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

Abstract

• A weak magnetic field can enhance ductile-mode cutting of CaF 2 single crystal. • Magnetic field direction shows anisotropy in affecting ductile–brittle transition. • A coupled analysis of magneto-plasticity and crystal plasticity is developed. • Dislocation behaviour and fracture toughness are revealed under magnetic field. • DFT computation corroborates the magnetic-induced dislocation depinning theory. 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 (CaF 2 ) to the magneto-plastic effect. The influence of magneto-plasticity on CaF 2 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 CaF 2 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.