Orientation relationships between M 2 C carbide and the austenite matrix in an Fe-Mn-AI-Mo-C alloy
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I.
INTRODUCTION
THE microstructures and the mechanical properties of Fe-Mn-A1-C alloys with a fully austenitic structure have been extensively studied.[~-~°~ After being solution heat-treated and aged at temperatures ranging from 450 °C to 600 °C f o r various times, these alloys can possess an outstanding combination o f strength and ductility, w h i c h is attributed to the precipitation o f extremely fine (Fe, Mn)3A1Cx carbides within the austenite matrix. When the alloys were aged at higher temperatures (600 °C to 750 °C), the (Fe, Mn)3A1Cx carbides appeared not only within the austenite matrix but also on the austenite grain boundaries, and they grew very quickly. This eventually leads to a sudden drop in ductility. E~-6J In order to improve the strength and the corrosion resistance, molybdenum has been added to Fe-Mn-A1-C alloys, l~ ~,~2.131 However, little information concerning the microstructure o f the Fe-Mn-AI-C alloys containing Mo has been provided. Recently, the present authors have made transmission electron microscopic investigations on the phase transformations in an Fe-24.6Mn-6.6A1-3.1Mo-l.0C alloy. t~4,~5,16~ The microstructure o f the as-quenched specimens consisted o f austenite and ferrite phases. No precipitates were found within either the austenite o r the ferrite by optical microscopy. However, when the asquenched specimens were aged at temperatures ranging from 450 °C to 800 °C, very complex carbide precipitation occurred within both the austenite and the ferrite matrices. Within the ferrite (ce) matrix, needlelike MzC and A12 a - M n precipitates were observed in the specimens aged at temperatures ranging from 400 °C to 650 °C. At higher aging temperatures, A12 c~-Mn gradually disappeared, leaving needlelike M2C precipitates. When the aging temperature was raised further to 750 °C, little o r no evidence o f either AI2 a-Mn or M2C needle precipitates CHANG-PIN C H O U , Professor, and SHANG-WEN P E N G , Graduate Student, are with the Department of Mechanical Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China. Manuscript submitted July 13, 1992. METALLURGICAL TRANSACTIONS A
could be found. The only precipitate observed within the ferrite matrix w a s M r C . The transition from M2C to M 6 C carbides in ferrite during aging at high temperatures has been reported in many alloy steels containing M o . [17-221 It is believed that a similar M2C ---> M 6 C transition occurred in the ferrite matrix o f the present alloy. A more detailed report on the complete phase transformations o f the Fe-Mn-A1-Mo-C alloy will be presented in a separate article. [411 Within the austenite matrix o f the alloy, Mz3C6 and M r C carbides were observed and identified. The orientation relationship between M 6 C and austenite can be parallel or twin-related, depending upon whether MrC is formed directly from the austenite o r indirectly from the e x i s t i n g M 2 3 C 6 carbide. L~5~ A further study o f the same alloy reveals that M2C carbide could be found within both the austenite a
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