Chunjiang Wang

Fabrication of MnBi/Bi composite using dilute master alloy solidification under high magnetic field gradients

By means of the application of a high gradient magnetic field during the solidification process of a dilute Bi–Mn master alloy, bulk MnBi/Bi composite magnets have been prepared. The observation on the microstructure indicates that MnBi grains aggregate at a specified end of the specimen with the local volume fraction of the MnBi phase above 40% and a high degree of orientation. This method avoids other unfavourable phases occurring in the solidified microstructure. The easy magnetization c axis of MnBi grains parallel to the imposed field is attributed to the rotation of magnetized MnBi grains under the effect of a high magnetic field, and the agglomeration of grains at the specified end results from the magnetization force induced by the field gradient. Simultaneously, the mutual action among magnetized grains makes the grain spacing uniform. Uniform orientation and the high degree of crystal ensure the improvement in the magnetic property of composite materials.

Mechanism of formation of aligned two-phase microstructure in a Fe-0.25wt%C alloy under high magnetic field gradients

The effect of a magnetic force on creating an aligned two-phase microstructure in a Fe-0.25wt%C alloy under magnetic field gradients has been investigated. Through the changes in the heating temperature, both dipolar interactions and magnetic forces work together during the austenite to ferrite transformation. The results showed that the aligned two-phase microstructure is not observed under the influence of the magnetic force alone. The ferrite grains are elongated due to dipole interactions at the early stages of their nucleation and growth and then the magnetic force turns the elongated ferrite grains, whose major axis is not parallel to the direction of magnetic force, to the direction of the field in the presence of magnetic field gradients.