Speaker
Description
Heat-treatable aluminium (Al) alloys obtain their strength from the nucleation of precipitates during artificial aging. The thermomechanical history of the alloy determine the type of precipitate phase that nucleate, their interface plane and their distribution. If the alloy is stored at room temperature after solution heat treatment, the hardness increases in time due to the formation of nano-sized clusters. The clusters that form are known to affect the subsequent precipitation process during artificial ageing by either promoting or impeding the nucleation of hardening precipitates. Hence, for design of better alloys it is important to understand the crystal structure and to quantify the distribution of these clusters. The main cluster type in the Al-Zn-Mg alloy system is the GPI zones and their crystal structure has been a subject for debate for decades. One of the most powerful tools to determine the crystal structure of precipitates in Al alloys is transmission electron microscopy (TEM). The GPI zones however are small compared to the thickness of a typical TEM specimen, making it challenging to detect them. In this work, we elucidate the structure of the GPI zones [1]. Based on atomically resolved high-angle annular dark-field scanning TEM images, we found a unique fundamental building block for the clusters. The unit is described by partial substitution of Mg and Zn on the fcc Al matrix positions and its surrounding truncated cube octahedral (TCO) shell with a possible interstitial position at the centre of the TCO. A simple set of principles describes how these units arrange to form larger clusters. Density functional theory calculations, scanning precession electron diffraction and simulated diffraction patterns support the proposed atomic models. The newfound undestanding is believed to play a major role in the future development of age-hardenable Al alloys.
[1] A. Lervik et al. Acta Materialia (2021). DOI: 10.1016/j.actamat.2020.116574
| Speaker Country | Norway |
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