Alberto Castellero

Effects of chemical composition on nanocrystallization kinetics, microstructure and magnetic properties of finemet-type amorphous alloys

In this study, the kinetics of nanocrystallization of amorphous Fe73.5Si13.5B9Nb3Cu1 (F1) and Fe77Si11B9Nb2.4Cu0.6 (F2) alloys is investigated. The microstructure and magnetic properties of the nanocrystalline alloys are compared. The crystallization temperature of F2 alloy is shifted towards lower temperatures with respect to F1. Thus, the crystalline volume fraction and the crystalline grain size at specific annealing temperature for the F2 alloy are higher than for the F1 alloy, accounting for the higher coercive force of F2 alloy with respect to the one of F1 alloy. According to isoconversional methods, the activation energy for crystallization is variable as a function of transformed fraction because of the continuous changes in chemical composition during the transformation. Mean values of 350 and 290 kJ/mol are obtained for F1 and F2, respectively. Microstructural observations confirm that minor changes in chemical composition affect the kinetics and final microstructure of the nanocrystalline alloy, that determine the observed magnetic properties.

Nanoporous microtubes obtained from a Cu-Ni metallic wire

Nanoporous microtubes of a nickel-copper alloy were obtained from a Cu-44Ni-1Mn (wt%) commercial wire (200 μm diameter). A new synthesis method was established through three steps: 1) partial oxidation of the wire at 1173 K in air, 2) removal of the inner unoxidized core by chemical etching, 3) reduction in 10 bar hydrogen atmosphere. During oxidation, the segregation of Cu and Ni occurred because of their different diffusion coefficients in the corresponding oxides. As a consequence, pores were formed by Kirkendall effect and due to selective chemical etching of the different oxides. Additional porosity formed because of volume contraction during reduction with hydrogen. After reduction, the microtube shows a composition gradient from the inner wall (almost pure nickel) to the outer wall (almost pure copper). The process allowed to obtain microtubes with tuneable wall thickness and inner pores around 180 ± 80 nm. The morphological features developed suggest improved capillarity properties for applications in MEMS.

Calorimetry of Undercooled Metals and Alloys

A calorimetric approach has been employed to study undercooling of metals and alloys using a high temperature differential scanning instrument. The samples are kept under alumina powder, the entire cell is evacuated several times, and heated or cooled under flowing helium. Undercooling can exceed 200 K (as found for pure Ni and Fe). Ni-B, Fe-B, Pd-Si alloys around eutectic compositions, as well as several multicomponent glass-forming alloys, were also significantly undercooled and the solidification of metastable phases was detected. The heat of fusion at the eutectic temperature and the heats of solidification in the undercooling regime have been determined. The thermal data are compatible with the existence of a substantial excess specific heat of the melt for glass-forming alloys whereas the specific heat difference between liquid and crystal phases is nil for pure metals. The undercooling could be extended further when the samples were previously fluxed in molten B 2 O 3 or boro-silicates.

Preparation and Characterization of Fe-Based Metallic Glasses with Pure and Raw Elements

Amorphous alloys with a composition (at.%) Fe48Cr15Mo14C15B6Y2 were prepared by using either pure elements (alloy B1) or a commercial AISI430 steel as a base material (B2). When prepared from pure elements, alloy (B1) could be cast in plate form with a fixed thickness of 2 mm and variable lengths between 10 and 20 mm by means of copper-mold injection in an air atmosphere. In the case of alloy B2, prepared by using commercial grade raw materials, rods of 2 mm diameter are obtained. Ribbons (B1 and B2) of width 5 mm and thickness about 30 μm are prepared from the arc-melted ingots using a single roller melt spinner at a wheel speed of 40 m/s. The thermal and structural properties of the samples are measured by a combination of differential scanning calorimetry (DSC), x-ray diffraction and scanning electron microscopy. Chemical compositions are checked by energy dispersive spectroscopy analysis. X-ray diffraction and scanning electron microscopy observations confirm that an amorphous structure is obtained in all the samples. A minor fraction of crystalline phases (oxides and carbides) is detected on the as-cast surface. Values of hardness and Young modulus were measured by nanoindentation for both the alloys. The effects of adverse casting conditions (such as air atmosphere, non-conventional injection copper mold casting and the partial replacement of pure elements with commercial grade raw materials) on the glass formation and properties of the alloy are discussed.

