Enhanced densification of cavitated dispersion-strengthened aluminum by thermal cycling
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I. INTRODUCTION
DESPITE their outstanding creep resistance, oxide-dispersion-strengthened aluminum materials have limited creep ductility due to the formation and subsequent growth and linkage of creep cavities. A recent investigation comparing the creep-rupture behavior of pure aluminum and various dispersion-strengthened aluminum alloys is given in Reference 1. One approach to extending the creep life of various engineering components is ex situ treatments to close creep cavities. The technical literature contains many examples of the shrinkage and closure of cavities formed during deformation by isothermal heat treatment with or without superimposed hydrostatic pressure.[2–7] Although isothermal heat treatment at ambient pressure is simple and inexpensive, the time required to fully close creep cavities is often prohibitive. Hot isostatic pressing can more rapidly close porosity, but at an increased cost. Shiozawa and Weertman[8] showed that cavity shrinkage is strongly affected by the internal residual stress state, potentially increasing the rate of densification or causing cavity growth rather than shrinkage. In the present article, we demonstrate that by thermally cycling dispersion-strengthened aluminum at ambient pressure, creep cavities can be eliminated more rapidly than by isothermal annealing. The enhanced densification rates observed during cycling are discussed in light of a diffusional densification model, giving special consideration to thermalexpansion-mismatch stresses. The effect of intermittent thermal-cycling treatments on the isothermal creep-rupture behavior of dispersion-strengthened aluminum is also investigated and discussed. II. EXPERIMENTAL The experimental material was as-cast dispersionstrengthened-cast aluminum (DSC-Al) with about 23 vol CHRISTOPHER SCHUH, Graduate Student, and DAVID C. DUNAND, Associate Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. BING Q. HAN, Postdoctoral Researcher, formerly with the Department of Materials Science and Engineering, Northwestern University, is with the Department of Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Manuscript submitted January 25, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
pct alumina dispersoids in a 99.9 pct pure Al matrix, from Chesapeake Composites Corp. (Newcastle, DE). Microstructural characteristics as well as ambient and elevated temperature mechanical properties of this material can be found in References 1 and 9 through 11. The dispersoid particles are approximately spherical with an average diameter of about 0.3 mm and are well dispersed in large matrix grains (,2 to 10 mm diameter).
A. Cavity Shrinkage Experiments The specimens tested in these experiments were previously crept to failure at various temperatures and uniaxial tensile stresses in a separate study by Han and Dunand,[12] as shown in Table I. The gage pieces were separated from the heads with a diamond blade, and the material in the vicinity of the frac
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