Changes in the structure and microhardness of rapidly solidified foils of aluminum alloy 1421 during their annealing

Abstract

In this work, the relationship between the microstructure and microhardness of Al–Mg–Li–Zr–Sc alloy (1421 Al) prepared by ultrafast quenching from the melt has been studied. The following methods are used in studying the rapidly solidified (RS) alloy: scanning electron microscopy integrated with energy dispersive X-ray microanalysis, the method of nuclear reaction analysis, and the measurement of microhardness changes during isochronal annealing. The intercept method is applied to determine the size of secondary phases, their volume fraction, and the specific surface area of the interface boundaries in the samples. It is established that the as-quenched rapidly solidified alloy foils are composed of an aluminum-based supersaturated solid solution. It is found that lithium, the content of which reaches 9.0 at %, is unevenly distributed over the subsurface region of foils. After annealing at 300°C, precipitates of (Sc, Zr)-containing phase are detected in the structure of foils in addition to magnesium-containing phases. Nonmonotonic changes in the microhardness are observed during isochronal annealing of the foils in the temperature ranges of 50–100°C, 150–210°C, 230–340°C, which are associated with the precipitation of metastable and stable phases. It is found that heating of the alloy foils to 340°C leads to an increase in the microhardness by 23%, and a sharp decrease in the microhardness takes place at temperatures above 400°C.