Room-temperature cleavage fracture of FeMnAlC steels
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Alloys of the FeMnAlC system have been extensively studied in recent years, and perhaps their more attractive aspect for structural application is the possibility of obtaining different combinations of mechanical strength, ductility, and fracture toughness over a wide temperature range. Above 850 °C, alloys with 28 to 30 pct Mn, 8 to 10 pct Al and 0.8 to 1 pct C (usual composition ranges) present a supersaturated austenitic structure. After quenching to room temperature, although the austenitic structure is not fully preserved,[1] the resulting properties are high ductility (60 to 70 pct total elongation) and reasonable mechanical strength (ultimate tensile strength (UTS) around 800 to 900 MPa). This austenitic phase undergoes a series of decomposition reactions during isothermal holding within the temperature range 350 °C to 700 °C.[1] When aging at temperatures ranging from 500 °C to 750 °C, a fine distribution of (Fe,Mn)3AlC carbide ( phase) appears, resulting in a significant improvement in mechanical strength.[2,3] For structural purposes, a good combination of mechanical strength and fracture toughness is desirable, and so far the most commonly used treatment is that involving quenching from temperatures within the austenitic field and subsequent aging for 16 hours at 550 °C.[4] The resulting hardness and mechanical strength are high (Hv of 340 and UTS around 1200 to 1400 MPa), but fracture behavior may be unacceptable for critical applications. Experiments concerning controlled cooling from O. ACSELRAD and L.C. PEREIRA, Professors, are with the Metallurgy and Materials Engineering Department, Federal University of Rio de Janeiro, 21949-900 Rio de Janeiro, Brazil. J. DILLE, Research Engineer, and J.-L. DELPLANCKE, Director, are with the Science des Matériaux et Electrochimie, Université Libre de Bruxelles, 1050 Brussels, Belgium. Manuscript submitted November 18, 2003.
METALLURGICAL AND MATERIALS TRANSACTIONS A
solubilization temperature down to room temperature are under way, as an attempt to produce a good combination of strength and fracture toughness. In this article, we discuss a specific feature of fracture behavior revealed by specimens submitted to one of the controlled cooling conditions under study. A 10-kg ingot of Fe-28Mn-8.5Al-1C steel with 1.25 pct silicon was prepared in a vacuum induction furnace under controlled argon ambient. It was then homogenized for 6 hours at 1150 °C and then hot rolled to a final thickness of 13 mm. After cutting into billets, the billets were solution treated at 1050 °C for 2 hours and water quenched. A PHILIPS* CM 20 scanning transmission electron micro*PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah.
scope (200 kV), a MEV-ZEISS 940-A electron scanning microscope and a TMX 2010 Discoverer Topometrix Scanning Probe Microscope (atomic force microscopy) were applied for microstructural characterization. Fracture toughness was characterized by KA, the apparent fracture toughness,[5] using conventional Charpy 10 10 mm test specimens. The KA valu
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