Undergraduates
Aluminum alloys containing Mg and Si as main alloying elements (6xxx series alloys) are used extensively as structural materials due to their formability, mechanical strength, and corrosion resistance. These alloys display a large increase in hardness upon aging at elevated temperatures, which is attributed to the formation of nanosized metastable clusters and semi-coherent precipitates that disrupt dislocation movement in the fcc Al host lattice. The form, structure and strengthening properties of age-hardening precipitates depend on the alloy composition and the thermo-mechanical history of the material.
Dependent on formation and interfacial energies, vacancy concentration, lattice coherence and strain fields in the host lattice, solutes cluster and form precipitates through substitutional diffusion. All metastable precipitates are coherent with the Al host lattice along <001>Al and are needles/rods parallel to these directions. Coherency stresses arise in the {001} Al planes, normal to the needle direction, around the precipitates.
Our target is to understand more of the fundamental physics responsible for solute nucleation, clustering, phase transformations and precipitation in these alloys. Being able to generate accurate physical models at the atomistic scale, we can design the right alloy for the right applications ' and advanced alloy design can be used to establish quantitative relationships between structure and properties to tailor materials with the desired properties.
We have mapped and determined most precipitate phases in Al-Mg-Si, including Cu and Ge additions. The progress has been made possible by combining advanced transmission electron microscopy techniques (quantitative nanobeam diffraction and high angle annular dark field scanning TEM) with atom probe tomography and calculations based on density functional theory [1]. In this talk I will give the background, present the different experimental and theoretical methods used and show examples of recent and ongoing work in our group [2, 3].
[1] Hasting HS, Frseth AG, Andersen SJ, Vissers R, Walmsley JC, Marioara CD, Danoix F, Lefebvre W, Holmestad R J. Appl. Phys. 106, 123527, 2009.
[2] Torster M, Ehlers FJH, Marioara CD, SJ Andersen, Holmestad R, Phil. Mag., DOI: 10.1080/14786435.2012.693214.
[3] Bjrge R, Dwyer C, Weyland M, Nakashima PNH, Marioara CD, Andersen SJ, Etheridge J, Holmestad R, Acta Mat., 60, 3239, 2012.
* Co-workers are C.D. Marioara, F.J.H. Ehlers, M. Torster, R. Bjrge, T. Saito, S. Wenner, J. Ryset and S.J. Andersen from SINTEF, NTNU and Hydro Al in Norway.
