Microstructural Evolution During Normal/Abnormal Grain Growth in Austenitic Stainless Steel
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INTRODUCTION
AUSTENITIC stainless steels (ASSs) exhibit excellent corrosion resistance, weldability, and versatile mechanical properties.[1,2] Since the properties of a material depend on the microstructure, the microstructural evolution during grain growth at elevated temperatures has to be investigated. There are two grain growth modes: (1) normal grain growth (NGG) and (II) abnormal grain growth (AGG) or secondary recrystallization. NGG is defined as a uniform increase in the average grain size of a polycrystalline aggregate by grain boundary migration as a result of annihilation of smaller grains. Therefore, the distribution of grain size remains relatively uniform and a monomodal grain-size distribution will be attained. Conversely, AGG is characterized by a bimodal grain-size distribution as a result of the rapid growth of a small number of large grains.[3–9] Apparently, secondary recrystallization occurs when NGG is restrained. The main factors which lead to MOHAMMAD SHIRDEL, Master Student, is with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran. HAMED MIRZADEH, Assistant Professor, is with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, and also with the Advanced Metalforming and Thermomechanical Processing Laboratory, School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran. Contact e-mail: [email protected] MOHAMMAD HABIBI PARSA, Professor, is with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, and also with the Advanced Metalforming and Thermomechanical Processing Laboratory, School of Metallurgy andMaterials Engineering, University of Tehran, and Center of Excellence for High Performance Materials, School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran. Manuscript submitted March 11, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
AGG are second-phase particles,[7,10] texture[3] and surface effects,[11] and grain boundary faceting.[12–14] Some research works have been published on the kinetics of grain growth of different materials during exposure to high temperatures.[15–19] However, the microstructural evolution during grain growth and corresponding kinetics analysis, for the separation and determination of the grain growth mode, in ASSs has received less attention and a general conclusion based on the reported research works cannot be obtained. There are several criteria to distinguish between AGG and NGG. For instance, according to one criterion, in NGG the majority of grains are less than two to three times the mean grain size,[20] whereas based on another criterion, AGG commences when the ratio of Dmax/Daverage becomes >5,[10] where Dmax and Daverage are the sizes of the maximum observed grain and the average grain size, respectively. Rios[21,22] has established a useful criterion, which considers that when the relative size of the candidate grain for abnormal
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