Microstructure and Mechanical Properties of Co21Cr22Cu22Fe21Ni14 Processed by High Pressure Torsion and Annealing

The strengthening mechanisms of Co21Cr22Cu22Fe21Ni14 multiple-principal element alloy processed by high pressure torsion (HPT) and annealing were examined. Two face-centered cubic (FCC) phases were observed in the as-cast alloy; one was a Cu-rich phase and the other was a Cu-lean one. In the HPT process, the microhardness increased from 190 HV to 470 HV at a strain of 157 due to strain hardening and grain refinement hardening. X-ray diffraction showed that the lattice parameters of the two FCC phases became closer to each other at higher HPT strain, indicating the alloying of Cu into the Cu-lean matrix. The HPT processed specimens were annealed at 500A degrees C, 550A degrees C, 600A degrees C, and 650A degrees C. The microhardness increased to 540 HV after annealing at temperatures lower than 650A degrees C, whereas it decreased when the specimen was annealed at 650A degrees C. The mean grain size of the specimens annealed at temperatures lower than 650A degrees C was much smaller than 100 nm, and Cu-rich clusters with sizes ranging from 2 nm to 32 nm were distributed homogeneously. The reasons for the formation of the Cu-rich nano-clusters were discussed from a perspective of the positive mixing enthalpy of Cu in the alloy and thermalenergy for Cu diffusion at a given temperature. The dissolution and partitioning of two FCC phases played a key role in strengthening the Co21Cr22Cu22Fe21Ni14 system.