Recrystallization in oxide-dispersion strengthened mechanically alloyed sheet steel
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INTRODUCTION
INCOLOY* MA-956 is an oxide-dispersion-strengthened *INCOLOY is a trademark of INCO Alloys International, Inc., Huntington, WV.
(ODS) iron-base superalloy derived from the KANTHAL** **KANTHAL is a trademark of the Kanthal Corporation, Bethel, CT.
alloy (Fe-20 pct Cr-5 pct A1-2 pct Co) used for furnace heating elementsY ,21MA-956 contains a fine, stable yttrium oxide dispersion and is produced by a powder metallurgy, mechanical alloying (MA) process. This alloy was originally targeted for aerospace applications, such as gas turbine engines that require operations in excess of 1200 ~ Recently, there has been increased use of MA-956 as a replacement material for furnace heat shields, rails, and baskets where traditionally materials such as molybdenum have been used.t61 The MA-956 matrix is essentially a single-phase, bodycentered-cubic (bcc) stainless steel with a melting point of 1480 ~ The aluminum addition allows for the formation of an extremely adherent and stable aluminum oxide outer layer which provides excellent oxidation, sulfidation, and carburization protection at elevated temperatures, t41 The Y203 oxide dispersion, consisting of particles in the range of 30 to 50 nm in diameter, improves the high-temperature creep and stress rupture life by inhibiting dislocation motion.m High-temperature strength is further enhanced by the
R.C. KLUG, is a Research Specialist at the Allegheny Ludlum Corporation, Technical Center, Brackenridge, PA 15014-1597. G. KRAUSS, John Henry Moore, Professor and D.K. MATLOCK, Armco Foundation Fogarty Professor, are with the Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted June 9, 1995. METALLURGICAL AND MATERIALSTRANSACTIONS A
development of a coarse-grain microstructure with a high grain aspect ratio (GAR), or high ratio of grain length to width, oriented in the loading direction.iT,8] The coarse grains desirable for the high-temperature performance, often several millimeters in size, are produced by recrystallization during the annealing of the cold-rolled sheet. The coarse-grain microstructural development of MA956 sheet, and the European equivalent PM 2000, has been noted and discussed by a number of authors, t9-141 but the mechanisms of evolution have not been systematically evaluated. The coarse-grain microstructure may form by conventional recrystallization and grain growth, secondary recrystallization, or other discontinuous grain-growth mechanismsY ,~3] Coarse-grain development in the presence of stable second-phase particles, such as yttrium oxide, has been the subject of considerable research. Analyses of the effects of particles on recrystallization t~5.~61have shown that fine particle sizes, less than 0.1 /xm, and fine particle spacings, less than 1.0/zm, stabilize boundaries and substructures and increase the temperatures needed for recrystallization. The latter behavior also may be related to deformation and subsequent rec
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