Ultrafast Atomic Diffusion Paths in Fine-Grained Nickel Obtained by Spark Plasma Sintering
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allurgy represents a growing industry where complex shapes of a large range of materials (pure metals and alloys) can be processed in a short time and with excellent production value.[1–3] It is highly used in different domains such as aerospace electronics, nuclear energy industry, and technologies for medical or dental use and especially in the automobile industry.[4–6] Different sintering techniques enable the fabrication of custom-made microstructures including ultrafine grained (UFG), bimodal, or harmonic structures.[7–9] Such microstructures are produced using powders of different characteristics. A practical tool to modify the morphology, size, chemical nature, and microstructure of powders is mechanical milling (MM). Mechanical milling is a technique where high strain is achieved from the collisions between the milling media and the powder.
LUCI´A GARCI´A DE LA CRUZ, BERNADETTE DOMENGES, and ERIC HUG are with the Normandie University, UNICAEN, ENSICAEN, CNRS, Laboratoire CRISMAT, 6 Bvd du mare´chal Juin 14050 Caen, France. Contact e-mail: [email protected] SERGIY V. DIVINSKI and GERHARD WILDE are with the Institute of Materials Physics, University of Mu¨nster, Wilhelm-Klemm-Str. 10, 48149 Munster, Germany. Manuscript submitted November 4, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Adjusting the different milling parameters,[10] powders with the required characteristics can be obtained. In the last decade, excellent mechanical properties of the above mentioned microstructures have been highlighted[11–14] but studies concerning self-diffusion behavior are lacking. Knowledge on self-diffusion in sintered powders is extremely important to increase the basic understanding of specific grain boundary structures and their kinetic properties and for a better understanding of the sintering process. For example, systematic studies of sintering stages in nanocrystalline FeNi alloys[15] were convincingly explained in terms of specific diffusion paths.[16] In most cases, the custom-made structures are prepared from MM powders that display UFG microstructures.[17] SPD-processed metals are known to display enhanced diffusivity due to the formation of the so-called ‘‘non-equilibrium’’ or ‘‘deformation-modified’’ grain boundaries (GBs) that are in a high-energy state.[18–20] They are generated by plastic deformation and display an excessive density of extrinsic grain boundary dislocations.[21,22] This is the case for nickel samples prepared by equal channel angular pressing (ECAP)[23] and high pressure torsion (HPT).[24] Studies on nickel, as a model material, are very convenient as a large body of literature can be used to compare the measured results. The highly deformed nature of powders prepared by MM makes it pertinent to think that MM could also entail the formation of
deformation-modified GBs[25] similarly to SPD-processed materials. In fact, MM has been used to enhance hydrogenation of Mg alloys,[26,27] providing similar results as the ECAP processed samples.[28] Relaxation of deformation-modified GBs can be achieved a
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