Abstract
Nanocrystalline Mg-7.4%Al powder was prepared by mechanical alloying using a high-energy mill. The evolution of the various phases and their microstructure, including size and morphology of the powder particles in the course of milling and during subsequent annealing, were investigated in detail. Room temperature milling leads to a rather heterogeneous microstructure consisting of two distinct regions: Al-free Mg cores and Mg-Al intermixed areas. As a result, the material is mechanically heterogeneous with the Mg cores displaying low hardness (40–50 HV) and the Mg-Al intermixed regions showing high hardness of about 170 HV. The Mg cores disappear and the microstructure becomes (also mechanically) homogeneous after subsequent cryo-milling. Rietveld structure refinement reveals that the crystallite size of the milled powders decreases with increasing the milling time reaching a minimum value of about 30 nm. This is corroborated by transmission electron microscopy confirming an average grain size of ~25 nm.
Highlights
Mg-based alloys have significant importance as lightweight structural materials due to their high specific strength [1,2]
Two main approaches have been extensively used for the production of nanocrystalline materials: (i) the bottom-up approach, which consists of building the nanostructure atom-by-atom or layer-by-layer, such as in inert gas condensation, chemical vapor condensation, or pulse electron deposition [7,8,9]; and (ii) the top-down approach, such as mechanical alloying (MA), that consists of alloying combined with the structural decomposition of a coarse-grained microstructure into a nanostructure [10,11]
(Figure 1b), arising from the ductile character of the starting Mg and Al powders. This behavior drastically changes for the material milled for 40 h, where the particle size is reduced to about 70 μm
Summary
Mg-based alloys have significant importance as lightweight structural materials due to their high specific strength [1,2]. (i) the bottom-up approach, which consists of building the nanostructure atom-by-atom or layer-by-layer, such as in inert gas condensation, chemical vapor condensation, or pulse electron deposition [7,8,9]; and (ii) the top-down approach, such as mechanical alloying (MA), that consists of alloying combined with the structural decomposition of a coarse-grained microstructure into a nanostructure [10,11]. This results from two essential processes taking place during milling: cold-welding, and fracturing of the cold-welded particles due to high-energy collisions [10,11]. The Mg-7.4%Al alloy was selected in order to balance γ-Al12Mg17 phase precipitation and solid solution in the present experimental set-up
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