Effect of a weak magnetic field on ductile-brittle transition in micro-cutting of single-crystal calcium fluoride

TL;DR

This study demonstrates that a weak magnetic field can enhance ductile-mode micro-cutting of brittle single-crystal calcium fluoride by inducing magneto-plasticity, reducing surface damage, and increasing the ductile-brittle transition, with anisotropic effects modeled analytically.

Contribution

It provides experimental evidence and an analytical model for the anisotropic magneto-plastic effect on micro-cutting of CaF2, revealing potential for magnetic-assisted machining of non-magnetic materials.

Findings

Magnetic field weakens surface pile-up and suppresses crack propagation.

Magneto-plasticity increases ductile-brittle transition from 512 nm to 664-806 nm.

Subsurface damage layer becomes thinner under magnetic influence.

Abstract

Magneto-plasticity occurs when a weak magnetic field alters material plasticity and offers a viable solution to enhance ductile-mode cutting of brittle materials. This study demonstrates the susceptibility of non-magnetic single-crystal calcium fluoride (CaF2) to the magneto-plastic effect. The influence of magneto-plasticity on CaF2 was confirmed in micro-deformation tests under a weak magnetic field of 20 mT. The surface pile-up effect was weakened by 10-15 nm along with an enlarged plastic zone and suppressed crack propagation under the influence of the magnetic field. Micro-cutting tests along different crystal orientations on the (111) plane of CaF2 revealed an increase in the ductile-brittle transition of the machined surface with the aid of magneto-plasticity where the largest increase in ductile-brittle transition occurred along the [11-2] orientation from 512 nm to a range of 664-806 nm. Meanwhile, the subsurface damage layer was concurrently thinner under magnetic influence. An anisotropic influence of the magnetic field relative to the single-crystal orientation and the cutting direction was also observed. An analytical model was derived to determine an orientation factor M that successfully describes the anisotropy while considering the single-crystal dislocation behaviour, material fracture toughness, and the orientation of the magnetic field. Previously suggested theoretical mechanism of magneto-plasticity via formation of non-singlet electronic states in defected configurations was confirmed with density functional theory calculations. The successful findings on the influence of a weak magnetic field on plasticity present an opportunity for the adoption of magnetic-assisted micro-cutting of non-magnetic materials.