The Coarsening Behavior Of Particles In A Liquid-solid System Of Mn-Sb Alloy In High Magnetic Fields

Coarsening process is the core issues of materials processing like semi-solid processing, liquid synthesis, immiscible alloy solidification, precipitation strengthening, liquid-state sintering, inclusions control and so on. A good understanding of the basic laws of particles coarsening process is beneficial of controlling particles coarsening processing and changing the micro structure of materials to improve its performance effectively. A high magnetic field is applied to the a liquid/solid binary system in semi-solid state during isothermal annealing in this paper, hoping to control the coarsening behavior of particles in the liquid phase through a high magnetic field. In this study, the experiment of isothermal annealing in semi-solid state in high magnetic fields of hypoeutectic Mn-Sb alloy were carried out. The microstructure evolution of the alloy and particles size and distribution of MnSb were investigated. the action mechanism of high magnetic fields was analysed.The main findings are as follows:(1) With the extension of the holding time, MnSb dendrites in the alloy specimens after isothermal annealing in semi-solid state at OT and 1T experienced fragmentation, globalization and the coarsening process and formed the microstructure in which MnSb particles and MnSb/Sb eutectic structure coexisted. A special microstructure in which MnSb particles, the primary Sb and eutectic structure coexisted were formed in the alloy after quenching of isothermal annealing in semi-solid state at the magnetic field of 4,8,11.5T. With the extension of the holding time, the microstructure transformed into a distribution of MnSb particles in MnSb/Sb eutectic matrix.(2) The special microstructure appeared after isothermal annealing in semi-solid state is due to convection suppression caused by Lorentz force and the dendrite arm. Besides, the solute accumulation of Mn caused by the local magnetic field gradients induced by the magnetized MnSb particles is another reason for the appearance of this structure.(3) The coarsening kinetics of MnSb primary phases corresponds to Y " ∝ t. The average size of MnSb particles at the magnetic field of OT and 1T increases with annealing time t as t1/3, as predicted by LSW theory. The size distribution of MnSb particles under this condition resemble the steady-state distribution of LSW theory. The particles coarsening at the magnetic field of OT and 1T is mainly diffusion controlled. The average size of particles at the magnetic field of 11.5T increases with annealing time t as t1/4. Particle coarsening rate increased.The size distributions of the MnSb particles at 11.5T deviates from the LSW theory.(4) The contact and mergence caused by the magnetic interaction between the particles and the change of Mn solute concentration caused by local magnetic field gradients induced by the magnetized MnSb particles is the main reasons for the deviation of particles size distribution from LSW theory.