Austenite Grain Growth in High Manganese Steels
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NTRODUCTION
HIGH-MANGANESE steels (HMS) offer an excellent combination of strength and ductility because of their high and sustained work-hardening rate. This is attributed to the formation of epsilon martensite (transformation-induced plasticity, TRIP) and deformation twins (twinning-induced plasticity). As a result of their mechanical properties, high-Mn austenitic steels have recently gained much attention as materials of choice for a new generation of ultra-high-strength steels (UHSSs).[1–4] Possible applications of high-Mn steels (HMS) in automobile part fabrication include structural reinforcement, anti-intrusion parts (longitudinal beam, cross members), high-stretch flangeability components (suspension dome) and high-strength (roof rail, A-pillar, B-pillar) components. While the recrystallization and mechanical properties of HMS have been extensively reported in the literature,[1–4] much less attention has been devoted to grain growth processes in these alloys. The present contribution primarily aims at evaluating the kinetics of austenite grain growth in a high-Mn TWIP steel at high temperatures. Bhattacharyya et al.[5] reported that austenite grain growth kinetics in HMS are sluggish
MADHUMANTI BHATTACHARYYA, GARY R. PURDY, and HATEM S. ZUROB are with the Department of Materials Science and Engineering, McMaster University, 1280 Main St W., Hamilton, ON L8S 4L7, Canada. Contact e-mails: [email protected]; [email protected] YVES BRECHET is with the University Grenoble Alpes, CNRS, Grenoble INP SIMaP, 38000 Grenoble, France. Manuscript submitted January 16, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
relative to those of plain low-Mn carbon steels. The sluggish kinetics were correlated to the presence of a high fraction of special boundaries, primarily annealing twins, within the microstructure. The interactions of coherent annealing twins with random HAGBs are of interest here, as these interactions are considered to slow down subsequent grain growth kinetics. According to the growth accident model, nucleation of an annealing twin takes place only if the free energy of a high-energy random HAGB is lowered by the formation of a twin boundary and its related grain boundary segment of lower energy on the original random HAGB.[6–10] A comprehensive measurement of the angles formed between the twinned parts of the grain boundary near a grain corner in OFHC copper showed that the twinned parts are lower in energy than the random HAGB segment. This was also supported by thermal etching experiments performed on the grain boundaries near grain corners with annealing twins located at a specific distance. Of 167 observations, 103 cases showed that the relative width of the thermal grooves formed at twin-influenced random HAGB segments appeared narrower, indicating lowering of the boundary energy.[6] Some other workers also supported the same view of energy reduction of HAGBs as a result of annealing twin interaction.[11,12] A TEM and diffraction electron microscopy observation of the grain boundary triple juncti
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