M. Palumbo

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.

Thermodynamic Properties of CMSX-4 Superalloy: Results from the ThermoLab Project

This contribution reports the results of calorimetric measurements of the enthalpy of fusion and liquid specific heat carried out in different laboratories as part of the ground campaign of the Thermolab project. Different equipments and calibration methods have been used and critically evaluated. Thermodynamic calculations using the Thermocalc software have been performed and a comparison has been carried out with the experimental results.

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.

Hydrogen absorption and desorption in rapidly solidified Mg- Al alloys

The addition of Al to Mg has been indicated as a suitable way to destabilise the hydride phase, in order to bring the absorption and desorption reactions close to reasonable temperatures and pressure values for hydrogen storage. Rapid solidification is known to refine the microstructure of Mg-Al alloys and it might improve the H2 absorption/desorption kinetics. In this paper, the interaction of H2 with rapidly solidified Mg-Al alloys have been studied for three different composition: Mg38.5Al61.5, Mg69Al31 and Mg72Al28. For Mg72Al28, no significant changes in the microstructure have been obtained by rapid solidification. In Mg69Al31, a significant grain refinement has been observed, whereas, for Mg38.5Al61.5, the formation of a metastable hexagonal phase has been found. In all cases, a disproportionation reaction has been observed after H2 absorption, leading to MgH2. After heating up to 430 °C the hydrogenated samples, a main desorption reaction from MgH2 has been observed, which brings again to the starting phases. Experimental results have been discussed on the basis of a thermodynamic assessment of the Mg-Al-H system.

Al-Rare Earth-Transition Metal Alloys: Fragility of Melts and Resistance to Crystallization

The stability of Al-transition metal (TM)-rare earth (RE) alloys is considered with reference to transport and thermodynamic properties of the melt. The mobility of species or groups of atoms is described above the melting point and in the proximity of the glass transition. Viscosity is analysed at first in the frame of the strong/fragile classification of liquids. Al-based glass-formers are shown to be fragile systems. Evidences that for some molecular and metallic glass-formers the Stokes-Einstein equation does not hold below and above T g are reviewed. The alloys are, however, resistant to crystallization on rapid quenching. Some of them display also peculiar devitrification behaviour, often, but not always, with formation of primary compact nanocrystals. Analysing the transformation paths it can be inferred that mobility is belated by composition gradients in the amorphous matrix. The resistance to crystallization may result from the shape and relative position of free energy curves because crystal and liquid phases are such that the driving force for nucleation of intermetallics does not increase steadily but tends to level off on undercooling.

Thermodynamic and Kinetic Modelling of Primary Crystallisation in Amorphous Alloys

The aim of this paper is to apply thermodynamic and kinetic modelling to primary crystallisation in metallic glasses. The CALPHAD method has been used to obtain an assessment of stable and metastable phase diagrams. Amorphous phases have been described as undercooled liquids. The moving boundary model has been applied to simulate the growth of nuclei into the amorphous matrix. A thermodynamic assessment of stable and metastable phase diagrams is reported for Fe-B and Al-Ni-Ce systems. From calculated thermodynamic functions, driving forces for nucleation have been estimated as a function of temperature and composition. The growth of bcc Fe into a Fe85B15 amorphous alloy has been simulated at 650 K. A composition gradient at the crystal/amorphous interface is obtained at the beginning of the treatment, which progressively overlaps with that of the next crystal. It has been shown that the crystal growth enriches in B the residual amorphous matrix, hindering further nucleation. The growth at 553 K of an fcc-Al nanocrystal into an Al87Ni10Ce3 amorphous alloy has been simulated and the effect of the diffusion coefficients of the various elements on composition profiles has been evidenced.