# Unified Interpretations of Two Kinds of Needle-Shaped Precipitates Using Transmission Electron Microscopy and Small-Angle Neutron Scattering in Aged Al–Mg2Si(-Cu) Alloys

## TL;DR

This study uses electron microscopy and neutron scattering to analyze needle-shaped precipitates in aluminum alloys, revealing a new phase that affects hardness.

## Contribution

The study identifies and characterizes a new precipitate phase (R phase) in Al–Mg2Si alloys and shows how Cu addition influences its formation and properties.

## Key findings

- The R phase forms as a needle-shaped precipitate and partially aligns along the axis of random-type precipitates.
- Cu addition stabilizes and promotes the growth of the R phase, enhancing hardness in the alloy.
- The R phase is present even in Cu-free alloys but with lower volume fraction and slower growth.

## Abstract

This study investigates the nanostructural properties of pseudo-binary Al–1.0Mg2Si (mass%) alloys with and without 0.5Cu using transmission electron microscopy (TEM) and small-angle neutron scattering (SANS). The TEM results show that both alloys exhibit extra electron diffraction spots related to MgSiMg second clusters at peak-aged conditions. High-resolution TEM images have revealed that the second cluster exists as a needle-shaped precipitate that is shorter and thicker than the β″ phase. We found that the second cluster, which we referred to as the R phase in this paper, is more likely to form partially along the longitudinal axis of a random-type precipitate. Thus, the atomic arrangement in the random-type precipitate is not completely random. SANS is used to quantify the size and volume fraction of the observed needle-shaped precipitates since the R phase is difficult to observe with TEM. The R phase forms even in the Cu-free alloy, but the volume fraction is low, and the growth and formation are retarded near the peak-aged conditions. Undoubtedly, the Cu addition has the effect of stabilizing the growth of the R phase and also promoting its formation. Therefore, the R phase also contributes to the increase in hardness at both under- and peak-aged conditions in the Cu-containing alloy in addition to the strengthening β″ phases.