E. Bosco

Thermodynamic analysis of glass formation in Fe-B system

Abstract A thermodynamic optimisation of the Fe-B system has been carried out in order to model metastable phases. Amorphous and Fe 3 B metastable phases have been considered in the assessment in order to describe, from a thermodynamic point of view, the well known glass forming ability of this system. An excess specific heat has been considered for the liquid alloys in order to take into account ordering processes on undercooling. The glass transition has been considered as a second order transition between undercooled liquid and amorphous phases. The effect of lattice stabilities on assessment has been discussed considering different descriptions for the free energy of undercooled pure liquid Fe. The calculated stable and metastable phase diagrams are in good agreement with experimental data, as well as invariant equilibria and thermodynamic properties. The results evidence the existence of an excess specific heat for glass forming Fe-B liquid alloys, which allows a reasonable description of thermodynamic functions for all phases in a wide temperature range.

Driving forces for crystal nucleation in Fe–B liquid and amorphous alloys

Abstract The Fe–B system has been reassessed in order to model metastable and amorphous phases. An excess specific heat term has been added to describe Gibbs free energy of the liquid phase on undercooling and glass transition has been considered as a second-order transition. In addition, a recently proposed description of pure Fe lattice stabilities has been used for this optimisation. Stable and metastable phase diagrams have been calculated, as well as thermodynamic properties, which turn out in agreement with experimental data. Thermodynamic driving forces for crystal nucleation have also been evaluated for liquid and amorphous phases. According to experimental findings, it was found that the first nucleating phase is b.c.c. Fe for B-poor amorphous alloys. However, for B-rich compositions, a competition between Fe2B and Fe3B nucleation has been evidenced. A full explanation of the crystallisation process requires the evaluation of interfacial energies.

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.

Rapid solidification of alloys

This paper reviews some applications of rapid solidification processing of alloys. With reference to original work of the authors, the production and properties of amorphous alloys, mainly iron based, of nanocrystalline aluminium alloys and of granular materials are described. A section is devoted to the latest advancement of thermodynamic modelling of metastable systems using CALPHAD softwares.

Solidification experiments for the study of phase selection in cast iron

The mechanical and thermal properties of cast iron depend on microstructure. There is a wealth of studies in the literature on metastable and stable eutectics and on graphite growth under various experimental conditions aimed at elucidating the cause of the formation of either lamellar, nodular or compact grains. However, the early stages of nucleation are difficult to isolate and the models for graphite nucleation are even controversial. In this work the problem is tackled with experiments of solidification at various rates and with different equipments: conventional cooling, copper moulding and melt spinning, with both Fe-C-Si alloys made from pure elements and industrial cast irons. Bulk ingots, cone shaped samples (with diameter ranging from 1 to 6 mm) and ribbons are obtained respectively. Experiments are made using either no additives or inoculants and spheroidizing agents. The microstructures are reported as a function of specimen shape and size (i. e. cooling rate). Additionally, alloys buttons are made by arc melting in clean atmosphere. The set of results provide information on metastable microstructures and the extent of undercooling of the alloys showing graphite shapes which can be related to the level of contaminants in the melt. Unusual microstructures were obtained which are explained in terms of ferrite-cementite metastable eutectic. Computer calculation of phase diagrams helps in explaining phase selection.

Logarithmic Relaxation of Resistance in Time of Annealed and Plastically Deformed Au 80 Fe 20

Rapidly solidified Au 80 Fe 20 ribbons were prepared either by melt spinning or by solidstate quenching of a homogenised master alloy. The as-quenched sample displays a paramagnetic behavior indicating a perfect solid solution of Fe in the Au matrix. Subsequent anneals have been performed to induce the precipitation of Fe particles. X-ray diffraction technique have been exploited to determine the alloy microstructure. The structural stability have been studied by measuring electrical properties in isothermal and tempering condition. The variation of magnetisation and electrical resistance have been measured after submitting the samples to plastic deformation. A logarithmic relaxation of the electrical resistance is observed in all studied samples after deformation. Magnetic hysteresis loops have been measured in as-quenched and annealed samples at different temperatures. X-ray diffraction and magnetic measurements indicate that thermal treatments have been successful in inducing the precipitation of both bcc and fcc Fe clusters.