Research On The Nanoindentation Creep And Plastic Deformation Behavior Of The Single-phase Al_0.3CoCrFeNi High-entropy Alloy

The high-entropy alloys(HEAs) with unique compositions, microstructures, and adjustable properties have attracted increasing attentions to the materials-science community. HEAs have been reported to possess many unique properties, such as outstanding wear resistance, high softening and fracture-toughness resistance, high magnetic saturation, good extrinsic ductility, good fatigue, oxidation, and corrosion resistance. These excellent properties are associated with the four core effects for HEAs summarized by Yeh: high-entropy effects, sluggish diffusion, severe lattice distortion, and cocktail effects. Until now, the research of HEAs has been done in several areas. But the nanoindentation-creep behavior and incipient plasticity of the single-crystal or coarse-grained HEA are seldom reported. In this paper, we use the nanoindentation experiment, which is different from the traditional tensile-test to probe the creep behavior and incipient plasticity of the coarse-grained Al0.3CoCrFeNi HEA on the submicron scale.Nanoindentation-creep behavior were studied on a coarse-grained Al0.3CoCrFeNi HEA with a single face-centered-cubic structure. The effects of grain boundary(GB),indentation size, and loading rate on creep behavior were investigated. The experimental results show that the hardness, creep depth, creep-strain rate, and stress exponent are all dependent on the distances from GB. It is inferred that GBs can affect the deformation behavior of the area at a distance of 5 μm from GB. The creep behavior of the area at a distance of 2 μm from GB is likely dominated by grain-boundary sliding, but the dominant deformation mechanism in the area(> 5 μm) is the dislocation slip due to its large matrix.The indentation creep experiments show a remarkable indentation-size effect in the grain interior at different maximum indentation loads for the Al0.3CoCrFeNi alloy. The dominant creep mechanism is dislocation slip at high indentation loads and self-diffusion at low indentation loads. An obvious loading-rate sensitivity of creep behavior is found at different loading rates in the grain interior of the alloy. High loading rate can lead to a large creep deformation characteristic with a high stress exponent during the holding time.The phenomenon of the displacement burst during the holding time in the grain interior onthe alloy is observed for the first time.The incipient plasticity of a single face-centered cubic Al0.3CoCrFeNi high-entropy alloy was investigated using nanoindentation by measuring the first pop-in event. A large number of independent measurements revealed that the cumulative distribution of the maximum shear stress was strongly dependent on the applied loading rate. The relationship between activation volume and maximum shear stress can be obtained through amendments to the statistical model. The local nanoscale heterogeneities can lead to different dislocation nucleation sites for the incipient plasticity of the alloy. Then, the bimodal distribution of the probability distribution of the maximum shear stress will appear for the alloy.