Cluster structure in amorphous Ti-Ni-Cu alloys subjected to high-pressure torsion deformation

Abstract

The features of the internal structure's evolution of melt-spun (MS) amorphous Ti50Ni25Cu25 alloy samples subjected to high-pressure torsion (HPT) at 20 and 150 °C were revealed by TEM and STEM – in the planar direction and the cross-sectional direction of the HPT-processed disc-shaped samples.In STEM HAADF observations in the planar direction, areas with brightness contrast with a size of 20 nm are revealed. We assume them to be “amorphous nanoclusters”. In dark-field TEM images, in many areas nanocrystals (with sizes about 5 nm) are revealed, resulting from strain-induced nanocrystallization. In STEM HAADF images, there can be seen nanoclusters (with sizes about 20 nm) separated by bright boundaries. Groups of shear bands (SBs) are revealed on the cross-sectional directions on lamellae cut from the HPT-processed samples. Nanocrystals form directly in the shear bands as a result of strain-induced nanocrystallization. Amorphous nanoclusters are also observed in the amorphous phase surrounding the SBs. Formation of the amorphous nanocluster in the areas surrounding the shear bands has been discovered for the first time. Atomic force microscopy (AFM) was used to study the surface morphology of foils prepared by ion polishing from the initial amorphous ribbons and HPT-processed samples. A hollows-like surface morphology in the HPT-processed state is revealed by AFM, which is reflected as nanocluster contrast in TEM images. The hollows-like surface morphology could be explained by the specifics of etching processes in the HPT-processed state. Brittle fracture of melt-spun Ti50Ni25Cu25 in the as-cast state and after HPT processing is determined during tensile tests. However, observable changes in the fracture surfaces correspond to an increased local ductility in samples subjected to HPT, in comparison with the initial state.