High Throughput Analysis of Alloy Corrosion Across Composition Space: AlxFeyNi1-X-Y (x = 0 → 1, y = 0 → 1-x)

Improving our fundamental understanding of the oxidation of multicomponent alumina-forming alloys is crucial to their ongoing development.  In this work, high-throughput methods were developed to study oxidation of Al_xFe_yNi_1-x-y alloys in dry air at 427 °C using composition spread alloy films (CSAFs) as combinatorial libraries (x = 0 → 1, y = 0 → [1-x]).  CSAFs were deposited under ultra-high vacuum conditions using a rotating shadow mask, electron beam evaporation tool.  The Al_xFe_yNi_1-x-y CSAFs we first analysed using EDX to map their local compositions across the library, then exposed to oxidation conditions, before subsequent analysis using spatially resolved EDX, Raman spectroscopy, and XPS depth profiling.  These allowing mapping of the oxygen uptake (Figure 1), the local phases formed and the distribution of the metal and oxide phases into the depth of the CSAF.  This work used Al_xFe_yNi_1-x-y CSAFs to determine the critical Al concentration, N_Al^*(x, y), for establishment of a passivating Al₂O₃ scale continuously across Al_xFe_yNi_1-x-y composition space (x = 0 → 1, y = 0 → [1-x]) in both dry air and a 10% H₂O/air mixture at 427 °C.  The results divide the Al_xFe_yNi_1-x-y composition space into four regions of phenomenologically distinct oxidation behaviour.  At high an Al fraction, the alloy is passivated by either a surface Al₂O₃ scale or a subsurface Al₂O₃ scale.  The boundary defining N_Al^*(x, y) was determined across the entire continuous Al_xFe_yNi_1-x-y composition space.  The presence of H₂O vapour in an oxidizing environment increases the value of N_Al^* required to establish a passivating Al₂O₃ scale in multicomponent alumina-forming alloys.  The N_Al^*(x, y) in this environment was significantly higher across much of composition space than that measured in dry air.  Physical insights from the observed differences between passivation in dry and humid air have been considered using a modified Wagner-Maak model. 

Figure 1.  Map of oxygen uptake into a Al_xFe_yNi_1-x-y CSAF following exposure to dry air at 427 ^oC for 4 hrs.