Segregation assisted grain boundary precipitation in a model Al-Zn-Mg-Cu alloy

Segregation assisted grain boundary precipitation in a model Al-Zn-Mg-Cu alloy

The objective of the project is to investigate grain boundary precipitation in comparison to bulk precipitation in a model Al-Zn-Mg-Cu alloy during aging.

Al-Zn-Mg-(Cu) Al-alloys are well known to become highly susceptible to stress corrosion cracking and subject to brittle intergranular fracture under certain conditions of heat treatment. The embrittlement has been attributed to the formation of the coarse, incoherent precipitates along grain boundaries and existing of precipitation free zones in the vicinity of grain boundaries, which leads to softening and substantially reduces the fracture resistance of these alloys. Hence, understanding the composition evolution of grain boundaries and grain boundary precipitation at near-atomic scale in these alloys is crucial to tailor mechanical properties and to increase resistance to corrosion and stress corrosion cracking.

The objective of the project is to investigate grain boundary precipitation in comparison to bulk precipitation in a model Al-Zn-Mg-Cu alloy during aging. We follow the kinetics of solutes segregation to grain boundaries, the formation and growth of the precipitates in the bulk and on the grain boundaries and the evolution of PFZs, so as to unveil the effects of solutes segregation on the grain boundary precipitation in a model Al-Zn-Mg-Cu alloy in a more holistic fashion. Atom probe tomography (APT), transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD) microstructural characterizations are combined with hardness measurements in this study.

Atom probe analysis for a sample in the as-quenched state: (a), EBSD IPF map showing the grain boundary (GB) for site-specific APT tip preparation; (b), A desorption map showing indexed crystallographic poles of two grains; (c), Atom maps of all elements in the as-quenched state; (d), Composition profile across the grain boundary in a 20nm-diameter cylinder. (Al, Zn, Mg, and Cu are depicted as grey, dark cyan, olive, and dark red, respectively)

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