Characterization on nanocrystallization and amorphization evolution in Zr–X alloys during ARB process

Abstract

The gradual evolution of the nanocrystallization and amorphization processes in various Zr–X-based binary, ternary, and pentanary alloys during accumulative roll bonding (ARB) are characterized by Scanning and transmission electron microscopy (SEM and TEM), in correlation with X-ray diffraction (XRD) results. It appears that compatible initial foil hardness would be most beneficial to the nanocrystallization and amorphization processes; the influence would overwhelm the atomic size effect. Based on SEM and XRD results, after 40 and 80 cycles, the foil can mostly be thinned to around 0.1–1 μm, the grain size can be refined to 30 ± 20 and 10 ± 5 nm, and the amorphous volume fraction reaches 40 ± 10 and 70 ± 20%, respectively, depending on the relative hardness. The local spatial distributions of the nanocrystalline (nc) and amorphous (amf) phases are seen under TEM to be non-uniform, varying significantly in size and quantity in different regions. The diffraction spots and rings in the TEM diffraction patterns are still originated from the pure elements, meaning that the nanocrystalline phases are those unmixed hard particles left from the previous severe deformation and diffusion processes. A critical size of the nanocrystalline phases around 3 nm is consistently observed in all binary, ternary, and pentanary Zr–X-based alloys, below the critical size a sudden transformation from the nanocrystalline to amorphous state would occur.