Phase transformation in an Fe-10.1Al-28.6Mn-0.46C alloy
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I.
INTRODUCTION
IN order to develop a new high-strength-high-ductility alloy steel, considerable attention has been concentrated on the development of Fe-A1-Mn-C alloys, t~-~6~ In this alloy system, manganese and carbon are austenite stabilizers whereas aluminum is a ferrite former. The amount of manganese necessary to produce a face-centered cubic (fcc) structure depends on the contents of aluminum and carbon in the alloy. In general, the fcc structure can be stabilized by increasing the contents of manganese and carbon or decreasing the content of aluminum. Therefore, through a proper combination of aluminum, manganese, and carbon, an alloy with a fully austenitic structure can be achieved. After being solution heat-treated and aged at temperatures ranging from 450 ~ to 650 ~ for moderate times, the austenitic alloy can possess a remarkable combination of strength and ductility, which is attributed to the formation of extremely fine (Fe, Mn)3A1C carbides (K phase) within the austenite matrix, t~-~~ The (Fe, Mn)3A1C carbide has an ordered fcc crystal structure which belongs to the L'12 type. t~'~2'~3~ After prolonged aging within this temperature range, (Fe, Mn)3AIC carbides were found to precipitate not only coherently within the austenite matrix, but also on the grain boundaries in the form of coarser particles. Besides the precipitation of (Fe, Mn)3A1C carbides, A13 /3-Mn precipitates could also be observed to form on the grain boundaries through a 31---> a + /3-Mn transition. The formation of (Fe, Mn)3AIC carbides and fl-Mn precipitates on the grain boundaries resulted in the embrittlement of the alloy. In the Fe-AI-Mn-C alloys with lower carbon contents, the microstructure containing duplex (a + 3") phases could be o b s e r v e d . H4,15,161 Both K-phase and fl-Mn precipitates were also examined in the aged specimens. However, C.C. WU and J.S. CHOU, Graduate Students, Institute of Mechanical Engineering, and T.F. LIU, Professor, Institute of Materials Science and Engineering, are with the National Chiao Tung University, Hsinchu, Taiwan 30049, Republic of China. Manuscript submitted October 12, 1989. METALLURGICAL TRANSACTIONS A
all of the microstructural developments in (a + y) duplex Fe-A1-Mn-C alloys were studied principally by means of X-ray diffraction and optical microscopy. [14,~5,16~Little transmission electron microscopy (TEM) information concerning the microstructural development of (a + 3,) duplex Fe-A1-Mn-C alloys has been provided. The present investigation, using optical microscopy and transmission electron microscopy, is an attempt to clarify the phase transitions in an Fe-10.1A1-28.6Mn-0.46C alloy aged at temperatures ranging from 450 ~ to 750 ~ In the present study, the ordered D03 phase was observed in as-quenched specimens. This result is quite different from that reported by o t h e r w o r k e r s [14,15,161 in which only ferrite and austenite phases were observed in the as-quenched specimen. Besides, a detailed study of the 3'--~ a + fl-Mn and 3" a + K-phase transitions was also done.
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