Abstract
Nanoindentation tests were conducted near the grain boundary (GB) of the Al-Mg-Si alloy, and the influence of GB character on the aging precipitation behavior and the mechanical properties was confirmed. After obtaining the GB characters by electron back scattered diffraction (EBSD) analysis, nanoindentation tests were carried out on under-aged, peak-aged, and over-aged samples. And then, the indentation areas were observed by back scattered electrons imaging (BSE) in order to identify indentation positions with respect to the GB. In this study, for the GB character, focusing on the rotation angle, the high-angle GB (HAGB) and the low-angle GB (LAGB) were selected. In addition, coincident site lattice GBs (CSL) were selected as the special GB. In the 180°C under-aged samples, the nano-hardness near GB is higher than that far from GB, while 180°C peak-aged samples, the nano-hardness is lower than that far from GB. Then the range near the GB where the hardness changes was larger at HAGB than at LAGB and CSL3. This suggests that the GB character affects the aging precipitation behavior and mechanical properties.
Highlights
Al-Mg-Si alloys are superior in workability and corrosion resistance as well as bake hardenability compared to other aluminum alloys and are used as body panels for automobiles
It can be confirmed that the range near the low-angle GB (LAGB) and the Σ3 coincident site lattice GBs (CSL) grain boundary (GB) where the hardness increases, or decreases is smaller than that near the highangle GB (HAGB)
Nano-indentation and transmission electron microscopy (TEM) observation were performed on Al-Mg-Si alloys to investigate how GB characteristics affect aging precipitation behavior and local mechanical behavior
Summary
Al-Mg-Si alloys are superior in workability and corrosion resistance as well as bake hardenability (age hardenability) compared to other aluminum alloys and are used as body panels for automobiles. When an aging heat treatment is applied to an Al-MgSi alloy, it is proposed that the aging precipitates transition in the following process [1,2]: SSSS→Mg-Si cluster→GP zone→β′′→β'→β. Precipitates that are predominantly precipitated in the alloy differ depending on the aging temperature and time, and β′′ is a precipitate involved in the automobiles body manufacturing process. As the heat treatment is continued, β′′ eventually transits to β', a rod-like semi-coherent precipitate, and to plate-like non-coherent precipitate, β (Mg2Si), the final stable phase. They have a smaller contribution to strength as compared to β′′ and are rarely used as the main reinforcing phase. It is necessary to clarify the nature of GB precipitates
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