastic interaction of polarized dislocation arrays (generalized pile-up or bow-out model) at precipitate obstacles. Such effects are ignored in continuum and crystal plasticity models.