High-energy grinding of FeMo powders
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Iron-molybdenum powders ground in a planetary ball mill under different operating conditions were studied by x-ray diffraction line profile analysis using a recently developed whole powder pattern modeling approach. The evolution of the microstructure, expressed in terms of size distribution of coherent scattering domains, average dislocation density, and edge/screw character, shows the importance of the main process parameters: the ratio between jar and main disk rotation speeds, and ball milling time. A characteristic three-stage process is observed, with work hardening followed by particle flattening/bending before nanocrystalline grains form by a fragmentation process triggered by localized deformation. The relationship between lattice defect density and domain size suggests a progressive transition between statistically stored to geometrically necessary dislocations, with the latter mostly present as excess dislocations at the nanodomain boundary.
I. INTRODUCTION
Two basic parameters are usually used to describe the microstructure of a nanocrystalline material: grain size and content of lattice defects, such as vacancies, dislocations, faulting, etc. Transmission electron microscopy (TEM) is certainly a fundamental tool in these studies, even though a quantitative and statistically reliable microstructure evaluation can be rather difficult for highly deformed materials. In these materials, the density of defects can be so high that defects and regions of coherent scattering (crystallites) can be difficult to single out and quantify. In any case, a reliable analysis requires a lengthy (and possibly interfering1) sample preparation, and the collection and evaluation of a large number of micrographs.1,2 Moreover, exposure to a high-energy electron beam can be responsible for annealing and microstructure evolution during the TEM observation.3 As an alternative or complementary technique, x-ray diffraction (XRD) line profile analysis (LPA) is frequently used to study the average microstructural evolution of nanostructured materials.4,5 Even though it cannot give visual evidence as TEM, LPA is much faster and statistically very robust, as it analyzes a huge number of domains simultaneously. Despite the high interest in LPA, considerable advances in this field during the past decade were not apparently considered beyond a restricted community of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0224 1744 J. Mater. Res., Vol. 22, No. 6, Jun 2007 http://journals.cambridge.org Downloaded: 14 May 2014
experts.6,7 The majority of (even recent) studies on nanocrystalline materials is in fact based on the application of highly simplified so-called integral breadth methods like the Scherrer formula (SF) or the WilliamsonHall (WH) plot,8,9 dating back to the beginning/middle of last century. This is probably the reason for the common thought that information available through LPA is rather poor and arbitrary, as pointed out in a recent review: “. . . The width of the Bragg ref
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