In Situ Observation of Austenite Growth During Continuous Heating in Very-Low-Carbon Fe-Mn and Ni Alloys
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INTRODUCTION
LOCAL equilibrium at migrating phase interfaces is a useful assumption for analyzing the kinetics of diffusional phase transformations in metallic materials. Interfacial phenomena, such as solute drag, have long been recognized to be one of the principal causes for the deviation from local equilibrium at migrating interfaces and have attracted considerable attention in recent years.[1,2] During transformation, the difference in free energy between parent and product phases is dissipated by atom diffusion, interfacial friction that arises out of reconstruction of the crystal lattice, and solute drag if a significant amount of solute is segregated at the phase interfaces.[3] Thus, to study the mobility of the a/c phase boundary, some authors chose massive ferrite transformation in Fe-X alloys to avoid long-range diffusion of solute and/or extrinsic friction at the boundary (solute drag).[4–6] The alloy element X is added because the transformation is too fast in unalloyed iron. More recently, the a/c boundary mobility was studied using ferrite-to-austenite and austenite-to-ferrite transformation below Ae3 in ultralow-carbon steel,[7] and cyclic transformation in low-carbon steel,[8] coupled with simulation that involved the so-called effective mobility.[8–10] In these studies, the evolution of volume fraction of massive ferrite was monitored experimentally
M. ENOMOTO is with Ibaraki University, 2-1-1, Bunkyo, Mito, 3108512 Japan. Contact e-mail: [email protected] X.L. WAN is with State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, 947 Heping Avenue, Qingshan District, Wuhan 430081, China. Manuscript submitted March 20, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
by differential scanning calorimetry (DSC) and/or dilatometry from which the average boundary velocity was evaluated. On the other hand, Hamada et al.[11] carried out in situ observation of massive ferrite growth during continuous cooling under a confocal scanning laser microscope (CSLM), which allows us to observe the microstructure evolution in steel at high temperatures with high resolution.[12,13] They reported the mobility of the a/c phase boundary somewhat greater than those determined by DSC or dilatometry, albeit the amount of differences varied among alloys studied. In this report, in situ observation of austenite growth during continuous heating was carried out under CSLM using the same alloys as those of Hamada et al. It will be shown that the a/c boundary mobility obtained from the growth of massive austenite is of a similar order of magnitude as that previously reported in the growth of massive ferrite.[11]
II.
EXPERIMENTAL PROCEDURE
The alloys were vacuum-induction melted using high-purity electrolytic iron, high-purity graphite, manganese, and nickel. The chemical compositions of alloys are shown in Table I. After hot rolling, they were sealed into a silica tube and were homogenized at 1523 K (1250 C) for 48 hours. The details of the specimen preparation procedur
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