Seeing the unseen: uncover the bulk heterogeneous deformation processes in metallic glasses through surface temperature decoding

Deformation processes in various materials are inhomogeneous in space and jerky in time, with the shear banding in bulk metallic glasses (BMGs) as a quintessential example, but there is a lack of in situ, nondestructive observations of such processes on the appropriate spatio-temporal scales. This work solves this long-lasting difficulty by the integration of in situ infrared (IR) measurements and innovative signal processing algorithms. A spatio-temporal unmixing method is developed to identify the discrete surface ‘hot-spots’ that are responsible for the initiation and propagation of macroscopic shear bands during the serrated flow. The use of a thermal-electric analogy further identifies depths of these hot-spots, whose magnitudes and locations evolve as the successive shearing process repeats on the major shear band. Seeing the previously ‘unseen’ localized heat sources and their 3D evolution patterns, both in situ and inside the bulk, reveals for the first time how the coupled structural/thermal softening mechanisms govern the heterogeneous deformation processes in BMGs.