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20XV International Symposium on Self-Propagating High-Temperature Synthesis
THE MECHANICAL ACTIVATION OF METALLIC POWDER, AN ESSENTIAL ROUTE TO PREPARE DENSE NANOSTRUCTURED MATERIALS BY SPS
S. Le Gallet" and F. Bernard*"
aLab. ICB UMR 6303 CNRS/UBFC, Dijon, 21078 France *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10045
The fabrication of dense nanostructured materials is a main challenge of many researchers. Indeed, it is often reported an enhancement of mechanical properties versus the grain size. However, the number of processes to produce them is not large [1]. It is well known the powder metallurgy is an interesting route to produce dense nanostructured materials starting from nanopowders or from nanostructured powders [1]. These latter were obtained according to different routes: (i) from individual nanopowders prepared from chemical processes (i.e. solgel, hydrothermal method), (ii) from nanostructured powders prepared by ball milling (i.e. planetary ball mill, attritor). The main difficulty with the use of ultra-fine powders concerns the management of the oxygen contamination (i.e. the highly reactivity) at the surface of the metallic powders. One solution to limit such reactivity is the use of ball milling process.
The main interest of the ball milling process is to form large agglomerates with a micrometric size composed of nanocrystallites. Such a microstructure is obtained after repeated fracture/welding processes induced during the milling (Fig. 1).
Fig. 1. SEM observations of Ni commercial powder (left part) and the same powders after a ball milling (right) showing the agglomeration process (central part) [2].
Consequently, a non-equilibrium stage is obtained because of the reduction of the crystallite size and the multiplication of structural defects such as stacking faults, twins or dislocations (Fig. 2).
(a) (b) (c)
Fig. 2. SEM observation of Cu milled powder, (a) large view, (b) zoom on one Cu agglomerate and (c) TEM observation on Cu milled powder showing twins [3].
iSHS 2019
Moscow, Russia
The main interest of such a situation is to modify the heat treatment conditions like a reduction of SHS ignition or the sintering temperatures, an increasing of the sintering reactivity and, then it is possible to limit the grain growth without reducing the densification stage using especially the SPS (spark plasma sintering) technology in which an electric current is used to heat rapidly the powder by Joule effect.
In this work, two solutions using sequential mechanical and field activations will be investigated to:
(i) synthesize and simultaneously densify nanostructured powder mixture [3, 4]. It concerns the production of intermetallics by reactive sintering including an SHS reaction. As an example, solid reactants (Mo and Si) in a stoichiometric ratio are milled for producing mechanically activated agglomerates composed of Mo and Si nanocrystallites. The green body is then exposed to a temperature (i.e. the current) at which the reaction occurs and, to a uniaxial pressure to consolidate the product. Thus, the control of SPS processing parameters allowed obtaining dense nano-organized MoSi2 compounds. Specifically, an increase of the heating rate makes it possible to control the chemical composition by avoiding the formation of secondary phases such as Mo5Si3.
(ii) consolidate different nanostructured metallic powders prepared by ball milling for enhanced mechanical properties [5, 6]. The rapidity of the SPS process leads to a limited grain growth. A mechanical activation on commercial Ni powders was accomplished by high energy planetary mill in order to reduce the crystallite size and to increase structural defects. Consequently, a dense nanostructured Ni with an Rp0.2% of 498 MPa and an elongation of 28% was obtained whereas a dense microstructured Ni with a true elongation of 42% and yield strength of 230 MPa has been produced. Finally, from a best control of the ball milling conditions, it is possible to manage the best compromise between yield strength and ductility.
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2. O. Boytsov, E. Gaffet, F. Bernard, A. Ustinov, Correlation between milling parameters and microstructure characteristics of nanocrystalline copper powder prepared via a high energy planetary ball mill. J. Alloys Compd., 2007, vol. 432, pp. 103-110.
3. Z.A. Munir, E. Gaffet, F. Charlot, F. Bernard, One-step synthesis and consolidation of nano-phase materials, US Patent N°6 200 515 B1, 13 Mars 2001.
4. G. Cabouro, S. Chevalier, S. Le Gallet, E. Gaffet, Yu. Grin, F. Bernard, Dense MoSi2 produced by reactive flash sintering: control of Mo/Si agglomerates prepared by high energy ball milling, Powder Technol, 2011, vol. 208, pp. 526-531.
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6. F. Naimi, L. Minier, S. Le Gallet, H. Couque, F. Bernard, Dense nanostructured nickel produced by SPS from mechanically activated powders: enhancement of mechanical properties. J. Nanomater., 2013, ID 674843.
