Nucleation and Precipitation Strengthening in Dilute Al-Ti and Al-Zr Alloys
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THE Group 4 transition metals (Ti, Zr, or Hf) constitute a group of alloying additions to Al that show particular promise for developing creep-resistant, thermally stable Al-based alloys.[1] In each of these systems, an ordered Al3M (M = Ti, Zr, or Hf) trialuminide may be precipitated from a supersaturated solid solution during post-solidification aging. While the equilibrium structure of these Al3M trialuminides is tetragonal (D022 or D023), decomposition of supersaturated Al-M solid solutions occurs initially by the formation of nanometer-scale metastable cubic Al3M (L12) precipitates exhibiting small lattice parameter mismatches with a-Al, which transform to their respective equilibrium tetragonal structures after prolonged aging (approximately 100 to 1000 hours) at elevated temperatures (>450 C). Moreover, these transition elements are anomalously slow diffusers in a-Al, with very limited equilibrium solid solubilities, enabling precipitated Al3M to be resistant to Ostwald ripening KEITH E. KNIPLING, formerly with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, National Research Council Associate, is with the Naval Research Laboratory, Washington, DC 20375-5320, USA. Contact e-mail: [email protected] DAVID C. DUNAND, James and Margie Krebs Professor of Materials Science and Engineering, is with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA. DAVID N. SEIDMAN, Walter P. Murphy Professor of Materials Science and Engineering, is with the Department of Materials Science and Engineering and Northwestern University, Evanston, IL 60208-3108, USA. Center for Atom-Probe Tomography (NUCAPT), Northwestern University. Manuscript submitted December 15, 2006. Article published online September 13, 2007. 2552—VOLUME 38A, OCTOBER 2007
in accordance with volume diffusion-controlled coarsening models.[1] For high-temperature applications, Ti seems especially promising as an alloying addition among the Group 4 elements because it has the smallest atomic weight and the smallest diffusivity in a-Al.[1] Indeed, Al alloys reinforced by Al3Ti dispersions have garnered considerable interest as potential lightweight, hightemperature structural materials, and the thermal stability and strength at high temperatures of these alloys is well documented.[2–18] These studies, however, investigated alloys prepared by rapid solidification processing (RSP)[2–8] or mechanical alloying (MA)[2,9–18] techniques, whose nonequilibrium processing routes circumvent the difficulties encountered during conventional solidification of Al-Ti alloys. The problems with casting these alloys arise from the fact that Al3M (M = Ti, Zr, or Hf) exhibits a peritectic phase equilibrium with the terminal a-Al solid solution. The first solid to form under equilibrium conditions is the properitectic, or primary, Al3M ordered phase (for alloys in the peritectic composition range, i.e., those enriched beyond the minimum liquid solubility of solute). These
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