Effect of Si addition on the microstructure of an Fe-8.0AI-29.0Mn-0.90C alloy
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I.
INTRODUCTION
RECENTLY, a considerable amount of interest has been expressed in the possibility of developing Fe-A1-Mn-C alloys ashigh-strength, high-ductility alloy steels. Resuits concerning the microstructures and the mechanical properties have been given in several publications, tl-8] It is found from these results that high strength and high ductility can be achieved by controlling the contents of aluminum, manganese, and carbon so as to obtain a fully austenitic structure and by performing the heat treatment, including solution heat treatment, quenching, and aging, so as to obtain fine (Fe, Mn)3A1C carbides precipitated coherently within the austenite matrix. The (Fe, Mn)3A1C carbide (K phase) has an ordered fcc structure which is of the L'12 type. [9,~~ In order to improve the strength, corrosion resistance, and high-temperature oxidation resistance, silicon has been added to the Fe-A1-Mn-C alloys. 02-2~ Based on these results, it can be generally concluded that the addition of silicon does achieve these results. In addition, the effects of silicon on the microstructures of the Fe-A1Mn-C alloys have also been studied principally by either optical microscopy or scanning electron microscopy ( S E M ) . tls'19,201 It was suggested that the addition of silicon enhanced the formation of single-phase ferrite, lls,191 In 1982, six kinds of Fe-A1-Mn-C-Si alloys were examined by Garcia et al.,[2~ in which they claimed to have observed the ordered B2 phase. However, no transmission electron microscopy (TEM) micrographs concerning the microstructure of the Fe-A1-Mn-C-Si alloy have been provided in the literature. The purpose of this work is an attempt to clarify the effect of silicon on the microstructure of an Fe-8.0A129.0Mn-0.90C alloy by TEM. Based on our present study, it is found that the silicon addition enhances the formation of (a + D03) phases, which is in disagreement T.F. LIU, Associate Professor, Institute of Materials Science and Engineering, and J.S. CHOU and C.C. WU, Graduate Students, Institute of Mechanical Engineering, are with National Chiao Tung University, Taiwan, Republic of China. Manuscript submitted September 21, 1989. METALLURGICAL TRANSACTIONS A
with other works. [18'19'20] It is noted here that the ordered D03 phase has never been observed in both Fe-A1-MnC and Fe-A1-Mn-C-Si alloy systems before. II.
EXPERIMENTAL PROCEDURE
Two alloys with Fe-8.0A1-29.0Mn-0.90C, one without Si (alloy A) and one with 1.50 wt pet Si (alloy B), were prepared in a vacuum induction furnace under a controlled protective argon atmosphere. After being homogenized at 1250 ~ for 14 hours, the ingots were hot forged and then cold rolled to a final thickness of 1.0 mm. The sheets were subsequently solution heat-treated at 1200 ~ for 1 hour and rapidly quenched into roomtemperature water. Aging was performed at temperatures ranging from 450 ~ to 1050 ~ in a salt bath. Electron microscopy specimens were prepared by means of a double-jet electropolisher with an electrolyte of 30 pet ethanol, 60 pct acetic acid, and 1
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