Structure and properties of a rapidly solidified Al-Li-Mn-Zr Alloy for high-temperature applications: Part I. inert gas
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I.
INTRODUCTION
A L - L i alloys for aerospace structures have been subject to extensive research in the past 20 years. [1"2'3] The objective of that research has been to improve performance in lowering densities and increasing strength. Lithium addition to an A1 matrix is known to provide a dramatic increase in elastic modulus (6 pc[ per each wt pct Li added) and a significant decrease in density (3 pct per each wt pct Li added). The stiffness improvement results from solution of Li in the matrix and from coherent precipitation of A13Li. The equilibrium solid solubility of Li in A1, however, is limited. In order to increase that solubility beyond the equilibrium limit, rapid solidification (RS) techniques are beneficial in producing stable alloys with extended Li solid solubility.[4] Addition of Li to A1 produces an age-hardenable alloy, with the following precipitation sequence: supersaturated solid solution ~ ' A13Li ~ ~ A1Li. After suitable heat treatment, the transition phase AIaLi forms in spherical particles having an ordered L12 crystal structure. [Sj A1-Li alloys have so far shown low ductility and fracture toughness. The reasons for that are strain localization effects within the A1-Li matrix, resulting from planar slip associated with the shearable nature of the ordered 3' precipitates. Identical strain localization effects may occur at grain boundaries, due to the presence of "soft" precipitate-free zones (PFZs)
MICHAEL RUHR, Graduate Student, and JOSEPH BARAM, Senior Lecturer, are with the Materials Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel. Manuscript submitted May 14, 1990. METALLURGICALTRANSACTIONS A
adjacent to them. Refining the grain size and adding fine hard dispersoids could provide a remedy to this crucial ductility problem. The introduction of transition metals like Zr and Mn yields grain refinement and also inhibits recrystallization, improving corrosion resistance and mechanical properties at elevated temperatures. Zirconium forms a very stable dispersoid, fl' A13Zr, as a result of low Zr solubility in AI. These dispersoids have an additional role in providing heterogeneous nucleation sites for the 6' precipitates, t6] Zirconium-containing alloys age more rapidly than a similar alloy containing no Zr. tT] Manganese has been investigated as a possible dispersoid formarion element through the A16Mn phase. An inert helium gas processed powder (2.5 wt pct Li, 2.8 wt pct Mn) has shown mechanical properties in the as-extruded condition at room temperature (yield: 494 MPa, ultimate: 516 MPa), slightly better than the 2024-type alloys of similar density and 13 pct higher specific modulus. [8] The presence of nonshearable dispersoids, such as ml6Mn, can be effective in dispersing slip and alleviating the problem associated with the planar slip. However, it has also been reported that the Mn dispersoids promote the formation of the stable ~ A1Li phase during aging, resuiting in consequent degradation of the mechanical properties.[9] Rapid solidification has th
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