A study of retained austenite in a fine-grained Fe-12Ni-0.25Ti alloy
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HE development of the retained austenite phase in ferritic steels has been the subject of a significant body of research in recent years. The motivation of this research comes from two sources: fundamental interest in the phase transformation behavior and the possibly beneficial effect of retained austenite on the mechanical properties. In an Fe-20Ni-0.8C alloy some austenite was observed to remain untransformed after continuous cooling to a subzero temperature. Kelly and Nutting' attributed the stability of this partially transformed austenite to the difficulties in the cooperative movement of atoms to form martensite. In low-carbon Fe -Ni based alloys, an over-aging (maraging steels) 2 or a tempering (low-nickel cryogenic steels) 5-6,25 process often yields stabilized austenite. During these processes the solute elements seem to play a significant role in the stabilization of the reverted austenite. The effect of ultrafine grain size on the stability of austenite was also reported.' Although there have been numerous reports on the mechanical properties of ferritic steels containing some volume fraction of retained austenite, the exact role of the austenite is not well understood. A deleterious effect on strength has been reported in maraging steels 4 8 ' ° A significant lowering of the ductilebrittle transition temperature has been observed in low-nickel cryogenic steels 5-6'25 An enhanced fracture toughness has been reported in a TRIP steel" as well as in a thermal-cycled maraging steel'' where metastable austenite undergoes martensitic transformation during testing. An improvement in the fracture toughness was also reported in an Fe-Cr-C alloy where interlath retained austenite films were present . n Recently a grain-refining technique for the Fe12Ni-0.25Tí cryogenic alloy system," which utilizes .
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SUN-KEUN HWANG, SUNGHO JIN, and J. W. MORRIS, JR., are Graduate Student, Assistant Research Engineer, and Associate Professor, respectively, Department of Materials Science and Engineering, University of California, Berkeley, CA 94720. Manuscript submitted February 24, 1975. METALLURGICAL TRANSACTIONS A
the y-a phase transformation through an alternate thermal-cycling process, has been developed. The alloy, after grain-refinement (to about 1 µm in diam), showed an unusual ductility at cryogenic temperatures. Retained austenite was introduced into this fine-grained, ductile matrix in the hope of achieving a further improvement in cryogenic ductility without sacrificing strength. Given the large grain boundary area, austenite reversion can take place easily. The objective of this paper is to present some results of a study of stabilized austenite formed in this way. The investigation was undertaken firstly to examine the morphology and stability of the retained austenite. Then, the cryogenic mechanical properties were studied after introducing a moderate amount of retained austenite. EXPERIMENTAL Low-carbon alloys of nominal composition Fe-12Ni0.25Ti were induction melted in an argon gas atmosphere, then cast into 9
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