Depth-dependent phase evolution and multicomponent nitride formation during plasma nitriding of a Zr-Ti-Nb-Al compositionally complex alloy

Plasma nitriding is a promising method for enhancing the surface mechanical properties of compositionally
complex alloys (CCAs), though the mechanisms governing phase evolution and property changes remain unclear. This study investigates the microstructural evolution and mechanical response of a Zr45Ti31.5Nb13.5Al10 alloy after plasma nitriding, combining experimental techniques with thermodynamic calculations. Mechanical properties were assessed via nanoindentation, including hardness, H/E, and H3/E2 ratios. Nitriding produces a chemically and structurally graded surface layer consisting of a near-surface FCC (B1-type) multicomponent nitride, a transition region with mixed FCC/HCP structure, and a BCC substrate. This gradient results from nitrogen diffusion and element-specific nitride formation, as confirmed by EBSD, TEM (SAED, EDS), and XRD analyses. Surface hardness increases more than fivefold, from ~5 GPa to ~28 GPa, due to a combination of nitride formation, solid-solution strengthening, and nanoscale microstructural refinement. The nitriding response is primarily controlled by strong nitride-forming elements (Zr, Ti, Nb), which facilitate uniform nitrogen incorporation and formation of a dense multicomponent nitride layer. Notably, aluminum is incorporated into the nitride lattice without forming a separate AlN phase. These results provide new insights into multicomponent nitride formation and highlight plasma nitriding as an effective strategy for tailoring surface properties of advanced CCAs.