Elevated temperature fracture of RS/PM alloy 8009: part i. fracture mechanics behavior
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I.
INTRODUCTION
A. Elevated-Temperature Aluminum Alloys EXTENSIVE research aims to develop aluminum alloys for aerospace applications at elevated temperatures as high as 400 ~ 1~-TJ For example, a mach 2.4 HighSpeed Civil Transport airframe will require light alloys that operate at temperatures between 125 ~ and 210 ~ for prolonged times.t81 Ingot metallurgy (IM) precipitation-hardened aluminum alloys (e.g., 2618, 2219, 2024, 2519, 2090, 2095, and 7075) are limited to applications below about 150 ~ due to microstructural instability. Advanced high-temperature aluminum alloys contain substantial concentrations of low-solubility, lowdiffusivity transition elements (e.g., Fe, Ce, Cr, Si, and V) and are processed using novel rapid solidification (RS) and powder metallurgy (PM) techniques to produce ultrafine grain-size, dispersion-strengthened microstructures.13 6.9] Such alloys include inert gas-atomized A1-Fe-Ce (8019, developed by ALCOA*) and AI-Fe*ALCOA is a trademark of the Aluminum Company of America, Pittsburgh, PA.
Mo-V (Pratt and Whitney, West Palm Beach, FL), meltspun AI-Fe-V-Si (8009, Allied-Signal, Morristown, NJ) and A1-Cr-Zr (Alcan, Banbury, UK), as well as WILLIAM C. PORR, Jr., formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia, is Materials Engineer, Fatigue and Fracture Branch, Code 614, the Naval Surface Warfare Center, Carderock Division, Annapolis, MD 214025067. RICHARD P. GANGLOFF, Professor of Materials Science and Engineering, is with the University of Virginia, Charlottesville, VA 229O3. Manuscript submitted April 9, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanically alloyed AI-Ti (Inco Alloys International, Huntington, WV). t9,~~ These alloys exhibit high stiffness and strength, as well as dispersoid and graincoarsening resistance at temperatures to 400 ~ A limitation of elevated-temperature aluminum alloys is that most exhibit decreased tensile elongation with increasing temperature from 25 ~ to 350 ~ 12'l~ This behavior is unusual compared with that of wroughtaluminum alloys such as 2618 l~~ and is due to the unique microstructural and compositional characteristics of RS/PM alloys. The uniaxial tensile deformation and fracture behavior of alloy 8009 was studied, with several mechanisms proposed to explain the ductility degradation with increasing temperature and decreasing loading rate. 113'141 These include reduced extrinsic delamination toughening, t~5,161 oxygen or hydrogen embrittlement from the moist-air environment, t~71 emhrittlement from dissolved hydrogen retained from PM processing, ttT'~sl dynamic strain aging (DSA) due to nonequilibrium solid-solution iron, t~4,~9-2~1 low work hardening and low strain-rate hardening with localized plastic deformation, [~2.22.23~and thermally-enhanced dispersoid-matrix or oxide-matrix interface decohesion. I24j Quantitative and mechanistically interpretable measures of tensile ductility, fracture toughness, and long-term crack-growth resistance are limited for hightemperature aluminum a
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