Microstructural evaluation of arc-melted Al-Li-Be alloys

Abstract

A microstructural investigation has been carried out on a series of small castings of arc-melted AI-Li-Be alloys. The alloy composition included amounts of lithium up to 3wt% and beryllium up to 10 wt%. Optical metallographic examination has revealed gross macrosegregation in the highly alloyed compositions. On the microstructural level, the structure consists of primary beryllium particles in a matrix of primary aluminium containing eutectic structure at the cell walls. A detailed Auger electron spectroscopy examination has been carried out on a section of an AI-3Li-10Be arc-cast alloy to determine the precise compositional variations. This approach is demonstrated to be a necessary prerequisite for selection of material for rapid solidification processing by techniques such as splat quenching.

Microstructural evaluation of rapidly solidified Al-Li-Be alloys

“…At low temperature, although beryllium has only a small solid solubility in aluminium, 0.03 wt %, at increased temperatures ( Fig. 1 of Wadsworth et al [2]), it shows an increasing liquid solubility, reaching approximately 10% by weight at 1025 ~ C. Therefore, it is possible to obtain by rapid solidification a microstructure in which the aluminium solid solution contain a high concentration of fine discrete particles (dispersoids) of beryllium. A fine dispersion of these particles can have at least three important effects upon the mechanical properties of the aluminium alloys: (i) a decrease in density, (ii) an increase of the elastic modulus, and (iii) an increase in strength.…”

“…The production of wrought A1-Li-Be alloys by conventional ingot casting having useful engineering properties is not possible [1,2]. This is because beryllium segregates in a coarse distribution upon freezing, and this results in poor ductility and toughness.…”

“…The rationale for the development of ternary AI-Li-Be alloys has been presented recently by Lockheed scientists [1,2]. It was shown that such alloys could have important applications for aerospace structures because of their high potential weight savings.…”

“…In fact, lithium and beryllium are the only elements that significantly have both of these effects when added to almninium. Recent studies of the various factors influencing weight savings in aerospace structures clearly emphasize the dominant importance of reduced density [2].…”