Enhanced thermal stability of nanocrystalline Cu composites processed by high-pressure torsion: The pinning effect of Al₂O₃, GO, and rGO/Al₂O₃ nanoparticles.

Abstract

Metal matrix composites with improved mechanical properties and thermal stability were produced using mechanical milling, spark plasma sintering (SPS) and high-pressure torsion (HPT). Three types of reinforcing particles were used GO, Al2O3 and rGO/Al2O3. All of the produced composites exhibit higher hardness and tensile strength than pure coper, reaching values of 250 Hv for Cu-GO, 240 Hv for Cu- Al2O3, 210 Hv for Cu- rGO/Al2O3 and 185 Hv for Cu after HPT. STEM analyses reveal that the HPT significantly refines the grain size of pure copper to ~210 nm, and even more in the Cu-based composites achieving grain sizes as small as ~55-75 nm. Pure Cu after HPT recrystalizes after annealing at 573 K. The Cu-Al₂O₃ composite demonstrated the best thermal stability with a hardness after annealing at 773 K of 220 Hv and a grain size of ~100 nm. The composite of Cu-GO after annealing at 773 K showed slight grain growth up to ~150 nm. The composite Cu-GO/Al2O3 exhibited improved microhardness and tensile strength up to 673 K and annealing of this composite at 773 K led to a bimodal microstructure. All of the composites annealed at 773 K had a hardness above 180 Hv.