Mechanical properties and microstructures of Al-Mg-Sc alloys

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I.

INTRODUCTION

T H E quest for new, higher performance aluminum alloys has been in progress at least since the discovery of practical methods for producing aluminum itself. Almost from the beginning, the usefulness of such alloys in highperformance aerospace structures was recognized, and research to identify combinations of alloys and processes that produced unique combinations of mechanical and physical properties has, of course, been ongoing since that time. In recent years, this search has concentrated on alloys containing substantial amounts of Li t1'2'3~ because of the combined improvements in density and elastic modulus that can be obtained. However, there are other methods available to the alloy design specialist for decreasing density, and these avenues deserve attention as well. Observe that decreased density has been achieved in the current generation of A1-Li alloys as much by reducing addition of high atomic number elements like Cu and Zn as by adding Li. A logical alloying alternative is Mg, which has substantial solid solubility and is a strong solid solution strengthener. Unfortunately, it has been difficult to develop A1-Mg alloys and processes that would produce competitive levels of strength despite the significant reduction in density that could be realized. For example, an alloy containing 6 pct Mg would exhibit a density improvement over AA7075 that is only slightly less than that obtained for an alloy with 2 pct Li. Alloys with substantially greater amounts of Mg have been investigated, and strengths approaching the regime of precipitation-hardening alloys have been obtained for a warm worked AI-10Mg alloy. [4,5,61 Unfortunately, the use of these extremely high Mg levels results in serious commercial processing challenges, and such alloys can be susceptible to stress corrosion cracking. Another effective method for significantly increasing the strength of A1-Mg alloys (as well as others) was discovered and patented more than 15 years ago. t7j This method relies on the use of scandium as an alloying addition, taking advantage of its unique precipitationhardening characteristics. Scandium combines with A1 to form a stable L12 phase, A13Sc, that precipitates coherently in a spherical configuration, tS~ Despite the rela-

RALPH R. SAWTELL, Project Manager, and CRAIG L. JENSEN, Senior Scientific Associate, are with the ALCOA Laboratories, Alcoa Center, PA 15069. Manuscript submitted March 24, 1989. METALLURGICAL TRANSACTIONS A

tively low solubility of Sc and, hence, limited volume fraction of the A13Sc phase, it produces a significant strength increment, t7,9-111 In fact, A13Sc is the most potent strengthener, on an equal atomic fraction basis, known in Al-base systems. The A13Sc precipitate is also extremely effective in stabilizing substructure, thus allowing the use of strain-hardening and stabilization treatments to push the strength of A1-Mg-Sc alloys to the levels achieved by traditional precipitation-hardening systems. Achieving high strength combined with low density is clearly a worth