Effect of initial grain size of austenite on hot-deformed structure of Ni-30Fe alloy
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I. INTRODUCTION
GRAIN refinement of ferrite has been actively studied for the improvement of strength and toughness in low-carbon steel.[1] It was reported that severe plastic deformation of austenite during thermomechanical processing (TMP) was very useful for the refinement of ferrite grains.[2,3,4] Typical applied strain by severe plastic deformation ranged from 0.7 to 5,[2,3,4] and these strain levels are much higher than the strain levels during conventional TMP, which are reported to be around 0.3 at most.[5] To understand the refinement mechanism of ferrite grains by severe plastic deformation of austenite, the evolution of hot-deformed structure of austenite by severe plastic deformation must be examined because the transformed structure of ferrite is closely related to the hot-deformed structure of austenite. The hot-deformed structure of austenite and its relation to the final microstructure during conventional TMP are well established.[5,6] Also, there have been some studies on the hot-deformed structures of austenite by severe plastic deformation.[7–10] Because the austenite in low-carbon steel transforms to ferrite, bainite, or martensite during cooling to room temperature, the stainless steel,[7] the Ni-Co alloy,[8] or the Ni-Fe alloy[9,10] was used as a model alloy for investigating the hot-deformed structure of austenite in most of the studies. Those studies reported that the severely deformed structures of austenite mainly consisted of dislocation walls or microbands. However, complicated microstructural evolutions accompanying the dynamic recrystallization (DRX) and the dynamic recovery hindered the quantitative characterization of the deformed structure of DONG-WOO SUH, Senior Researcher, is with the Materials Processing Department, Korea Institute of Machinery and Materials, Kyungnam, 641-010, Korea. Contact e-mail: [email protected] JAE-YOUNG CHO, Researcher, formerly with the Steel Research Center, National Institute for Materials Science, is with the Technical Research Laboratories, POSCO, Pohang, 790-785 Korea. KOTOBU NAGAI, Director-General, is with he Steel Research Center, National Institute for Materials Science, Tsukuba, 305-0047, Japan. Manuscript submitted October 28, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite by severe plastic deformation. Furthermore, the studies on the configuration of high-angle boundaries during the severe plastic deformation have been rarely reported, even though the high-angle boundaries in austenite are known as the most potent nucleation site for ferrite. We examined the deformed structure of austenite, using a Ni-30Fe alloy, under various deformation conditions.[11,12] The Ni-30Fe alloy is an austenitic alloy that does not transform during quenching, and thus, its hot-deformed structure can be retained at room temperature. The reported stacking fault energy of Ni-30Fe alloy is around 75 mJ/m2, which is similar to that of low-carbon steels.[13,14] The stacking fault energy has a great influence on the restoration processes during hot
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