Atomic insights into the effects of Al element on the nanoindentation behavior of single-crystal FeNiCoCr-based multicomponent alloys

Abstract

The effects of added Al element on the nanoindentation behavior of single-crystal FeNiCoCr-based multicomponent alloys (MCAs) loaded along different crystal orientations, were investigated via using molecular dynamic (MD) simulation. It is found that the added Al with a composition from ∼2.44 to ∼11.1 at. percentage, decreases the stacking fault energy (SFE) and enhances the lattice distortion of the face-centered cubic (FCC) alloys, accordingly affecting the dislocation-medium plastic behavior. Especially, via using the Hertz elastic theory to fit the load-displacement curve, the onset of incipient plasticity triggered by first dislocation nucleation exhibits a significant dependence on the Al concentration. As increasing the Al contents, the critical force for incipient plasticity decreases, mainly owing to the Al-enhanced lattice distortion reducing the atomic displacement necessary for homogeneous dislocation nucleation. Moreover, due to the different Schmidt factors and various slip systems activated for plasticity, dislocations are most difficult to be generated when the (111) lattice plane is indented, but easiest when the (100) plane is indented, leading to the orientation-dependence nano-mechanical response of these MCA. This suggests that whether the nanoindentation is easy in one orientation is trivially connected to the crystalline structure, and mostly has nothing to do with the chemical composition altered by the added Al. Besides, due to the lowered SFE value by the Al added to the alloy, the generation and propagation of extended dislocation structures (composed of Shockley partials and in-between stacking faults) that transfer most plastic strains on deformation, are significantly promoted, consequently increasing the plastic zone size beneath the indenter and lowering the nano-harness of the Al-doped specimens. The simulation results verify the strong composition dependence of mechanical properties for single-crystal MCAs and contribute to the composition design of MCAs for real applications.