NaBX 4-MgX 2 Composites (X= D, H) Investigated by In Situ Neutron Diffraction

Light element complex hydrides (e.g. NaBH 4) together with metal hydrides (e.g. MgH 2) are considered two primary classes of solid state hydrogen storage materials. In spite of drawbacks such as unfavorable thermodynamics and poor kinetics, enhancements may occur in reactive hydride composites by nanostructuring of reactant phases and formation of more stable product phases (e.g. MgB 2) which lower overall reaction enthalpy and allow reversibility. One potential system is based on mixing NaBH 4 and MgH 2 and subsequent ball milling, which in a 2:1 molar ratio can store considerable amounts of hydrogen by weight (up to 7.8 wt%). A study of the 2NaBX 4 + MgX 2 → MgB 2 + 2NaX + 4X 2 (X=D,H) reaction is assessed by means of insitu neutron diffraction with different combinations of hydrogen and deuterium on the X position. The desorption is established to begin at temperatures as low as 250 °C, starting with decomposition of nanostructured MgX 2 due to joint effects of nanostructured MgX 2 and its reducing effect at NaBX 4. Analyses of background profile, due to the high incoherent neutron scattering of hydrogen, as a function of temperature demonstrate direct correlation of H/D desorption reactions with relative phases amount.

The Importance of Interactions at the Molecular Level: A Spectroscopic Study of a New Composite Sorber Material

The functional properties of a new composite material having water vapor getter properties have been investigated by a large arsenal of characterization techniques. The composite system is originated by combining two constituents having very different chemical natures, a magnesium perchlorate (Mg(ClO4)2) salt and a polymeric acrylic matrix. In particular, Fourier transform infrared (FT-IR) and Raman spectroscopy have been fundamental to understand the type of interactions between the salt and the matrix in different hydration conditions. It was found that in the anhydrous composite system the dispersed Mg(ClO4)2 salt retains its molecular structure, because Mg2+ cations are still surrounded by their [ClO4]– counter-anions; at the same time, the salt and the polymeric matrix chemically interact each other at the molecular level. These interactions gradually vanish in the presence of water, and disappear in the fully hydrated composite system, where the Mg2+ cations are completely solvated by the water molecules.

Bulk Metallic Glasses

Rapid quenching techniques have been successfully applied since long time for the preparation of metallic glasses in ribbon form. Only in the recent years, the research activity addressed towards the synthesis of bulk metallic glasses (BMG), in form of ingots with a few millimetres in thickness. These materials can be obtained by casting techniques only for selected alloy compositions, characterised by a particularly high glass-forming tendency. Bulk amorphous alloys are characterised by a low modulus of elasticity and high yielding stress. The usual idea is that amorphous alloys undergo work softening and that deformation is concentrated in shear bands, which might be subjected to geometrical constraints, resulting in a substantial increase in hardness and wear resistance. The mechanical properties can be further improved by crystallisation. In fact, shear bands movement can be contrasted by incorporating a second phase in the material, which may be produced directly by controlled crystallisation. Soft magnetic properties have been obtained in Fe-based systems and they are strongly related to small variations in the microstructure, ranging from a fully amorphous phase to nanocrystalline phases with different crystal size. The high thermal stability of bulk metallic glasses makes possible the compression and shaping processes in the temperature range between glass transition and crystallisation. Aim of this paper is to present recent results on glass formation and properties of bulk metallic glasses with various compositions. Examples will be reported on Zr, Fe, Mg and Pd-based materials, focussing on mechanical and magnetic properties.