Tracer Measurements of Atomic Diffusion inside Shear Bands of a Bulk Metallic Glass

Atomic diffusion in deformed ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{40}{\mathrm{P}}_{20}$ bulk metallic glass containing a single family of deformation-induced shear bands was measured by the radiotracer technique. The significant, by orders of magnitude, enhancement of the diffusion rate with respect to that in the untransformed matrix suggests that the shear bands represent short-circuit diffusion paths. Correlations between diffusivity, viscosity, and the excess free volume distribution inside of shear bands are discussed.

Figure 1

Optical image of cross sections of the deformed ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{40}{\mathrm{P}}_{20}$ glasses (a) and the schematics of diffusion fluxes under the present experimental conditions (b). In (a), the images that were obtained for a sample reduced from 1.01 to 0.99 mm (true strain ${\epsilon }_1=0.02$; on the right) and a sample reduced from 1.02 to 0.91 mm (${\epsilon }_2=0.1$; on the left) are combined for a better comparison. In (b), local shearing across a shear band is depicted schematically.

Figure 2

Diffusion profiles of ${}^{110m}\mathrm{Ag}$ (closed symbols) at 473 K in deformed samples compared to diffusion in an undeformed sample. The ${}^{59}\mathrm{Fe}$ tracer (open symbols) was applied after the diffusion anneals and monitors experimental artifacts. Clearly, the diffusion in deformed samples is largely accelerated.

Figure 3

Schematic drawing of constitutive models of shear bands in deformed metallic glasses. The shaded areas represent the fast diffusion pathways and would correspond to a locally increased free volume fraction. Both scenarios—an enhanced diffusion across the whole shear band (a) or an enhanced diffusion limited to the boundary between shear band and amorphous matrix (b)—would be compatible to the measured diffusion profiles.