Ultrafine grained plates of Al-Mg-Si alloy obtained by Incremental Equal Channel Angular Pressing : microstructure and mechanical properties

In this study, an Al-Mg-Si alloy was processed using via Incremental Equal Channel Angular Pressing (I-ECAP) in order to obtain homogenous, ultrafine grained plates with low anisotropy of the mechanical properties. This was the first attempt to process an Al-Mg-Si alloy using this technique. Samples in the form of 3 mm-thick square plates were subjected to I-ECAP with the 90˚ rotation around the axis normal to the surface of the plate between passes. Samples were investigated first in their initial state, then after a single pass of I-ECAP and finally after four such passes. Analyses of the microstructure and mechanical properties demonstrated that the I-ECAP method can be successfully applied in Al-Mg-Si alloys. The average grain size decreased from 15 - 19 µm in the initial state to below 1 µm after four I-ECAP passes. The fraction of high angle grain boundaries in the sample subjected to four I-ECAP passes lay within 53-57 % depending on the examined plane. The mechanism of grain refinement in Al-Mg-Si alloy was found to be distinctly different from that in pure aluminium with the grain rotation being more prominent than the grain subdivision, which was attributed to lower stacking fault energy and the reduced mobility of dislocations in the alloy. The ultimate tensile strength increased more than twice, whereas the yield strength - more than threefold. Additionally, the plates processed by I-ECAP exhibited low anisotropy of mechanical properties (in plane and across the thickness) in comparison to other SPD processing methods, which makes them attractive for further processing and applications

Microstructural evolution of cryomilled Ti/Al mixture during high-pressure torsion

To provide insight into the influence of the length scale on the kinetics of phase evolution during severe plastic deformation, we studied the microstructure evolution of cryomilled Al and Ti mixture, which is further subjected to high-pressure torsion (HPT). The cryomilled microstructure consisted of elemental Al and Ti, and the subsequent HPT deformation at ambient temperature led to the solid state formation of Al-rich intermetallics. X-ray diffraction peaks originating from TiAl2 and TiAl3 were observed after one revolution of HPT, suggesting a shear strain-assisted formation of the intermetallics. A high resolution transmission electron microscope confirmed the formation of TiAl2 following HPT for one revolution. Further HPT straining led to microstructure refinement and a mixing of the Ti and Al, as well as of any phases formed initially. The solid state formation of the intermetallics and the overall evolution of the microstructure are discussed based on the generation of a high density of lattice defects that evolve under the strain conditions present during HP