Thermal stability of nanocrystalline CoCrFeNi multi-principal element alloy: Effect of the degree of severe plastic deformation

The effect of the degree of severe plastic deformation (SPD) on the thermal stability of a nanocrystalline CoCrFeNi multi-principal element alloy was studied. The SPD method of high-pressure torsion (HPT) was utilized to achieve the nanocrystalline microstructure. The structural stability was investigated near the centers and edges of the HPT-processed disks deformed for ½, 1, 5 and 10 turns. For almost all studied samples, two exothermic peaks in the temperature ranges of 600–750 and 750–950 K were observed by differential scanning calorimetry (DSC) between room temperature and 1000 K. The saturation released heat value for the first DSC peak was about 4 J/g that was achieved at the shear strain of ∼200. For the second exothermic peak, the released heat saturated at the shear strain of about 20 with the value of about 6–7 J/g. It was revealed that the first DSC peak is related to the annihilation of dislocations for low degree of deformation. At the same time, for edge parts of the disks processed by one or higher numbers of turns the vacancy annihilation has also a major contribution to the first exothermic peak. The annihilated vacancy concentration estimated from the released heat was between (0.6–0.9) × 10⁻³. The second DSC peak was related to the disappearance of grain boundaries due to recrystallization and annihilation of the remaining dislocations. The HPT-processed CoCrFeNi MPEA samples exhibited very high hardness values between 4000 and 5100 MPa, depending on the number of turns and the location along the disk radius. The hardness decreased only during the second exothermic peak when recrystallization occurred.