Papers by Keyword: Nanostructured

Abstract: The nanostructured metals and alloys are under intensive research worldwide and being developed into bulk forms for application. While these new materials offer record-high strength, their ductility is often inadequate and sometime rendering them unusable. Besides tailoring the nanostructure to achieve coexisting high strength and high ductility, to uncover the coherent property of this material is also important. This article reviews the recent researches finished in our lab. A set of nanostructured metals and alloys were synthesized by a direct current electrodeposition technique, and the effect of grain size and strain rate on the mechanical properties stressing on tensile ductility was systemically studied by tensile test at room temperature.

Abstract: The control and the improvement of the ductility of nano-structured structural steels is one of the key challenges in assessing the technological viability of this metallurgical strategy. In the present paper, it is shown that more rigorous definitions of the ductility are required in order to avoid possible confusions. After this preliminary work, a more transparent analysis is done concerning the effect of the microstructural scale showing clearly the weaknesses and the strengths of ultra-fine single phase steels. Finally, possible ways to overcome the main limitations are presented.

Abstract: Nanostructured metals which have nano-scale microstructure are classified into ultrafine grained metals and nanocrystalline metals. In recent years, many processing techniques have been developed for producing nanostructured metals. Nanostructured metals possess ultrahigh strength but the low ductility is an important limitation on development of these materials for structural applications. This paper overviews various methods of producing nanostructured metals and recent investigations of strength and ductility of nanostructured metals processed by sever plastic deformation.

Abstract: There is an increasing interest in the last years for materials with nanometric grain size because of the enhanced properties that could be achieved when reducing to the nanometric scale. Three coatings using conventional, nanostructured and bimodal (mixture of conventional and nano) WC-Co powders were obtained through High Velocity Oxygen-Fuel (HVOF) technique. The powders were sprayed under different spraying conditions in order to improve trybological properties for nanostructured powders. The relationship between spraying conditions and decomposition has been studied using in-flight measurements and XRD techniques. The nanostructured coating showed more decomposition than the other ones during spraying process, but this decomposition was reduced through the new conditions. Nanostructured coating showed more hardness using producer conditions, but the bimodal coating showed better abrasive and friction wear resistance. Nanostructured and bimodal coatings also provided an enhanced corrosion protection to the substrate when compared with the conventional one. An important improvement in the abrasive wear resistance for the nanostructured coating was obtained with the new conditions, due to the reduction in the decomposition.

Abstract: Severe plastic deformation (SPD) has received considerable attention for its capability to produce ultrafine and nano structured materials. On the one hand, SPD, especially in the forms of equal channel angular pressing (ECAP) and high pressure torsion (HPT) is able to refine bulk materials with coarse grain structures. On the other hand, SPD has been used to synthesise bulk materials from particles. It enables particles from nano to micro scales to be consolidated into fully dense materials at much lower temperatures and shorter times, compared to the conventional sintering processing. It is particularly relevant to consolidating particles with non-equilibrium microstructures and to producing complex multiphase alloys. In this summary, ECAP as an effective process to synthesise a range of light metal based materials from particles with various sizes and structures, including aluminium and aluminium composites, titanium and magnesium, will be demonstrated. Full density and good bonding are achieved easily with the application of a back pressure. Microstructures from nano to ultrafine scales have been produced, resulting in significantly enhanced strength. Simultaneous increase in ductility has also been achieved in some alloys by virtue of multi-scale structures.

Abstract: Severe plastic deformation (SPD) has been demonstrated to be the most efficient method to produce bulk metals with ultrafine grained (UFG, 100 nm < grain size d < 500 nm) and nanocrystalline (NC, d<100 nm) microstructures. Such metals exhibit some unique properties owing to their unusual microstructures such as high-energy, non-equilibrium grain boundaries. Efforts in the past two decades have focused on metals with face-centered cubic (fcc) structures. Recent experimental results have shown that UFG/NC metals with body-centered cubic (bcc) structures have some properties that are distinct from their fcc counterparts. Further, the majority of the fcc metals are very ductile and have relatively low melting points, making them easier to process using SPD. On the contrary, many bcc metals are refractory, and are very sensitive to interstitial impurities, rendering them difficult to work via SPD. In this article, we attempt to summarize the state-of-the-art of UFG/NC refractory metals processed by SPD, with focus on the microstructure and mechanical properties. Comparisons with UFG/NC fcc metals are made where appropriate. Outstanding issues and future directions are also addressed.

Abstract: The progress in bulk ultrafine and nanostructured materials through consolidation of particles by severe plastic deformation (SPD) is reviewed. The focus is on the processes of high pressure torsion (HPT) and equal channel angular pressing (ECAP) with or without the application of a back pressure. Various materials consolidated are described in terms of their densities, microstructures and mechanical properties. The important processing parameters and their effects on the resulting materials are discussed. It is shown that SPD consolidation of particles is an effective way of producing bulk nanostructured materials although much work is needed to understand the consolidation behaviour and to design the optimum compositions and microstructures.

Abstract: The microstructure characteristics of the spray-formed and melt-spun Al85Nd5Ni10 and Al89La6Ni5 alloys were studied. The spray forming process was demonstrated to produce a bulk scale hybrid composite consisting of amorphous and nanostructured phases directly without the need of an amorphous precursor. The spray-formed Al89La6Ni5 deposit (~1 mm in thickness) were partially amorphous, and the amorphous phase came from the undercooled liquid droplets upon deposition. The as-spray-formed Al85Nd5Ni10 deposit (~20 mm in thickness) was completely crystallized due to the devitrification of the retained amorphous phase to nano-scale secondary crystals upon deposition. Primary crystals (~1 μm) are dispersed uniformly in the bulk spray-formed amorphous/or partial amorphous composites and many distinctive deformation twins also are observed in the crystals, however, not twins found in the corresponding completely devitrified ribbon. This is mainly because of the stirring and impacting force offered by high velocity droplets during spray forming and the mismatch of thermal expansion coefficient between primary crystals and adjacent amorphous matrix.

Abstract: The compressive behavior was investigated on an electrodeposited nanocrystalline Ni-20%Fe alloy with a grain size of about 22 nm at room temperature (RT), 298 K, and the liquid nitrogen temperature (LN2T), 77 K. The sensitivity of the yield strength and plastic strain to the test temperature at different grain sizes was discussed. Moreover, through the Transmission Electron Microscopy (TEM) examination and microhardness measurement, the microstructures before and after the compression test at RT and LN2T were studied.

Abstract: The aim of this work is the achievement of nanostructured TiC–TiB2 powders through a metastability approach based on the Self–propagating High–temperature Synthesis (SHS) process followed by quench to obtain highly metastable powder agglomerates. An optimisation of the reaction stoichiometry was carried out in order to obtain products with approximately eutectic composition (i.e. 67%mol TiC – 33%mol TiB2). An optimized amount of sodium borate was used as gasifying additive to produce dispersed nanostructured powder agglomerates. The metastability of the nanocomposite powders obtained through the (SHS+quench) route was evaluated by annealing. The morphological evolution of the powders after thermal treatment yielded a recrystallisation with limited grain growth of the nanostructured TiC–TiB2 phases and demonstrated the metastability of the products obtained by the (SHS+quench) route.