Abstract
During ageing, 6xxx aluminium alloys will develop a microstructure characterised by needle-shaped Mg/Si-rich precipitates in the bulk, precipitate-free zones along the grain boundaries and larger Mg/Si-rich precipitates on the grain boundary. Depending on, among other things, the size of the precipitate-free zone, these alloys are prone to intergranular fracture. The role of the grain boundary precipitates during the initiation and propagation of the intergranular fracture is still not fully understood. Transmission Electron Microscopy has been used to characterise the grain boundaries and grain boundary precipitates. The precipitates were found to be of the β′ type surrounded by a layer of U2 structure. The atomic details of relevant interfaces of Al-β′ were characterised for further investigation. Density Functional Theory simulations were performed on the bulk precipitate structures and on the interfaces obtained experimentally. The decohesion energy of these interfaces was calculated and compared to bulk values. In addition, simulated tensile tests were performed in order to find values for the tensile strength σt. The dependence of the interfacial energy and tensile strength of β′ grain boundary precipitates were found to depend on the orientation and type of interface in addition to the amount of defects on the interface.
Highlights
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We investigate the role of grain boundary precipitates during intergranular fracture in Al-Mg-Si alloys by studying the decohesion energy and tensile strength of the interfaces between β0 grain boundary precipitates and bulk aluminium
This demonstrates that β0 is one type of precipitate that typically forms on such grain boundaries
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The fracture occurring in these alloys is ductile, and the crack propagation is found to be a combination of transgranular and intergranular fracture, depending on the microstructure [1,2,3]. The main strengthening precipitates in 6xxx series aluminium alloys belong to the β00 phase. Starting with the Guinier–Preston (GP) zones, all the precipitates have a main coherency direction with the Al matrix, along h100i Al directions. Metastable phase precipitation is dependent on whether the grain boundary plane contains a h100i Al direction from both or one of the adjacent grains This means that coherent metastable precipitation will occur along the grain boundaries with beneficial grain boundary planes. In overaged 6xxx alloys without Cu and Zn, phases such as β0 , U2 and B0 have been observed, with β0 as the most numerous [4]
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