Paola Rizzi

Phase selection in Al–TM–RE alloys: nanocrystalline Al versus intermetallics

Abstract Al–TM–RE (TM: transition metal, RE: rare earth metal) and Al–RE alloys were analysed with the aim of studying phase selection under various processing conditions. The metastable phases formed in binary Al-rich systems (notably Al–Sm) are revised for a unified interpretation of the literature. Two groups of alloys had different behaviour as for phase selection. In Al 88 Fe 9 Nd 3 and Al 87 Ni 10 Ce 3 stable intermetallic compounds form as primary phases during solidification. Nanocrystalline Al is formed at high undercooling. This may occur directly in rapid solidification or when fully amorphous materials are suitably annealed. In Al 90 Sm 8 Ni 2 and Al 90 Sm 8 Fe 2 metastable intermetallics form during rapid solidification together with a fraction of amorphous phase, whereas nanocrystalline Al forms on annealing the amorphous phase.

Calorimetric measurements on some undercooled metals and alloys

Abstract Undercooling was achieved directly in the cell of a high temperature calorimeter (Setaram HTDSC) for Ni, Fe, Cu, Pd and several alloys, using cooling rates between 1 and 15 K min −1 . The samples were immersed in alumina powder inside a standard alumina crucible under flowing helium. Ag, Au and Al were not undercooled significantly. The reproducibility of the measurements was within 1.5%. The heat of solidification of Ni at an undercooling of Δ T = 220 K was −17.5 ± 0.2 kJ mol −1 , which is the same absolute value of the heat of fusion at the equilibrium melting point T m . This implies that the specific heat of the undercooled liquid is very close to that of the crystalline solid in this temperature range. Fe appears to display a similar behavior at Δ T = 220 K. The difference between the heat of fusion at T m and the heat of solidification at an average value of ΔT = 95 K is significant for a Pd 77.5 Cu 6 Si 16.5 glass-forming alloy. From these data, we calculated an average specific heat difference between the liquid and crystal phases of 7 ± 5 J mol −1 K −1 . The enthalpy data for Pd 82 Si 18 comply with those of the ternary PdCuSi.

Thermodynamics of homogeneous crystal nucleation in Al-Re metallic glasses

In the present paper, thermodynamic data of the Al-Ce and the Al-Nd systems are analysed by a new CALPHAD assessment, in which special emphasis is put on the description of the short range order in the undercooled liquid. A reliable description of the thermodynamics of the undercooled liquid and the amorphous phase can only be achieved by considering thermodynamic data of each metastable state. The results demonstrate a stabilization of the liquid phase on undercooling up to the glass transition temperature. The thermodynamic properties of intermetallic compounds are also reassessed. From the calculated Gibbs energies, the driving force for nucleation of primary fcc-Al and Al 11 RE 3 crystals from the Al-rich liquid and amorphous phase have been computed as a function of temperature.

Nanoporous gold obtained from a metallic glass precursor used as substrate for surface-enhanced Raman scattering

Nanoporous gold (NPG) has been synthesized by electrochemical de-alloying a new precursor, amorphous Au30Cu38Ag7Pd5Si20 (at.%), starting from melt-spun ribbons. Ligaments ranging from 75 to 210 nm depending on the de-alloying time were obtained. Analytical and electrochemical evidence showed the ligaments contain residual Cu, Ag and Pd. Surface-enhanced Raman scattering from the NPG was investigated using pyridine and 4,4′-bi-pyridine as probe molecules. It was found that the activity is at maximum when the ribbon is fully de-alloyed although the ligaments then have a larger size. The enhancement is attributed to the small size of crystals in the ligaments, to their morphology and to trapped atoms.

Rapid solidification of immiscible alloys

Abstract Immiscible alloys have been rapidly solidified for the preparation of granular materials with giant magnetoresistance properties. Au-based (Au–Co and Au–Fe) and Cu-based (Cu–Co and Cu–Fe) systems have been investigated. Single supersaturated solid solution has been obtained for Au–Fe, whereas three FCC solid solutions with different Co content have been found for Au–Co. For Cu–Co and Cu–Fe a limit of solubility in Cu has been observed. Ni additions to Cu–Fe strongly enhance solid solubility. A thermodynamic analysis has been used to describe the competition between partition-less solidification and phase separation in undercooled liquid.

Microstructures in laser welded high strength steels

In this work, the effect of laser welding on the microstructure was studied for three Advanced High Strength Steels: transformation induced plasticity steel (TRIP), dual phase steel (DP) and martensitic steel. Two sheets of the same steel were laser welded and a microstructural study was performed by optical microscopy, scanning electron microscopy and X-ray diffraction. For all samples the welded zone was constituted by martensite and the heat affected zone shows a continuous change in microstructure depending on temperatures reached and on the different cooling rates. The change in mechanical properties in the welded area was followed by Vickers micro-hardness measurements. Quasi binary phase diagrams were calculated and, according to position of T0 lines, it was deduced that austenite is the primary phase forming during rapid solidification for all steels.

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.

Properties of FeNiB-based metallic glasses with primary BCC and FCC crystallisation products.

FeSiB tapes have long been commercialised for their excellent soft-magnetic properties but do not manifest a glass transition temperature Tg as crystallisation intervenes. In this work, we present the crystallisation and properties of two Fe-based glasses, which show a glass transition before crystallisation. Using Ni and Co substitution, we can design glasses that form primary FCC or BCC FeNiCo solutions in their first stage of crystallisation followed by the formation of the metastable (FeNiCo)4B (C6Cr23-type cF116) intermetallic phase. Thermal and structural and magnetic properties were compared during heat treatment.

Phase separation in multicomponent amorphous alloys

Abstract Metallic glasses have been obtained in the form of small ingots with the composition Zr40Ti14Cu11Ni10Be25, by means of water quenching. All glasses transform on annealing to a mixed state comprising a nanocrystalline phase embedded in an amorphous matrix. The transformation of the glasses occurs via a phase separation in the amorphous alloy, followed by the formation of the nanocrystalline phase. The process is analyzed by means of isothermal and non-isothermal differential scanning calorimetry (DSC) and X-ray diffraction. The isothermal heat of transformation has two steps as a function of temperature which are due to the transformation sequence. Similarly, non-isothermal traces have multiple signals, the intensities of which depend on the heating rate. A Ni36Fe32Ta7Si8B17 alloy has been shown to display glass forming tendency, as indicated by the reduced crystallization onset temperature of 0.6. Amorphous ribbons of this material reach a thickness of 100 μm. On annealing, an austenitic nanocrystalline phase is obtained through a nucleation and growth mechanism. In practice, the growth is limited and rarely proceeds beyond a particle size of 10 nm, as revealed by transmission electron microscopy. The present results allow a comparison of the DSC behaviour for alloys undergoing devitrification with different mechanisms.

Ferromagnetic resonance and superparamagnetic behavior of iron oxide nanoparticles injected in porous anodic alumina

Iron oxide nanoparticles with diameter around 10nm were produced and injected in 60μm thick anodic alumina membranes with pore diameters of 20 and 100nm. The structure, magnetic properties, and the ferromagnetic resonance frequency of the nanoparticles before and after injection into the columnar arrays were measured as a function of the out-of-plane applied field. The effect of dipolar interactions and clustering mechanisms of the injected nanoparticles on the static and radio frequency magnetic response is discussed.