Synthesis, characterization and mechanical behaviour of an in situ consolidated nanocrystalline FeCrNi alloy

IntroductionCurrently, nanocrystalline materials are being widelyinvestigated due to their unique properties. This interest hasspawned the development of various techniques to synthe-size materials exhibiting grain sizes (\20 nm) wherephysical properties are the most exotic. Such processesinclude: high-energy ball milling [1, 2], pulsed electrode-position [3], chemical vapour condensation [4], inert gascondensation [5] etc. However, nanocrystalline samplesproduced from these techniques are either in the formof powders or thin films. Successful consolidation to arte-fact free, fully dense material is required if structuralapplications are ever to be adopted. Many methods toconsolidate these materials (i.e., hot compaction [6],explosive consolidation [7], hot isostatic pressing [8], pre-annealing–compaction–sintering [9] etc.) have been repor-ted in the literature with partial success. In general, thesuccess of these techniques for consolidation hinges on theapplication of high temperatures and pressures, which leadto explosive grain growth in nanocrystalline materials.In situ consolidation technique as developed by Youssefet al. [10] seems promising for the synthesis of consolidatednanocrystalline materials as the step of sintering to bulkdensity is no longer a need and can be eliminated. However,this technique has not been reported widely in the literatureand has been limited to nanocrystalline Zn, Al, Al alloys,copper and copper-based alloys [10, 11]. This study showsthat in situ consolidation of a FeCrNi alloy is possible usingcombination of room temperature (RT) milling and liquidnitrogen temperature milling.Nanocrystalline materials produced by high-energy ballmilling have not been well characterized and the micro-structure of these materials is not well understood. Theprimary challenge in characterization is TEM samplepreparation from ball-milled powders. Nanocrystallinepowders as produced by ball milling are too coarse to beexamined under TEM and the sample preparation for TEManalysis on such powders is not a trivial task. In general,two methods are used for the preparation of conventionalmetallic samples for TEM analysis: electro jet polishingand dimpling followed by ion milling. Nanocrystallinematerials produced by consolidation of ball-milled pow-ders can be problematic for these preparation techniques asthe consolidated samples must first be mechanically thin-ned to \100 lm before being chemically thinned to thefinal thickness. If the material exhibits poor inter-particlebonding, the sample may not withstand the final thinning