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Nucleation and magnetism of supercooled Co-B metallic liquid under high magnetic field

Abstract : In the present work, a thorough investigation has been conducted on the Co-B alloys in terms of the microstructure evolution during non-equilibrium solidification with/without magnetic field and the temperature induced liquid-liquid structure transition. In this work, a Co-20at.%B hypereutectic alloys was undercooled by the melt fluxing technique and the microstructure was characterized by the back-scattering diffraction technique. A transition from hypereutectic to hypoeutectic was found at a critical undercooling of ∆T=119 K. When ∆T<119 K, a primary directional dendritic β-Co3B phase surrounded by the regular of α-Co+β-Co3B lamellar eutectics was found. When ∆T>119 K, the above hypereutectic microstructure changes into hypoeutectic structure with the α-Co phase as the primary phase. According to dendrite growth model, the transition from hypereutectic to hypoeutectic can be ascribed to the higher growth velocity of the α-Co phase than the β-Co3B phase, i.e., the growth-controlled mechanism. The current work shows also that there is a coupled zone skewed to the β-Co3B phase in the Co-B alloys system. Cyclic superheating and cooling were carried out for the undercooled hypereutectic Co-20at.%B, eutectic Co-18.5at.%B and hypoeutectic Co-17at.%B alloys. For each alloy, there is a critical overheating temperature Tc0 at which there is a sharp increase of the mean undercooling, i.e., the mean undercooling is about 80 °C when the overheating temperature is below Tc0, whereas the mean undercooling is about 200 °C when the overheating temperature is above Tc0. DSC measurements show that there is a thermal absorption peak in the heating process, the peak temperature of which is nearly equal to the critical overheating temperature, indicating that the temperature induced liquid-liquid structure transition does occur and should relate highly to nucleation in the undercooled Co-B eutectic melts. The effect of the liquid-liquid structure transition on nucleation was interpreted by the recent nucleation theory that considers the structures of overheated melts, and the composition-dependent overheating temperature was ascribed to the change of local favored structures. The present work provides further evidences for the liquid-liquid structure transition and is helpful for understanding solidification in undercooled melts. By in situ measuring the magnetization, the temperature induced liquid-liquid structure transition was further investigated. A magnetization anomaly in term of the non-Curie-Weiss temperature dependence of magnetization was observed in the overheated state, demonstrating a temperature induced liquid-liquid structure transition. This anomalous behavior was found to be a universal formula for the Co-B binary alloy system. A transition point (T0), two different Curie constants and two paramagnetic Curie temperatures (θ(LI), θ(LII)) corresponding to two distinct kinds of liquids (i.e., high-magnetization liquid I and low-magnetization liquid II) are determined. The Curie constant of liquid II was found much higher, which is attributed to the survived covalent bond below T0. The effects of magnetic field intensity on the liquid-liquid structure transition and paramagnetic Curie temperatures are studied. T0 and θ(LII) are found not sensitive to the field intensity, whereas, θ(LI) shifts to lower temperatures with the increasing magnetic field intensity. With the increased concentration of Co, T0, θ(LI) and θ(LII) shift to higher temperatures and the Curie constants for the liquid I and liquid II decrease. Based on the location of T0, a guideline was drawn above the liquidus in the Co-B phase diagram, which could provide a significant guidance to the practical melt treatment. Under an imposed magnetic field, a morphological alignment was found for the primary α-Co phase with its primary dendrite trunk or long axis paralleling to the direction of magnetic field. The primary α-Co phases are rod-like or spherical at relatively high undercooling, and the application of magnetic field is more conducive to obtain such kind of α-Co phases. The magnetic energy, magnetic torque and time required for rotation were analyzed theoretically to evaluate the magnetic alignment and alignment mechanisms. The dipolar forces between particles were calculated, based on which the phenomenon that the primary α-Co particles self-organize as chain-like stacking was described.
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https://tel.archives-ouvertes.fr/tel-02380643
Contributor : Yixuan He <>
Submitted on : Tuesday, November 26, 2019 - 12:32:19 PM
Last modification on : Wednesday, October 14, 2020 - 4:19:26 AM

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Yixuan He. Nucleation and magnetism of supercooled Co-B metallic liquid under high magnetic field. Physics [physics]. UGA (Université Grenoble Alpes), 2019. English. ⟨tel-02380643⟩

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