Microstructure and Defects Analysis in Solid-State Bonded Cu/Invar/Cu Hybrid Alloy Through High-Pressure Torsion

This study analyzes morphological features and defect microstructure in a Cu/Invar/Cu hybrid, which was solid-state-bonded through high-pressure torsion (HPT) processing under 6 GPa at room temperature for up to 30 turns, using optical microscopy, electron backscatter diffraction (EBSD) and positron annihilation lifetime spectroscopy (PALS). The results reveal a significant transition at the interfaces between Cu and Invar layers, demonstrating the excellent bonding yet limited mixture of this hybrid system even after extensive torsional shearing of HPT, introducing equivalent strain of over 600 by 30 turns. EBSD analysis shows that the Cu layer undergoes rapid saturation of dislocation density even at low equivalent strain, whereas the Invar layer continues to accumulate lattice defects, resulting in distinct strain accommodation mechanisms. The microhardness of 108 HV for Cu and 300 HV for Invar confirms the persistence of microstructural and mechanical inhomogeneities across the multilayer system. PALS reveals the presence of a dominant monovacancy–dislocation complex with 178–183 ps lifetime. Moreover, KAM-EBSD showed a strong influence of the highly accumulated dislocations on the microstructural evolution.