Abstract
Nanostructured metallic materials can be obtained by two major processing strategies: a bottom-up approach that starts with powdered metals to be mechanically and chemically compacted via different compaction methodologies, and a top-down approach that starts with bulk conventional metallic materials that are induced to a sometimes-extraordinary grain size reduction via different severe plastic deformation (SPD) methods. In the present study, a dual strategy was followed to obtain a sound and stable nanostructured commercially pure cobalt. Powdered cobalt of 2 μm was compacted by ball-milling (BM) followed by spark-plasma sintering (SPS) to obtain a bulk metallic material whose relative mass density reached a value of 95.8%. This process constituted a bottom-up strategy to obtain ultrafine submicrometer-grained bulk cobalt, and a top-down strategy of subjecting the BM + SPS submicrometer-grained cobalt to a specific SPD technique, namely equal-channel angular pressing (ECAP). The latter was carried out in one to four passes following so-called route BC, reaching 98.4% density and a nanometric-grained microstructure. The material was microstructurally and mechanically characterized by TEM (transmission electron microscope) and nanoindentation. The obtained results are a representative solid example for obtaining nanostructured metallic materials using both grain-refining strategies, bottom-up and top-down.
Highlights
In the mid-1980s, Gleiter [1] made the visionary argument that metals and alloys, in their nanocrystalline (NC) form, would have several appealing mechanical characteristics of potential significance for structural applications
In the bottom-up method, ultrafine powders are synthesized via various techniques and compacted into bulk form, using ways similar to those applied in conventional powder metallurgy (PM) [4]
The top-down process is based on severe plastic deformation (SPD) methods such as high-energy ball-milling (BM) [4], equal-channel angular pressing (ECAP) [5], high-pressure torsion (HPT) [6], accumulative roll-bonding (ARB) [7], surface mechanical attrition treatment (SMAT) [8], and dynamic severe plastic deformation (DSPD) [9]
Summary
In the mid-1980s, Gleiter [1] made the visionary argument that metals and alloys, in their nanocrystalline (NC) form, would have several appealing mechanical characteristics of potential significance for structural applications. NC metals and alloys, with grain sizes typically smaller than 100 nm, have been the subject of considerable research activity worldwide These materials have shown quite interesting mechanical properties, such as high strength, increased environmental resistance, improved strength and/or ductility with increasing strain rate, and, in some cases, superplastic deformation [2]. (i) grain size generated by the BM + SPS compaction process should possibly be kept within a submicrometer level, and (ii) the compacted cobalt porosity should reach a sufficient level to meet the typical properties of bulk commercially pure metals, and this has to be at least maintained, if not even reduced, after ECAP. In-depth process control of the UFG and, eventually, NC nature and of the relative densification of the compacted cobalt powder is an essential requirement for potential industrial application of the bulk Co produced by means of this approach
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