M. Baricco

Undercooling of NiB and FeB alloys and their metastable phase diagrams

Abstract NiB and Fe Balloy have been undercooled in the cell of a high temperature differential scanning calorimeter (DSC) obtaining thermal data for their melting and solidification. Primary solidification was suppressed up to an undercooling ΔT larger than 200 K: eutectic solidification was suppressed up to ΔT larger than 100 K. It is demonstrated, using the Lipton-Kurz-Trivedi model, that at low undercooling the dendrite growth process is described by the calorimeter trace owing to limited recalescence, whereas at high undercooling the signal is dominated by recalescence effects. The solidification microstructures are discussed in relation to the dendrite growth rate and the mechanism of eutectic freezing. For both NiB and FeB a metastable phase was found to solidify preferentially with respect to the stable eutectic. The metastable phase is Ni2Bh in NiB and very likely Fe3B in FeB. For each phase the enthalpy and entropy of formation are estimated and metastable phase diagrams are drawn. An evaluation of the excess specific heat of the liquid eutectics is made and the results conform to the value expected for glass-forming alloys.

dc Joule heating of amorphous metallic ribbons: Experimental aspects and model

A dc technique of Joule heating has been developed with the purpose of developing unconventional crystallization products in amorphous metallic ribbons. The thermal effects of a continuous current flow on Fe40Ni40B20 and Fe80Si10B10 amorphous ribbon strips are examined. The evolution of the temperature in samples submitted to different values of electrical current is followed by measuring the variations in the electrical resistance of the ribbon strips. A model of sample heating is developed and shown to predict both the steady‐state temperature of the ribbon strips and the law of approach to the steady state. A number of experimental data, obtained from multiple‐thermocouple measurements, x‐ray diffractometry, and differential scanning calorimetry, support the model’s assumptions. The effect of fluctuations in the room‐temperature resistance is discussed. The temperature behavior of samples submitted to free cooling after a Joule‐heating treatment is considered.

Mechanical alloying of the Al–Ti system

Abstract We have investigated the possibility of an amorphization reaction by mechanical alloying for two compositions of the Al–Ti binary system. While the Al-rich composition Al75Ti25 appears to give, after milling for 21 h, an Al(Ti) highly cubic phase, the Ti-rich composition Al25Ti75 does amorphize using various milling conditions. The progress of the amorphization as a function of time of milling was monitored by X-ray diffraction. At first, Al atoms diffuse into the host lattice of hexagonal Ti; subsequently, the milling accumulates a critical density of disorder that causes the Ti(Al) crystalline phase to collapse into an amorphous phase. The formation of amorphous alloys is discussed on the basis of thermodynamic models. The Miedema model is compared with a calculation of phase diagrams approach which has been modified to account for dependence of thermodynamic properties of the liquid upon temperature. T 0 curves are presented, showing a glass-forming range in agreement with experiments.

Study of the brittle behaviour of annealed Fe-6.5 wt%Si ribbons produced by planar flow casting

Abstract Rapidly solidified Fe-6.5wt%Si ribbons are fully ductile in the as-quenched state. However, the annealing process, required for optimization of the magnetic properties, generally leads to final brittle materials. In this article, a study is made of the embrittlement process, through mechanical tests and structural investigations on ribbons subjected to different cooling rates (1 °C min −1 ⩽T⩽ 1500 °C min −1 ). A ductile-to-brittle transition is observed for T∼ 1000 °C min −1 , where long range B 2 ordering takes place and the dislocation character changes from unitary to superlattice. An increase in the work hardening coefficients is correspondingly found, in association with enhanced hindering of the dislocation motion within the grains. The restricted glide and cross-slip capability of the dissociated superdislocations is consequently identified as the chief mechanism responsible for the buildup of internal stresses and eventual brittle fracture of the material.

Kinetics of the amorphous‐to‐nanocrystalline transformation in Fe73.5Cu1Nb3Si13.5B9

Isothermal measurements of the electrical resistance R were performed in samples of the alloy Fe73.5Cu1Nb3Si13.5B9 at temperatures where the nanocrystalline phase develops. At each temperature, nanocrystallization is associated to a very slow decrease of R(t) compared to conventional crystallization processes. X‐ray‐diffraction data were obtained on similar samples annealed at the same temperatures for selected times. The evolving nanocrystalline fraction turns out to be essentially proportional to the variation in the electrical resistance. The form of the kinetic law describing the R(t) decays is obtained by making use of a novel procedure, developed to analyze time‐dependent properties characterized by a complex behavior. Nanocrystallization in the considered alloy appears as initially described by a Johnson–Mehl–Avrami kinetics with exponent n≂4, suggesting that the early stages of the process involve homogeneous nucleation and three‐dimensional grain growth. For longer times, corresponding to the hea...

Soft Nanocrystalline Ferromagnetic-alloys With Improved Ductility Obtained Through Dc Joule Heating of Amorphous Ribbons

Nanocrystalline ferromagnetic materials of the type Fe-Nb-Cu-B-Si obtained by appropriate furnace annealing of amorphous ribbons display excellent soft magnetic properties but are very brittle. Dc Joule heating in vacuo allows nanocrystallization to occur in these systems well before the atomic rearrangements in the amorphous phase have been completed, thus partially avoiding the processes leading to sample embrittlement. In Fe73.5Cu1Nb3Si13.5B9, the average grain size of Joule-heated samples is significantly lower than in the furnace-annealed alloy. Nanocrystalline strips of this material having very high magnetic permeability and better ductility are therefore obtained. The advantages of this rapid heating technique are highlighted.

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.

Surface morphology and reactivity towards CO of MgO particles: FTIR and HRTEM studies

Abstract The morphology and adsorptive behaviours of two different MgO samples are compared. High resolution transmission electron micrographs indicate that MgO powders produced by decomposition of the hydroxide contain extremely rough particles, whereas nicely shaped and smooth cubelets are present in the MgO smoke sample. The proportion of surface sites exposed on (001) planes (five-coordinated) as compared with sites exposed on edges (four-coordinated) and corners (three-coordinated) is much larger on MgO smoke than on MgO ex-hydroxide. CO adsorption shows that at 300 K various families of polymeric anionic species are formed by interaction with basic sites in the lowest coordination (O 2− 3C ) in corner positions. The relative population of the different species depends on the morphology of the particles. CO adsorption at 77 K produces bands due to CO adsorbed on acidic Mg 2+ 5C , Mg 2+ 4C and Mg 2+ 3C sites, whose relative intensities also depend on the morphology of the particles.

Stripe domains and spin reorientation transition in Fe78B13Si9 thin films produced by rf sputtering

Magnetic thin films have been obtained by rf sputtering from an amorphous Fe78B13Si9 target. The samples have been produced with thickness t ranging in the interval 25–1000 nm. Microstructural investigations indicate that the films have different microstructures varying from fully amorphous to partially nanocrystalline with increasing t. Magnetic hysteresis loops have been measured by means of high-sensitive magnetometry. A tailorable spin reorientation transition (SRT) from in-plane single-domain-like to out-of-plane multidomain state with increasing film thickness was observed. Magnetic force microscopy images have been obtained for all samples indicating that for t≤80 nm the magnetization lies in the film plane. For larger thickness, a stripe domain pattern has been observed, indicating the presence of a magnetic anisotropy axis perpendicular to the film plane. In this work, SRT and stripe domain structure have been studied as a function of thickness and sample microstructure.

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.