Fatigue crack propagation and cryogenic fracture toughness behavior in powder metallurgy aluminum-lithium alloys

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METAL processing by conventional ingot metallurgy (11M) largely confines the chemical compositions of alloys to the maximum solid solubility limits of alloying elements in the base metal. The powder metallurgy (P1M) route, on the other hand, offers greater latitude in designing crystalline and amorphous alloys with unique nonequilibrium compositions using techniques like gas atomization, melt spinning, splat cooling, and mechanical alloying. [1-5] By rapid quenching from the melt or pure mixing of elemental powders at ambient temperatures, such methods can extend the solubility limits of alloying elements yet achieve a high degree of microstructural homogeneity and refinement, i.e., reduced segregation and ultrafine grain size. The resulting microstructures often display exceptionally high strength (both from dislocation substructure and grain size hardening), moderate toughness, and good stress corrosion resistance compared to corresponding 11M alloy structures.[2,3.4] Powder metallurgy processing has particular significance for lithium-containing aluminum alloys, which constitute a new series of ultralight, high-stiffness structural materials with excellent strength, cryogenic toughness, and fatigue resistance. [6,7,8] First, current 11M casting

K.T. VENKATESWARA RAO, Research Scientist, and R.O. RITCHIE, Professor, are with the Center for Advanced Materials, Lawrence Berkeley Laboratory, and the Department of Materials Science and Mineral Engineering. University of California, Berkeley, CA 94720. Manuscript submitted February 5, 1990. METALLURGICAL TRANSACTIONS A

practices limit their compositions to less than 3 wt pct lithium. However, since each percent of lithium added (up to 4 wt pct) leads to ~3 pct lower density and ~6 pet higher modulus, P 1M techniques offer the potential of developing higher lithium containing alloys, with consequent lower density and elevated strength and stiffnessY] Second, with P 1M processing, high-temperature AI-Li alloys can be made containing transition elements, like Ti, Zr, and Hf, and other intermetallic-forming elements, like Fe, V, Ce, and Co (these elements also have limited solubility in aluminum). Such alloying additions delay the aging kinetics by retarding diffusion in aluminum and contribute to elevated temperature strength through dispersion-hardening mechanisms.[9.1O] Third, I/M AI-Li aUoys often suffer from poor ductility and toughness, associated with the highly localized nature of plastic deformation within planar slip bands and weak solute-depleted zones surrounding heterogeneously nucleated grain-boundary particles; short-transverse properties, for example, are particularly poor in thick section plateY] In cast and rolled 11M plates and sheets, these limitations can be partly overcome by permanent stretching prior to aging to promote copious precipitation within the matrix. Such operations are not feasible in forgcd and extruded products, necessitating the use of P 1M processing to homogenize slip via dispersion hardening.[ll] Two techniques