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INTRODUCTION

THE continuous effort to design light and resistant structures is one of the major driving forces of the extensive research on nanocrystalline (nc) materials. In the past two decades, several studies aimed at the optimization of engineering parameters, e.g., increasing the hardness or the yield stress over density ratios.[1] However, difficulties in processing, brittle behavior, and reduced plasticity[2] so far limit the potentiality of these materials, except in a few cases where high values of yield strength (YS) and ductility were obtained.[3] Further problems were generated by stress-induced grain coarsening (mechanical instability), which was observed in different nc metals.[4–6] High energy ball milling (HEBM) followed by consolidation is a well-assessed procedure to produce bulk components made of nc materials: low costs, versatility, and predictability[7] are undisputed qualities of this procedure, although with the drawbacks of contamination[8] and limitation of the minimum grain size obtainable.[9] In fact, owing to the high yield strength (YS) of nc materials, consolidation usually requires very high pressures (r > 1 GPa)[10] or a compromise condition of high pressure–high temperature,[11] which inevitably coarsens the microstructure. An alternative, provided by electrodischarge compaction (EDC) techniques,[12] is to drastically increase the densification rates and limit the temperature rise time, thereby reducing recrystallization and grain coarsening effects. Capacitor discharge sintering (CDS)[13] is a new EDC technique based on the intense flow of a single pulse of high density electric currents through loose powders of conductive materials while a mechanical pressure is applied. The very short densification time, combined with the basic mechanisms ALESSANDRO FAIS, CEO, is with the EPoS srl, Strada delle Cacce 73, 10139 Turin, Italy, and also with the Department of Materials Science and Chemical Engineering, Turin Polytechnic, Corso Duca degli Abruzzi 24, 10129 Turin, Italy. MATTEO LEONI, Assistant Professor, and PAOLO SCARDI, Professor, are with the Department of Materials Engineering and Industrial Technologies, University of Trento, via Mesiano, 77, 38123 Trento, Italy. Contact e-mail: [email protected] Manuscript submitted March 16, 2011. Article published online May 17, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

involved in the CDS process, allows the processing of nc metals and alloys to full or near-full density with minimal grain coarsening.[14,15] Comparison with thermally controlled/thermally ‘‘more stable’’ techniques such as spark plasma sintering (SPS) has evidenced a marked difference between microstructures in a sintered bcc Fe-1.5 pct Mo alloy.[16] As shown by the electron backscattering diffraction (EBSD) maps in Figure 1, SPS (b) gives a bimodal size distribution with a coarser microstructure than CDS (a). Crystalline grain boundary orientation and local misorientation also appear remarkably different: if one focuses attention on a uniform grain-size area in the SPS sam