Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy

TL;DR

This study investigates the natural aging process in Al-Mg-Si alloys using positron annihilation lifetime spectroscopy, revealing multiple stages of clustering and precipitation with complex vacancy-solute interactions.

Contribution

It provides a detailed real-time analysis of the kinetics and stages of natural aging in Al-Mg-Si alloys, highlighting the role of vacancies and solute interactions.

Findings

Natural aging occurs in at least five stages.

Early clustering involves a positron trap with ~0.200 ns lifetime.

Mg influences both kinetics and lifetime changes during aging.

Abstract

The process of natural aging in pure ternary Al-Mg-Si alloys was studied by positron annihilation lifetime spectroscopy in real time in order to clarify the sequence and kinetics of clustering and precipitation. It was found that natural aging takes place in at least five stages in these alloys, four of which were directly observed. This is interpreted as the result of complex interactions between vacancies and solute atoms or clusters. One of the early stages of positron lifetime evolution coincides with a clustering process observed by differential scanning calorimetry (DSC) and involves the formation of a positron trap with \sim 0.200 ns lifetime. In later stages, a positron trap with a higher lifetime develops in coincidence with the DSC signal of a second clustering reaction. Mg governs both the kinetics and the lifetime change in this stage. Within the first 10 min after quenching, a period of nearly constant positron lifetime was found for those Mg-rich alloys that later show an insufficient hardness response to artificial aging, the so-called "negative effect." The various processes observed could be described by two effective activation energies that were found by varying the aging temperature from 10 to 37\degree C.