V. Provenzano

Overview of nanophase metals and alloys for gas sensors, getters, and hydrogen storage

Abstract Nanophase metallic materials produced by inert gas condensation and ball-milling, including zirconium and magnesium alloys, have been the subject of several exploratory studies on potential gas-reactive applications of nanophase metals. Such applications include gas sensing, vacuum gettering, and hydrogen storage. A brief overview is given of the background, rationale, and some results from the literature of gas-reactive applications of nanophase metals and alloys.

Void formation and suppression during high temperature oxidation of MCrAlY-type coatings☆

Abstract Oxidation studies have been conducted on cast Co-22Cr-11Al (CoCrAl) coating alloy in air at temperatures between 700 and 1000 °C. To investigate the role of oxygen-active element additions on the oxidation behavior of the MCrAlY-type coatings, some of the CoCrAl specimens were ion implanted with either yttrium, hafnium or cobalt prior to oxidation. The oxidized specimens were subsequently examined by various electron optical techniques including scanning transmission electron microscopy and electron microprobe analysis. It was observed that voids formed in the coating alloy at the metaloxide interface during oxidation. The average size and density of these interfacial voids were dependent on temperature and time. Ion-implanted yttrium or hafnium greatly reduced the rate of void growth for all experimental conditions; cobalt implantation had little effect. This behavior is explained by a vacancy kinetic model which invloves the preferential diffusion of either metal or oxygen ions through the growing oxide scale. The results of the present study have significant implications concerning the oxidation kinetics and the adherence of the oxide scale to the coating alloy.

Characterization of nanocrystalline palladium for solid state gas sensor applications

Abstract Nanocrystalline Pd has been synthesized by the inert gas condensation technique, consolidated into pellets, and characterized by x-ray diffraction and scanning electron microscopy. The hydrogen gas absorption/desorption properties of these materials were investigated to demonstrate their application to solid state gas sensor development. These nanocrystalline materials exhibited fast response to hydrogen gas and complete recovery up to exposure to several percent of hydrogen. Suppression of the α-βphase transition also was observed. The temperature dependence of the response/recovery rates indicate that the rate controlling step for absorption is the dissociative chemisorption of hydrogen on the surface of the Pd, whereas for desorption, diffusion of hydrogen out of the palladium metal was rate controlling.

Enhanced microhardness of copper-niobium nanocomposites

Abstract Nanocomposites of copper and niobium were prepared by the DC sputtering inert gas condensation technique. The Vickers microhardness, HV, of the as-deposited, cold-compacted nanocomposite pellets increases uniformly with increasing Nb concentration from the value for pure nanocrystalline Cu of about HV = 200 to HV = 400 near 90 wt % Nb. Specimens that have been sintered in hydrogen exhibit increased microhardness for all Nb concentrations, but rather than uniform increase in hardness with increasing Nb concentration as for the as-deposited material, there is a peak in the hardness versus concentration near 63 wt % (64 vol %, 54 at. %). Values as high as HV = 1330 were measured for specimens with approximately 63 wt % Nb that were sintered in hydrogen at 1000°C, near the melting point of the copper matrix. The implications of these results for high-temperature strength retention of nanocomposites are discussed.

Mechanical properties of nanostructured chromium

Abstract In this paper we present some initial results on pure chromium whose grain structure was refined by severe plastic deformation processing. These results include the microstructural features and the corresponding mechanical properties as a function of severe plastic deformation processing and annealing treatment. They show that severe plastic deformation is quite effective in reducing the grain size down to nanoscale dimensions with an attendant large increase in the hardness value; the hardness data is consistent with Hall-Petch strengthening. The annealing treatment demonstrated that the nanoscale microstructure is retained up to 400 °C. Further, the decrease in the hardness value with increasing annealing temperature is quite gradual up to 900 °C, suggesting that the microstructure of pure chromium, resulting from the severe deformation is not too unstable, but, it coarsens gradually with increasing temperature. Therefore, in the chromium alloys the deformed microstructure is expected to be even more stable due to the beneficial effect of the alloying additions in pinning the grain boundaries.

On dispersions of voids as sources of strength

Abstract Materials of reduced density and increased strength can be achieved by the creation of dispersed voids. A procedure for the manufacture of such materials is described and their characteristic strength estimated.

On the validation of a strengthening concept

Louat formulated a strengthening theory in which the four conditions specified above could be met in two phase materials in which the minor phase forms the matrix. Additionally, thermal stability is insured if the two phases are immiscible. It follows from Louats theory that material with superior strength can be obtained by embedding a high volume fraction (greater than 50%) of ultrafine particles in a ductile matrix. In contrast to conventional materials, these particle reinforced materials are predicted to retain much of their strength even when the matrix melts. In addition, the theory predicts that the strength of such materials increases with decreasing particle size. The results presented in this paper refer to the case of iron or copper particles embedded in a lead matrix

BOUNDS ON THE STRENGTH OF A MODEL NANOCOMPOSITE

Abstract Criteria for thermally stable, high-strength metallic nanocomposites in the context of Hall-Petch behavior of the constituent phases are examined. It is argued that both upper and lower bounds exist on the strength of such nanocomposites as a function of volume fraction and particle size of the component phases. Using a two-phase topological model, assuming Hall-Petch behaviors for the pure constituent phases, and accounting for thermally stable grain sizes in the context of the Zener drag model, we predict that an optimal combination of nanostructured particles in a metal matrix would consist of nanoscale particles with a volume fraction of about 25%, dispersed in a matrix of a metal with a high Hall-Petch coefficient, and the two phases being immiscible. The matrix phase grains would necessarily be up to ten times larger than the dispersed phase particle size, as a result of the trade-off of grain size stabilization versus Hall-Petch strengthening.

Reconsideration of a high-temperature, high-strength nanocomposite concept

Abstract A concept for high-strength, high-temperature nanocomposites proposed by N. Louat [Acta Metall. 33 , #1, 59–69 (1985)] is discussed in the context of several model metallic nanocomposite systems that have been tested: Cu-Nb, Ag-Ni, and Ag-Cu. It is argued that the conditions of the model that the particle and matrix phases be immiscible means that the particle-matrix interface is likely to be weakrelative to either constituent phase; therefore, the Louat model for strength enhancement and high-temperature strength retention cannot be met in practical nanocomposites.

Microporous fine-grained copper: Structure and properties

Abstract Powder metallurgy processing has been used to produce copper compacts with fine grain sizes (1-10 μm) that are pinned by submicron-size to micron-size gas-filled voids in the volume fraction range 0.05-0.2. The effect of subsequent heat treatment on the grain size and void size and shape was quantified. These changes strongly depended on whether the powder was consolidated using a coldpressing-and-sintering route, or hot pressing; thus the pressed and sintered compacts densified further whereas the hot-pressed compacts exhibited swelling during subsequent thermal exposure. Such materials were mechanically tested in compression and tension at room temperature, and high yield strength, attributed to grain-size strengthening, was recognized. Tensile ductility in excess of 20% was simultaneously obtained although some unusual features, atypical of fcc metals, including upper and lower yield points and a low work-hardening rate were noted. Approximate calculations examining the interaction of dislocations with a void pair, an assembly of voids and the particular case of all voids being located at grain boundaries indicate that direct strengthening due to the voids is not the principal contributor to the high strength; rather it is the refinement in grain size that is responsible for the observed yield strength level.