Achieving superior superplasticity in CoCrFeNiCu high entropy alloy via friction stir processing with an improved convex tool

The investigation of the superplastic behavior of HEAs is significant since superplastic forming shows a great potential in fabricating complex components of HEAs. In this study, the superplastic behavior of the CoCrFeNiCu HEA fabricated by friction stir processing (FSP) was first time investigated at the temperature range of 900–950 °C and strain rate range of 3 × 10−4–3 × 10−2 s−1. The ultrafine grained structure with 320 nm in grain size and 91% of high angle grain boundaries (HAGBs) was achieved by FSP with the improvement of stirring tool based on the characteristics of HEAs, which exhibited a good thermal stability when annealing at 900 °C. The FSP CoCrFeNiCu HEA exhibited a largest elongation of 620% and a low flow stress of 5 MPa at 950 °C and 3 × 10−3 s−1. The flow stress in this study is the lowest in all the HEAs reported during superplastic deformation, which shows the great potential in the practical superplastic forming application. This is the first time to report the excellent superplasticity of dual-phase FCC HEAs. The superplastic deformation mechanism was mainly dominated by grain boundary sliding (GBS), accommodated by the diffusion of Cu-rich second phase. The failure mechanism was related to the aggregation of cavities. The excellent superplastic property was attributed to the stable equiaxed ultrafine microstructure, the high HAGB ratio, and soft Cu-riched second phase, which promoted the occurrence of GBS and its accommodation behavior. This study provides a way for fabricating superplastic HEAs and theoretical basis for superplastic forming of HEAs.