Pearlitic Transformations in an Ultrafine-Grained Hypereutectoid Steel
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PEARLITE and martensite are two basic and very important constituents in steels for both hypo- and hypereutectoid compositions. Pearlite is formed when steels are heated to austenite temperature, austenitized, and then cooled in the furnace or in air. Pearlite consists of lamellar cementite (Fe3C) and ferrite formed in a eutectoid transformation,[1] which is a type of diffusive transformation. Martensite is formed by quenching or very rapid cooling in a diffusionless phase transformation from face-centered-cubic (fcc) austenite to bodycentered-cubic (bcc) martensite.[1–3] This was discovered over 100 years ago and holds true under normal conditions.[3] Our current work has addressed the question of whether this is still true if the grain size is refined to the micron or nanometer scale. Some experiments have shown that solid-phase transformations were indeed abnormal when the grain size was on the order of nanometers. An unknown bcc fi fcc transition was found in a nanometer-sized Nb80Al20 alloy.[4] The martensitic transformation could be restrained with grain refinement in Fe-Ni powder,[5–8] and the martensitic transformation initiation temperature, Ms, was lowered significantly with the reduction of austenite grain size.[9] Recent studies indicated that a reverse transformation of bcc ferrite to fcc austenite occurs during severe plastic deformation (SPD) of a pearlitic steel, resulting in a nanocrystalline structure.[10] Approximately 1.0 at. pct carbon can be dissolved in the nanocrystalline ferrite,[11] which is much higher than the balance resolvability, 0.021 pct, of ferrite with a larger grain size. Together, these experiments indicate YONGNING LIU, Professor, TAO HE, GUANGJIN PENG, and FULIANG LIAN, Graduate Students, are with the State Key Laboratory of Mechanical Behavior of Materials, School of Materials Science, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted May 16, 2010. Article published online January 27, 2011 2144—VOLUME 42A, AUGUST 2011
that the transformation law is indeed different in ultrafine-grained materials compared to coarse- or medium-grained materials. Because the study of the transformation requires samples to be reheated to high temperature, which causes the grains to grow, the reported solid-phase transformations in ultrafinegrained materials to date have mostly involved thin films (such as Al-Cu alloys[12]) or powders (such as Fe-Ni alloys[13,14]). However, phase transformations in bulk materials may be different from those in films and powders; in addition, alloy elements have a great influence on transformation. Thus, we designed an experiment to study the solid-phase transformation of an ultrafine-grained, pure hypereutectoid Fe-C alloy. The purpose of the selection of hypereutectoid composition was to prevent austenitic grain growth with more unresolved carbide during reheating in the c + Fe3C two-phase region. Usually, there are ways to produce bulk ultrafinegrained materials: one is SPD at room or eleva
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