Investigation of a novel sub-surface work hardening phenomenon in micro-turning of Zr-based bulk metallic glass

This study investigates the influence of the micro-turning operation on the subsurface deformation-induced transformation. The variations in nanohardness, shear transformation zone (STZ) volume, and free volume in the machined subsurface of the Zr-based bulk metallic glass (BMG) have been characterized. Nanoindentation studies reveal that the machined subsurface is strain hardened, a unique phenomenon that has not been reported in Zr-based BMG machining. The work-hardened zone extends until a depth of similar to 20 mu m from the surface. The quantitative evaluation of the deformation-induced hardening has been done experimentally by nanoindentation followed by the free volume characterization via Differential Scanning Calorimetry (DSC). The Shear Trans-formation Zone (STZ) volume approach has been used to understand the changes in the machined subsurface induced due to microturning. The STZ volume in the machined region is lower than that in the undeformed zone for all the machining conditions investigated in this work. The enthalpy change obtained using DSC analysis for estimating the change in free volume content of the machined regions correlates with the STZ volume.