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 mu m) as well as NbNi3, Ni2Y, Ni7Y2, and Ni3Y phases (grain size 3-5 mu m). 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.