Transmission electron microscopy investigation of boundaries between amorphous “grains” in Ni₅₀Nb₂₀Y₃₀ alloy

Some of the authors of this article recently demonstrated that severe plastic deformation permits the production of metallic alloys containing two coexisting amorphous phases from the crystalline multiphase Ni₆₀Nb₁₈Y₂₂ alloy. The aim herein is to provide a detailed description of the microstructure of this system of coexisting amorphous phases by transmission electron microscopy (TEM) analysis. As-cast Ni₅₀Nb₂₀Y₃₀ alloy was coarse grained and contained mainly NiY phase (grain size 25 μm) as well as NbNi₃, Ni₂Y, Ni₇Y₂, and Ni₃Y phases (grain size 3–5 μm). High-pressure torsion (4 GPa, 10 torsions) completely changed the structure. After severe plastic deformation (SPD), the sample contained two glassy phases and two other nanocrystalline NiY and Nb₁₅Ni₂ phases (grain size about 20 nm). Bright-field TEM micrographs showed fine, 5–10 nm, round bubbles of bright Y-rich amorphous phase embedded in darker Nb-rich “grains.” In turn, the dark Nb-rich grains were separated by layers of bright Y-rich amorphous phase a few nanometers thick. This stricture permits one to speak about mutual wetting of “grain boundaries” in both amorphous phases.