Mechanism of dislocation kinetics under magnetoplastic effect

Abstract

The plasticity of material is associated closely with the movement and proliferation of dislocation. Therefore, in the deformation and plasticity theory the dislocation kinetics is an important topic. In the case of no magnetic field, the conventional dislocation kinetics normally focuses on the dislocation microstructure, nucleation and mobility, and the inherent relationship between electron spin and plasticity is seldom concerned. As a matter of fact, the electron rotation is directionless and unordered in the absence of magnetic field, so the electron behavior will not take an apparent effect on the microstructure and properties of material. Nevertheless, in the presence of magnetic field the case is different. The magnetic field will influence the electron spin and, therefore, atomic rearrangement. The dislocation behavior and plasticity will also be affected by the magnetic field, which is called the magnetoplastic effect. In this paper, on the basis of magnetoplastic effect the dislocation kinetics involving dislocation stress, mobility and others is discussed both qualitatively and quantitatively. It has rarely reported currently in the literature. The pulsed magnetic field is first utilized to process solid nanometer alumina particulates reinforced aluminum matrix composites. The experimental results demonstrate that the dislocation density increases with magnetic induction intensity increasing from zero to 3 T. The phenomenon reveals the characteristic of plastic deformation in a processed sample. The further theoretical analysis displays that the generated magnetic force is not large enough to activate the dislocation movement. The fundamental reason lies in the magnetoplastic effect, that is, the magnetic field brings about the transition of electron spin in the radical pairs between paramagnetic dislocation cores and obstacles. The radical pairs tend to be conversed from the singlet state with high bonding energy to the triplet state with low bonding energy, therefore, the prerequisite energy for dislocation to surmount the obstacles will be lowed and the depinning tendency will be apparent. In a period of dislocation movement, the rate limiting consists in the dislocation stopping at the obstacle; on the contrary, the electron excitation and atomic arrangement governed by the magnetic field take negligible time. Hence, it can be seen that the performance of magnetic field is highly efficient. The critical magnetic induction intensity is calculated to be 3 T. That is, when the intensity is lower than 3 T, the magnetoplastic effect becomes strong with the increase of magnetic induction intensity and action time; when the intensity is higher than 3 T, the effect changes gently. Under this critical magnetic induction intensity, the dislocation velocity is deduced to be on the order of 10-3 m/s. Moreover, the dislocation length will be increased by two orders of magnitude. The displacement of dislocation is proportional to the square of magnetic induction intensity and action time of magnetic field. To sum up, the magnetic field treatment has been proved to be an efficient approach to improve the plasticity of material. The prospective research will focus on the mechanical properties of alloys or composites subjected to magnetic field, together with tensile stress so as to acquire the effect of magnetic field parameters of macro plasticity of materials.