| Summary: | 碩士 === 南台科技大學 === 奈米科技研究所 === 97 === The purpose of the present study is an attempt to examine the effects of aging treatments on microstructures and mechanical properties of the dual-phase Fe-15Mn -8Al-0.7C Alloy by means of optical microscopy (OM), hardness(HRC) tester, and transmission electron microscopy (TEM).
Based on our TEM examinations, it was concluded below. (1) In the as-quenched condition, the microstructure of the Fe-15Mn-8Al-0.7C alloy was the mixture of (ferrite + austenite) phases. TEM examinations revealed that no carbides precipitated within the austenite matrix, however, (B2+D03) phases could be found within the ferrite region. (2) When the Fe-15Mn-8Al-0.7C alloy was aged at 550℃ and 650℃ for 24 hours, the (Fe,Mn)3AlCx carbides not only precipitated within the austenite matrix but also the formed on the γ/γ grain boundary. Along with the growth of (Fe,Mn)3AlCx carbides, the phase decomposition of γ→κ'+α occurred.(3) When the alloy was aged at 550℃ and 650℃, the (Fe,Mn) 3AlCx carbides formed firstly on the γ/γ grain boundaries. Along with the growth of (Fe,Mn) 3AlCx carbides, the phase transformation sequence of γ→κ'+α occurred. However, when the alloy was aged at 750℃, ferrite particles were observed to form firstly on the γ/γ grain boundaries. Along with the growth of ferrite particles, the phase decomposition of γ→α+κ' occurred. Therefore, the morphologies and chemical compositions of (Fe,Mn) 3AlCx carbides and ferrite particles on the γ/γ grain boundaries in the alloy aged at 750℃ were quite different from those observed in the alloy aged at 550℃ and 650℃. This result has never been reported by other workers in the Fe-Mn-Al-C alloy system before. (4) When the alloy was aged from 450℃ to 650℃, the ordering phase transformation ofα→α+B2→α+D03 could be found within the ferrite region. The transition temperatures ofα→α+B2 and α+B2→α+D03 were found to be within the narrow range of 525℃ and 550℃, which were also quite different from those observed in the Fe-Mn-Al-C alloy before by other researchers. (5) When the alloy was solution treated and aged from 450℃ to 750℃, the hardness values of the alloy increased significantly with the aging temperature from 450℃ up to 550℃ and then dropped slowly from 550℃ to 750℃. TEM observations revealed that the alloy with the particle size of the (Fe,Mn) 3AlCx carbides ranging from 6 to 10 nm could possess the optimal strengthen effect.
Keyword: Fe-Mn-Al-C alloy, dual phases Microstructure, B2 and D03
ordered phases, ordering phase transformation,
phase decomposition sequence, (Fe,Mn)3AlCx carbides,
surface hardness.
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