Understanding the influence of free-volume generation, thermal instabilities and fracture energy on shear localization in micromachining of bulk metallic glass

Abstract

Bulk metallic glasses (BMGs) are a new class of amorphous metallic alloys having enhanced mechanical and tribological properties. However, the chip formation behavior of BMGs is not as well established as that of the crystalline materials, which hinders its prospective applications. In this paper, an orthogonal micromachining experiment is conducted, and chip morphology, force signature, hardness, and enthalpy of the machined surface are analyzed. The chips show segmented morphology with distinct fractures. The chip exhibits primary shear zones along with several secondary shear bands emanating within a single segmentation, which evidenced shear localization. Additionally, the machined surface exhibits softening as compared to the undeformed material. The shear localization, responsible for chip segmentation in BMG machining, is assumed to be driven primarily by free volume and the shear zone temperature. Hence, a localized chip segmentation model has been formulated by incorporating the effects of temperature and free volume generation-induced instabilities. The proposed chip formation model accounts for the contributions of the fracture and the new surface creation energies, in addition to the shear and friction energies used traditionally. The contribution of fracture energy was observed to be significant in the chip formation of BMGs. To differentiate between the relative contributions of temperature-induced instabilities and free volume generation on shear localization, four distinct regimes were identified based on the critical value of free volume flow coefficient (FVFC) and heat flow coefficients (HFC). The findings demonstrate that if the FVFC is greater than the critical value, significant shear localization takes place. Conversely, the shear localization becomes less pronounced if the FVFC is smaller than the critical values. Thus, it can be deduced that temperature has an insignificant role in shear localization, and free volume acts as the main driver.