Multi-principal element alloys are considered a promising option for high-temperature applications due to their potential for excellent oxidation resistance. However, processing conditions play a crucial role in determining the final microstructure and, consequently, the material's properties. This study examines the impact of the microstructure on the mechanical properties and high-temperature oxidation behavior of a non-equiatomic AlCrCoFeNi _2.1 eutectic multi-principal element alloy. The alloy was fabricated using traditional arc melting and a powder metallurgy (PM) technique, specifically, spark plasma sintering (SPS). While the fabrication method had a minimal effect on the primary phases present, it significantly impacted the microstructure morphology. All the samples consisted primarily of a mixture of body-centered cubic and face-centered cubic phases. However, the arc-melted alloy exhibited a typical lamellar structure, whereas the SPS samples displayed globular structures in certain regions. Using a PM method significantly refined the grain size. PM routes enabled the microstructure of the samples to be tailored to improve mechanical properties and oxidation resistance. During oxidation tests, the samples fabricated using PM exhibited more homogeneous oxide layers.

The influence of the consolidation route on the microstructure and high-temperature oxidation behaviour of the non-equiatomic Fe _1.5 Cr ₁ Al _0.75 Mo _0.1 Ti _0.1 Ni _0.25 multi-principal element alloy (MPEA6) was systematically investigated. Samples were fabricated by casting, Spark Plasma Sintering (SPS), and Electric Resistance Sintering (ERS) in order to isolate processing-induced effects while preserving constant composition. Although all routes preserved the BCC–B2 phase constitution, significant differences were observed in grain size, precipitate morphology, and structural homogeneity. Spark Plasma Sintering allowed controlled grain refinement and tuning of B2 precipitate size through adjustment of sintering conditions, whereas ERS produced ultrafine B2 precipitates (30–40 nm) but introduced intrinsic porosity gradients. Hardness measurements revealed a competition between grain boundary strengthening and precipitation strengthening, reflecting the combined influence of grain size and B2 dispersion. Oxidation tests at 800°C for 48 h showed that the SPS-processed condition with refined and homogeneous microstructure developed thinner and more uniform oxide scales compared to the cast alloy. These results demonstrate that oxidation resistance in MPEA6 is governed not only by nominal composition but also by the processing-controlled microstructural state. Processing, therefore, emerges as a key design parameter to tailor both mechanical and environmental performance in Al-containing MPEAs.

Eutectic alloys stand out for their ability to combine high strength and good ductility; a behaviour rooted in their characteristic two-phase microstructure—lamellar or globular—formed at a constant solidification temperature that minimizes segregation and suppresses brittle phases. Their low interfacial energy limits microcrack propagation, while interfacial sliding and dislocation blocking at phase boundaries enhance both strength and toughness. In this work, we investigate how controlled microstructural modifications influence the behaviour of the eutectic high-entropy alloy AlCoCrFeNi2.1, composed of B2 (Ni–Al-rich) and L12 (Co–Fe–Ni-rich) phases. Because these phases exhibit distinct mechanical responses, microconstituent morphology becomes a design parameter. Powder metallurgy is the only processing route capable of providing the level of microstructural control required in this study. It preserves the rapidly solidified eutectic architecture of gas-atomised powders while allowing its intentional transformation during consolidation. Two strategies were implemented: (i) tuning the thermal–electrical input in Spark Plasma Sintering (SPS) and Electrical Resistance Sintering (ERS), and (ii) engineering the particle size distribution, including a bimodal design that enhances surface-energy-driven morphological transitions. SPS enables a gradual lamellar-to-globular evolution, whereas ERS induces ultrafast transformations governed by current intensity. The bimodal PSD significantly accelerates globularisation at lower energy input. EBSD-KAM (Electron Backscatter Diffraction—Kernel Average Misorientation) mapping identifies the lamellar B2 phase as metastable and highly strained, while globular B2 domains show reduced dislocation density. Nanoindentation confirms that intrinsic phase properties remain unchanged, whereas microhardness scales with morphology and lamellar spacing. These results demonstrate that the macroscopic mechanical response is governed by microstructure, establishing powder metallurgy as a uniquely powerful pathway for microstructure-driven design in eutectic HEAs.

Powder metallurgy (PM) provides a distinctive processing framework for high-entropy and complex concentrated alloys by decoupling microstructural evolution from liquid-state solidification and shifting control toward solid-state kinetics. By avoiding melting, PM enables precise regulation of diffusion length scales, defect populations, and thermal exposure, making it a particularly powerful route for accessing refined, heterogeneous, and metastable microstructures that are difficult or impossible to achieve through conventional casting or melt-based additive manufacturing. This chapter presents a mechanistic overview of PM as a microstructural design platform, emphasizing the role of powder feedstock characteristics and consolidation kinetics in defining final material architectures. The influence of different powder production routes – including gas atomization, mechanical alloying, cryomilling, and blended-elemental approaches – is discussed in terms of microstructural inheritance, defect engineering, and chemical heterogeneity. Particular attention is given to consolidation strategies such as spark plasma sintering, hot isostatic pressing, and hybrid routes, highlighting how diffusion-limited processing enables the retention of nanocrystalline grain structures, metastable solid solutions, amorphous-derived states, chemical gradients, and architectured grain morphologies. The chapter further analyses how these PM-enabled microstructures translate into distinctive processing–microstructure–property relationships, including enhanced strength–ductility synergies, architecture-driven strain hardening, improved high-temperature stability, and functional responses governed by local disorder. Finally, current limitations and future research directions in PM processing are outlined, underscoring the importance of integrating powder design with kinetic control to fully exploit the capabilities of PM as an enabling technology for advanced multicomponent alloys.