Macro-scale structural homogeneity and mass density of bulk metallic glasses revealed by their rough surfaces and ultra-small angle neutron scattering (KIST-USANS)*

It has been known that bulk metallic glasses (BMGs) are homogeneous at the atomic level. We have questioned whether the homogeneity of metallic glasses confirmed at the atomic can be sustained at a much larger scale. Ultra-small angle neutron scattering (USANS) enables us to measure the structure up to tens of micrometers.
Bulk metallic ribbons have inherent rough surfaces. The non-trivial surface scatterings could mislead USANS data as the homogeneous materials have inner heterogeneity. We demonstrate that the rough surface of melt-spun amorphous ribbons can be utilized to determine their homogeneity and mass density using the USANS. The scattering of the Cu₅₀Zr₅₀ amorphous alloy disappeared under the surface neutron contrast-matched environment, indicating that the scattering originated not from its internal structure but from the surface. This confirms the homogeneity at the atomic level sustains to a micrometer. On the other hand, the crystallized alloy showed a strong scattering under the matching environment due to the structural heterogeneity inside the alloy. This technique can apply to the bulk samples when the transmission is high enough, not causing multiple scattering.