Microstructural Evolution in cryomilled Inconel 625
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Microstructural Evolution in cryomilled Inconel 625 Jianhong He and Enrique J. Lavernia Department of Chemical and Biochemical Engineering and Materials Science, University of California Irvine, Irvine, CA 92697-2575 ABSTRACT Nanocrystalline Inconel 625 alloy, with a uniform distribution of grains, was synthesized using cryogenic mechanical milling. Microstructures of the powder, cryomilled for different times, were investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that both the average powder particle size and average grain size approached constant values as cryomilling time increased to 8 hours. The TEM observations indicated that grains in the cryomilled powder were deformed into elongated grains with a high density of deformation faults, and then fractured via cyclic impact loading in random directions. The fractured fragments from the elongated coarse grains formed nanoscale grains. The occurrence of the elongated grains, from development to disappearance during intermediate stages of milling, suggested that repeated strain fatigue and fracture caused by the cyclic impact loading in random directions, and cold welding were responsible for the formation of a nanocrystalline structure. 1.
INTRODUCTION
J. Benjamin and his colleagues stated that the mechanical milling/alloying involves repeated welding, fracturing and re-welding of powder particles under a highly energetic ball charge [1-3]. In terms of milling mechanisms, however, Benjamin and his coworkers primarily studied inter-particle behavior, whereas the issue of grain size evolution was not addressed [1-3]. The formation of a nanocrystalline structure is thought to evolve from the development of dislocation cell structures within shear bands [4]. Plastic deformation leads to the formation of dislocation cell within shear bands, then dislocation cells transform into low-angle grain boundaries, and finally form nanocrystalline grains surrounded by high-angle grain boundaries via grain rotation [4-6]. In such a dislocation cell mechanism, the contribution of the fracture and welding processes in the powder particles to the formation of a nanocrystalline structure has heretofore never been studied. It is anticipated that the welded fragments of the original coarse grains should form new grains, although Benjamin and his co-workers did not explicitly demonstrate the relationship between the repeated fracturing-welding process and the refinement of grains. In view of the above findings, the primary objective of the present investigation is to perform structural evaluation of cryomilled materials. Particular attention is paid to enhancing our understanding of the mechanisms that govern the evolution of microstructure in the cryomilled powder during distinct milling stages.
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EXPERIMENTAL PROCEDURE
Commercially available gas atomized Inconel 625 (Diamalloy 1005 AMDRY 625) powder were mechanically cryomilled in a attritor at a rate of 180 rpm up to 20 hours
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