Tailoring microstructure, residual stress evolution, and dynamic mechanical response of plasma-nitrided FeCoCrNiAl _x HEA claddings on AISI 4140 steel through Al content control

In this study, FeCoCrNiAl _x high-entropy alloy (HEA) cladding layers with controlled Al contents were deposited on AISI 4140 steel to regulate nitrogen-induced phase evolution and the resulting mechanical performance during plasma nitriding. The results demonstrate that Al addition serves as an effective microstructural regulator, modifying the nitride chemistry, spatial distribution, residual stress state, and deformation behavior of the nitrided surface layers. After plasma nitriding, all HEA cladding layers retain an FCC solid-solution structure, while increasing Al content promotes the evolution from localized Cr-containing nitride formation toward a refined AlN/CrN-containing heterogeneous architecture accompanied by suppressed Fe ₄ N formation. This Al-induced phase evolution progressively enhances the strengthening capability of the nitrided layers, increasing the compressive residual stress from −330 MPa in the Al-free coating to a maximum of −710 MPa at intermediate Al addition, while the dynamic yield stress increases from 980 MPa to 1420 MPa with increasing Al content. However, further Al enrichment increases the fraction of rigid nitride-containing regions and reduces deformation compatibility within the nitrided layer, resulting in localized strain concentration and decreased impact damage tolerance. Among the investigated compositions, the FeCoCrNiAl ₀ . ₅ HEA cladding layer achieves the most favorable balance between strengthening and deformation accommodation, exhibiting the highest Charpy impact energy of 61 J. This superior performance originates from the formation of a refined and well-distributed AlN/CrN-containing structure, which provides effective load-bearing capability while preserving sufficient plastic accommodation for energy dissipation. These findings demonstrate that the mechanical response of plasma-nitrided HEA-coated steels is governed not simply by maximizing nitride formation or hardness, but by tailoring the Al-dependent relationship among nitride morphology, residual stress evolution, and deformation compatibility.