High-Modulus Aluminum Alloys with Beryllium and Magnesium

Abstract

Al – Be – Mg alloys possess a valuable combination of physical and mechanical properties the most important of which is the high modulus of elasticity. Commercial Al – Be alloys bearing 5 – 80% Be are hypereutectic. Hypereutectic alloys of aluminum with beryllium (over 1.15% Be) have a binary structure containing primary dendrites of a solid solution of aluminum in beryllium (the Be phase) and a eutectic that does not have a typical eutectic structure and is represented by pure aluminum. This is explainable by the fact that beryllium in an amount of 1.15%, which should enter the eutectic, is segregated on the primary crystallizing dendrites of the beryllium phase. The presence of a very soft and low-strength aluminum phase in the structure explains the relatively low strength characteristics of these alloys. Elements that dissolve in aluminum but are insoluble in beryllium and do not form compounds with it are used as additives simultaneously improving the strength and ductility of Al – Be alloys. Such elements are magnesium and zinc. The most promising are alloys of the Al – Be – Mg (ABM) system lying in the double-phase range. Their structure is composed of a primary crystallizing beryllium phase (Be) and a hardened Al solid solution of magnesium in aluminum. When the composition of an alloy goes outside the double-phase region, its mechanical properties worsen noticeably due to the appearance of coarse segregations of a brittle Al – Mg – Be-phase represented by a solid solution based on a -phase of the Al – Mg system. For this very reason the strength of the alloys (and the ductility) first grows with the content of magnesium and then decreases (Fig. 1). The maximums on the curves directly depend on the maximum solubility of magnesium in the Al-phase. At the same content of beryllium the strength of Al – Be – Mg alloys is much higher than that of Al – Be alloys (American Lokeloy alloys) (Fig. 2). For example, at 30% Be the ultimate rupture strength r increases from 200 MPa (binary alloy) to 500 MPa (ternary alloy), and at 50% Be it increases from 300 MPa to almost 600 MPa, respectively. The Al – Be – Mg alloys bearing up to 70% Be are superior to binary Al – Be alloys not only with respect to the strength but also with respect to the elongation and the modulus of elasticity. The modulus of elasticity (E ) of ternary alloys is 15 – 30 GPa higher than that of binary alloys, though the limited amount of magnesium introduced into the composition has a modulus of elasticity (45 GPa) even lower than that of aluminum. The explanation of these regularities should be sought for in the physical properties of the structural components of the alloys. In binary alloys they are represented by a mixture of two quite inhomogeneous phases. In ternary alloys the inhomogeneity of the phases is lower. In addition, magnesium promotes considerable disintegration of the structural components both in the crystallization process and in subsequent deformation of the ingots. This is especially vividly ilMetal Science and Heat Treatment Vol. 45, Nos. 9 – 10, 2003