Formation and annealing behavior of nanocrystalline ferrite in Fe-0.89C spheroidite steel produced by ball milling
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1. INTRODUCTION
NANOCRYSTALLINE materials have attracted considerable scientific interest in the last few decades because of their unusual properties, which are normally attributed to an ultrafine grain size in the nanometer range and a large volume fraction of grain boundaries.[1,2] Various methods have been developed to obtain nanocrystalline materials, such as ball milling,[3,4,5] crystallization from an amorphous state,[6,7] equal-channel angular pressing,[8,9] torsion straining under high pressure,[10,11] etc. Among these, ball milling is the simplest method and can effectively refine the grains down to the nanometer scale for most metals, alloys, and intermetallics.[3–5,12–14] Therefore, ball milling has been widely used to produce nanocrystalline materials. The general understanding of the nanocrystallization mechanism is that ball milling results in the deformation of milled powders, leading to gradual grain refinement, and results in the final nanocrystalline structure. Fecht et al.[15] proposed that shear bands might be a precursor for nanocrystalline structure formation by studying the ball milling of an AlRu compound. Huang et al.[16] also found the existence of shear bands and a number of twins in a study on ball-milled Cu powders. The formation of subgrain was suggested to take place either in the shear bands, at the tip of them, at the tip of the twin boundaries, or at the edge of the larger grains. Therefore, it is expected that the shear band is a necessary precursor for nanocrystalline formation. However, in recent study on cryomilling Zn powder, the formation of a large number of small grains (2 to 6 nm) in the very early cryomilling stage was explained by a dynamic recrystallization mechanism.[17] Y. XU, Associate Professor, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, P.R. China. Contact e-mail: [email protected] Z.G. LIU, formerly Assistant Professor, Department of Production Systems Engineering, Toyohashi University of Technology, is Guest Scientist, Materials Division, Faculty of Engineering, Ulm University, D-89081 Ulm, Germany. M. UMEMOTO, Professor, and K. TSUCHIYA, Associate Professor, are with the Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi 4418580, Japan. Manuscript submitted October 19, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Although the direct microstructural observations by transmission electron microscopy (TEM) were carried out for the powders that had completed nanocrystallization, the structural change during ball milling was usually estimated by indirect methods such as grain-size estimation by X-ray diffraction line broadening. The description of the nanocrystallization process is less convincing due to the absence of direct microstructural observations. Therefore, the clear and detailed process of microstructural evolution during nanocrystallization has not been well understood. As the most widely used material, steel has been well studied to improve its mechanical properti
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