Gradient plastic zone model in equiatomic face-centered cubic alloys

Indentation tests at various depths confirm that the scale factor, f, which was proposed to modify the Nix-Gao model, is governed only by intrinsic properties of materials, but the connection between f and intrinsic properties is still unknown. From this, a connection is established between f and pertinent parameters, such as activation volume, V, and theoretical strength, G. Moreover, considering the interaction between adjacent indentations, we define a critical scale factor f(eff) to describe the strengthening interface, which is found proportional to indentation depth for the given material. It is also found that larger f(eff) is preferred for high-strength high-entropy alloys and/or medium-entropy alloys than pure metals. Combining f and f(eff) of Ni, CoNi, CoCrNi and FeCoCrNi metals and alloys, a model describing dislocation density distribution in indentation plastic zone is established, in which the plastic zone includes three parts, i.e., the indentation zone of equivalent conical indenter, the detectable dislocation enhancement region, and the low dislocation density region. The inconsonant trends of f and f(eff) can be explained by the existing of dislocation saturation zone in the plastic zone. The gradient plastic zone model proposed here graphically depicts the dislocations motion, as well as its reinforcement effect. Furthermore, this model lends credence to modify the framework which describes the mechanical response of materials under nanoindentation.