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.