Hybrid and ultrafine-grained materials produced by high pressure torsion extrusion

The development of advanced structural materials by creating designed structures and architectures is one of the main areas of scientific work in the field of modern materials science.
High pressure torsion extrusion (HPTE), as one of the methods of severe plastic deformation, was used to process ultrafine-grained (UFG) samples of pure copper and hybrid samples. HPTE-processing of hybrid samples with straight Fe wires in the straighty configuration embedded in the copper matrix leads to the creation of a helical architecture of the wires and a significant change of the wire cross section shape. The helical configuration (number of loops and pitch) can be easily varied by changing the HPTE-processing parameters (extrusion and rotation rate).
Results of finite element modeling, experimentally validated using copper samples with aluminium markers, were analyzed to gain a profound understanding of the influence of processing temperature and properties of the processed materials on the strain distribution in the bulk billet processed by HPTE. The calculations were performed for pure copper for HPTE regimes with resulting strains in a range between 0.3 and 12.0 at deformation temperatures of 25 and 100°C. ... mehrIt was established that the accumulated strain in HPTE can be as high as ~4 even in the middle of the round bar billet, which shows the high efficiency of HPTE to obtain severe plastic deformation. By the comparison of the calculated strain distributions with experimentally measured ones in copper samples the spreading of the deformation zone along the height of the billet, caused by its sliding in the die could be revealed. It was established that the sliding increases with increasing deformation temperature and in creating accumulated strain. X-ray tomography was used to visualize the change in the shape of wire markers inserted in the billets prior to HPTE processing.
The potential of the HPTE method for obtaining high strength in bulk structural materials has been demonstrated. The helical architecture of iron reinforcements leads to a substantial enhancement in the plasticity of copper processed through severe plastic deformation (SPD). In particular, an extension of the stage of uniform elongation from ~1 to ~4% has been obtained.
The microstructure of the HPTE-processed Cu shows a gradient structure, consisting of fine grains in the central area and of ultrafine grains on the edge and in the middle-radius zone. A detailed analysis of the tensile characteristics for the samples with gradient structure showed that the strength of copper after HPTE exhibiting a structure is similar to that of copper after other SPD techniques that result in a homogeneous UFG structure. The analysis of the contributions of various strengthening mechanisms revealed that the main strengthening factor in the HPTE-processed copper arises from high and low-angle grain boundaries, which act as effective obstacles to dislocation motion, as discussed by the Hall-Petch relationship.