Morphology and kinetics of discontinuous precipitation and dissolution in an Fe-8.5Al-27Mn-1.0Si-0.92C alloy
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I. INTRODUCTION
DISCONTINUOUS precipitation (DP) is a solid-state reaction in which an initially homogeneous, supersaturated solid solution is decomposed into a lamellar two-phase microstructure in the region behind the migrating reaction front.[1,2] This reaction starts preferentially at a large-angle grain boundary. By heating the alloy after partial decomposition through DP to near or above the solvus temperature, the lamellar DP will dissolve under a discontinuous dissolution (DD) reaction, while the reaction front between the DP and the matrix will migrate backwards as well.[2,3,4] Owing to merits, such as high strength, high corrosion and oxidation resistance, and low cost, the FeAlMn alloy has been considered as a promising substitute alloy for the more expensive austenite stainless steels.[5] In order to attain optimal alloy design of the FeAlMn system, it is necessary to understand its phase transformations under various aging processes.[6,7] Related investigations have been extensively conducted; among which, the grain-boundary precipitation in this alloy system has been most exhaustively studied.[4–14] In a fully austenitized Fe-9Al-9.5Mn0.96C-6.77Ni, James found a DP of (ferrite ⫹ carbide) phases after aging at 873 K.[8] The precipitated carbide was a mixed compound, (FeMn)3AlC, with a cubic structure. The result was confirmed by Krivonogov et al. in a study of the Fe-9Al-29Mn-0.9C alloy,[9] which showed that the quenching austenite of this alloy would decompose at temperatures below 1023 K, first, into a coherent metastable continuous precipitation ( ⬘ phase Fe3AlC0.66), which possessed an ordered fcc-perovskite-type structure, and was oriented in the 具100典 crystallographic direction. After prolonged aging, a DP of ␣ (ferrite) ⫹  -Mn ⫹ occurred at the austenite grain boundaries. The phase was identified as a coarse (Fe, Mn)3AlCx carbide. The initially formed ⬘ precipitates grew simultaneously in the austenite grains and SHIH-YING CHANG and LUNG-CHUAN TSAO, Post-doctorates, and TUNG-HAN CHUANG, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, 1 Roosevelt Road, Sec. 4, Taipei 106, Taiwan. SHIUAN-SHENG WANG, Lecturer, is with the Department of Mechanical Engineering, De-Lin Institute of Technology, Tu-Cheng, Taipei 236, Taiwan. Contact e-mail: [email protected] Manuscript submitted July 3, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
were transformed into incoherent coarse particles of the phase (Fe, Mn)3AlCx . At higher temperatures ranging from 1023 to 1123 K and already in the initial stage of decomposition, the phase (Fe, Mn)3AlCx precipitated preferentially at grain boundaries. As the aging time increased, the DP formed at grain boundaries was (␣ ⫹ ) phases in the absence of the  -Mn phase. Similar structural changes were noted by Storchak and Drachinskaya[10] in an FeAlMn single-crystal specimen with the same composition as the polycrystalline alloy employed by Krivonogov et al. The existence of a  -Mn phase in FeAlMn allo
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