Microstructure Characterization of Metallic Materials Processed by Equal Channel Angular Pressing (ECAP): An Electron Backscatter Diffraction (EBSD) Analysis

Abstract

This overview article discusses the Equal Channel Angular Pressing (ECAP) processing of different metallic materials. Particular emphasis is given to the microstructural evolution from the coarse grain (CG) to the ultrafine-grained (UFG) states throughout the electron backscattering diffraction (EBSD) technique. Iron-based alloys, such as duplex and 1020 low-carbon steels reached higher hardening with a lower deformation and lower non-ultrafine average grain sizes than the ultrafine pure iron condition due to fast grain fragmentation, i.e., more geometrically necessary dislocation (GND) grouping. Moreover, due to the magnesium adhesion, copper alloys reached superior mechanical properties compared to pure copper even when the initial grain size for as-cast alloys was over 1000 µm. On the other hand, low melting temperature (T_MP) materials processed at 250°C, like the ZK60 magnesium and AA6082 aluminum alloys (i.e., homologous temperatures (T_H) of 0.38T_MP and 0.37T_MP, respectively), showed grain refinement without reaching the ultrafine regime and mechanical softening due to the static and dynamic recrystallization phenomena. CP titanium also displayed heterogeneous grain sizes with average values of above 1 µm after four ECAP passes for temperatures ranging between 150°C and 400°C (T_H between 0.09T_MP − 0.24T_MP). The evolution of the GNDs suggested that the initial deformation stages of severe plastic deformation (SPD) by ECAP produced the most notorious density increments from 10¹² m⁻² to 10¹⁴ m⁻², which level up at high deformations (more than four ECAP passes) around 10¹⁴–10¹⁵ m⁻², explaining the fast and slow grain size reduction rates, respectively. The ECAP processing on different metallic material systems showed a larger grain fragmentation capacity in high melting points and alloyed materials, giving rise to steep yield strength increases and low ductility. The low ductility and grain size saturation correspond to a low capacity to create new grain boundaries manifested by the GNDs saturation in the UFG range.