Study Of Room Nanoindentation Behavior Of Al_xCoCrFeNi High-entropy Alloys

As a new class of alloy materials,high-entropy alloys?HEAs?generally consist of at least five equiatomic alloying elements with the concentration in the range of 35⁵0 at.%.Compared with traditional metals,HEAs mostly possess many fascinating and excellent mechanical properties,which are often attributed to a high configuration entropy of mixing and solid solution hardening.HEAs are widely utilized in many application fields.For instance,HEAs are often applied as coating materials in the high-temperature environment.Hence,it is very important to understand the micro-mechanism of the plastic behavior in high-entropy alloys.The paper mainly studied the nanoindentation behavior of AlCoCrFeNi high-entropy alloy in room temperature.Nanoindentation tests with continuous stiffness measurement technique were measured to investigate the deformation behavior of AlCoCrFeNi high-entropy alloy under four indentation strain rates.Results showed that the creep behavior exhibited remarkable strain rate dependence.In-situ scanning images showed a conspicuous pile-up around the indents,indicating that an extremely localized plastic deformation occurred during the nanoindentation.Under different strain rates,elastic modulus basically remained unchanged,while the hardness decreased with increasing indentation depth due to the indentation size effect.Furthermore,the modulus and hardness of AlCoCrFeNi HEAs were greater than that of the AlxCoCrFeNi?x = 0.3,0.5?at the strain rate of 0.2s-1.The structure of as-cast Al_0.75 CoCrFeNi HEA was characterized by using X-ray diffraction.The Al_0.75 CoCrFeNi HEA consists of body centered-cubic?BCC?solid solution and face centered-cubic?FCC?solid solution.Nanoindentation tests were utilized to investigate the indentation deformation of the FCC and BCC crystals in the Al_0.75 CoCrFe Ni HEA.Both the contact modulus and the hardness of the FCC and BCC crystals increased with the increase in the indentation loadings and then tended to be constant for large indentationloadings.The modulus of the FCC phase was greater than that of the BCC phase,while the nanohardness of the BCC phase was the greater one.The plastic energy dissipated of the FCC and BCC crystals in the nanoindentation both increased with the indentation loadings.It was found that a power-law relationship can be used to fit the dependence of the energy dissipated in the indentation on the indentation loadings.For the same indentation loading,the plastic energy dissipated of the FCC crystals was greater than that of the BCC phase.The nanoindentation behavior of AlCoCrFe Ni high-entropy alloy was studied by combining the numerical simulation with the reverse analysis method.The finite element analysis software,ABAQUS was used in indentation to determine the plastic properties of the AlCoCrFeNi HEAs by comparing the test data acquired with the Berkvich indenter with the results of numerical simulation.It was found that the unique power-law constitutive equation obtained hereby can reasonably describe the plastic deformation behavior of the AlCoCrFeNi HEAs.