Creep and rupture properties of an austenitic Fe-30Mn-9Al-1C alloy
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I.
INTRODUCTION
THE Fe-Mn-Al-C system alloy was initially developed as a possible substitute for the conventional Fe-Ni-Cr stainless steel in view of its high strength, good toughness, light weight, and low cost. In this alloy system, manganese and carbon are added to form the austenitic structure which is essential for good mechanical properties at both elevated and cryogenic temperatures. Carbon is known to be more effective than manganese in stabilizing and strengthening the austenitic phase. Aluminum is a ferrite stabilizer, and the addition of this element to the alloy system can increase its oxidation resistance. It has been shown that through the proper choice of alloy composition in the Fe-Mn-Al-C system, alloys with high strength and good ductility could be produced.[1,2,3] For example, Kim et al. developed a highstrength and excellent toughness cryogenic alloy, Fe-30Mn5Al-0.3C-0.1Nb.[4,5] A particularly interesting behavior of this alloy is its increase in elongation with decreasing temperature, in the range of 77 to 298 K, which makes it a candidate engineering material for cryogenic applications. However, Altstetter et al.[6] indicated that the Fe-Mn-Al-C alloys exhibit poor corrosion resistance, which makes them unacceptable as replacements for conventional stainless steels in aqueous environments. Austenitic Fe-Mn-Al-C alloys containing 25 to 30 wt pct Mn, 8 to 10 wt pct Al, and 0.8 to 1.0 wt pct C have received considerable attention recently.[7–13] According to the work of Kayak,[2] the Fe-(25 to 30)Mn-(8 to 10)Al-1C alloy represents one of the best combinations of strength and ductility available in the Fe-Mn-Al-C system. Furthermore, these alloys exhibit superior oxidation resistance due to the high concentration of aluminum and, thus, may hold some S.M. ZHU, Graduate Assistant, and S.C. TJONG, Associate Professor, are with the Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong. Manuscript submitted May 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
promise for applications at elevated temperatures. A series of studies on the tensile deformation of Fe-Mn-Al-C alloys at ambient and elevated temperatures have revealed the occurrence of high work hardening during deformation.[14–21] In a very recent study,[21] we have identified the temperature domains of the work hardening mechanisms of Fe-Mn-Al-C alloys, i.e., (1) strain-induced martensitic transformation tends to occur at cryogenic temperatures; (2) strain-induced twinning prevails at ambient temperatures; (3) dynamic strain aging becomes more pronounced at intermediate temperatures, where the alloys exhibit serrated flow and negative strain rate sensitivity; and (4) strain-enhanced precipitation hardening predominates at high temperatures. However, there appears to be no published literature on the high-temperature creep and rupture behavior of Fe-MnAl-C alloys. In the present study, an austenitic Fe-30Mn-9Al-1C alloy was crept at 923, 948, and 973 K in the stress range of 50 to 350 MPa. The cre
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