Abstract
Compositionally complex alloys (CCAs) have emerged as a class of alloys which are characterized as containing multiple elements at meaningful concentrations. The development of alloys containing such a diverse elemental composition shows promise for the targeted co-optimization of multiple properties simultaneously, where different elements influence various properties, often synergistically. In particular, there is interest in the design of low-cost, lightweight, and corrosion-resistant alloys for marine applications. Promising approaches have been undertaken, especially surrounding the usage of “secondary passivators” and/or “lightweighting elements” - species which simultaneously lower alloy density and improve Cr-based passivity. While secondary passivation and lightweighting are third element effects primarily targeted herein, other reported effects, such as short-range ordering or activity enhancement, are also discussed according to relevant literature findings. Alongside improving performance, diverse CCA compositional spaces require careful consideration of phase behavior. Such consideration is infrequent in the current literature base, commonly confounding conclusions made regarding compositional effects on passivity. Such confounding effects represent potent research opportunities that are largely unexplored, particularly surrounding the compositional optimization of the aforementioned triad of desirable properties while being explicitly mindful about phase stability. By controlling for single-phase behavior and designing consistent experimental alloy sets, previously unknown elemental roles and beneficial aspects were elucidated for solid-solution FCC and BCC alloys within the FeCoNiCrAlSiTi elemental palette.
Therefore, this thesis will approach the discovery and design of corrosion-resistant compositionally complex alloys in two thrusts: (1) the delineation of phase-, density-, and passivity-based roles for elements in the single-phase FeCoNiCrAlSiTi system, and (2) the development of a data-driven, phase-aware workflow for the design of both single-phase and duplex (FCC+BCC) alloys with low density, low cost, and enhanced corrosion resistance.
For Thrust 1, specific LWE effects on density, passivity, and localized breakdown were ascertained through the design of phase-consistent ternary ([Fe, Co, Ni]90-xCr10[Al, Si, Ti]x) and CCA (Ni43Fe37Cr10[Al, Si, Ti]10) solid solution alloys. Results indicate that Ti provides the greatest per-at.% benefit for passivity, self-healing, and localized corrosion resistance among LWE, and Al provides the greatest density reduction among LWE. In particular, alloys containing high (≥5 at.%) Ti were found to achieve Epit values superior to that of SS304, despite the novel CCAs containing half the Cr content of the commercial alloy. The utility of such phase- and compositionally-controlled alloy sets was further demonstrated through the determination of novel experimentally-determined compositional rules for corrosion resistant alloy design.
For Thrust 2, a machine learning model predicting a self-healing metric (the h-value) was first developed, then utilized in a novel CALPHAD/ML design workflow, yielding two novel single-phase CCAs which offer significant density reduction and corrosion resistance improvements relative to SS304. Finally, the validated CALPHAD/ML workflow was adapted to design duplex alloys, where phases were designed to avoid preferential attack. Following the targeted design of one duplex alloy, a “tie-line” approach was developed and validated through the fabrication of a set of alloys containing 0% to 100% FCC phase, where the FCC and BCC per-phase compositions were maintained. Such phase-varying, phase composition-invariant alloys allowed for the decoupling of microstructural and compositional effects on localized and passive state corrosion, where compositional effects were demonstrated to dominate for passive state corrosion characteristics (e.g., icrit) and interphase boundary area prevalence was closely tied to localized corrosion characteristics (e.g., Epit). The duplex alloys are noted to achieve corrosion resistances broadly improved relative to SS304 across environments while having substantially lower densities and Cr amounts.
Overall, this thesis presents a robust, phase-aware experimental approach for the specific delineation of elemental effects and compositional guidelines for enhanced resistance to uniform and localized corrosion within CCAs. Such powerful observed compositional effects enabled the development of a novel CALPHAD/ML approach for the targeted design of single-phase, low-density, corrosion-resistant CCAs within the FeCoNiCrAlSiTi elemental palette. Furthermore, a novel analytical expression relating the h-value to a closed-form equation using compositional features, akin to the well-known PREN, was developed for CCA design using symbolic regression. This work is intended to be extendable to other alloy systems and desired phases, with the overall design approach proven to be adaptable to multiphase alloys in the final chapter.
