Characterization of Cu-Nb-Cu heterostructure fabricated by high-pressure torsion

High-pressure torsion (HPT) processing disrupts the thermodynamic equilibrium in immiscible systems and often produces nonequilibrium microstructures with unique properties. This study investigates the microstructural evolution and mechanical behaviour of a Cu-Nb immiscible alloy subjected to HPT under 6 GPa compressive stress. The HPT processing was performed on stacked Cu-Nb-Cu layers by up to 200 turns and this produced mechanically alloyed, homogenized disks free of porosity or cavities. Microstructural characterization using X-ray diffraction and scanning electron microscopy, coupled with energy-dispersive X-ray spectroscopy, revealed a stepwise evolution, including the reduction of segregation layers, the formation of nonequilibrium Cu-17 at. %Nb solid solution in the disc processed at 200 HPT turns and an increased Nb insertion into the Cu lattice. Additionally, grain refinement and residual strain increments were observed with increasing torsional turns. Thereafter, the mechanical properties were evaluated using hardness mapping and tensile testing. The material exhibited strain hardening behaviour and achieved an ultimate tensile strength (UTS) exceeding 1.25 GPa. Following post-deformation annealing, the UTS decreased to ~700 MPa due to recrystallization and recovery. These results provide a preliminary understanding of microstructural transformations and their impact on the mechanical properties of immiscible systems subjected to extreme deformation. • HPT process is able to produce bulk disk of immiscible Cu-Nb defect-free mechanically alloyed sample after 200 turns. • Homogeneity of the composition increases by increasing number of turns. Edge is easily homogenized while centre needs more shear strain provided by applying further turns. • Residual strain and crystallite size are increased and reduced respectively by increasing number of turns. • Negative work-hardening behaviour was observed after tensile properties of 200 turns with UTS of 1250 MPa.