Comprehensive review of Co-Cr-Ni-Fe high entropy alloys: fundamentals, properties, and recent advancements

Abstract Co–Cr–Ni–Fe high-entropy and complex concentrated alloys (HEAs/CCAs) constitute a versatile materials platform offering a unique combination of compositional tunability, mechanical robustness and potential environmental resilience. This review critically consolidates recent progress in alloy design, processing, microstructural engineering and characterization for Co–Cr–Ni–Fe systems, with particular emphasis on powder-metallurgy (mechanical alloying, sintering, spark-plasma sintering) and additive-manufacturing routes. These techniques enable refined grain structures, controlled precipitation and near-homogeneous chemistries that enhance strength and hardness, yet they introduce processing challenges notably oxygen uptake, residual porosity and process-induced anisotropy that must be mitigated to preserve ductility and long-term stability. Mechanistic sections synthesise the roles of stacking-fault energy, twinning/TRIP, precipitation (L1₂, D0₂₂) and carbide formation in setting strength–ductility trade-offs and delineate the compositional and thermal conditions that promote deleterious σ/TCP phase formation. The review further highlights the imperative for statistically representative, multiscale characterization (TEM, APT, EBSD, X-ray tomography) and the integration of predictive tools (CALPHAD, first-principles calculations and data-driven screening) to accelerate composition–processing–microstructure–property linkage. Remaining barriers to application include scalable, reproducible processing protocols that retain engineered microstructures, comprehensive service-relevant testing (hydrogen embrittlement, oxidation, creep, fatigue), and rigorous techno-economic and life-cycle assessments. To expedite industrial translation, the field should prioritise coordinated alloy design, stringent powder and atmosphere control, optimized consolidation and AM parameter windows, and interlaboratory benchmarking. Collectively, these efforts will enable the deliberate development of Co–Cr–Ni–Fe alloys suited to demanding structural and energy applications. Unlike prior descriptive reviews, this work critically interrogates prevailing assumptions in high-entropy alloy research, particularly the validity of the so-called ‘core effects’, the limits of entropy-driven stabilization, and the role of local chemical ordering in phase selection. A quantitative meta-analysis of processing routes is incorporated to establish statistically meaningful correlations between synthesis pathways and resulting microstructures and properties. Furthermore, emerging directions in hybrid alloy design and composite architectures based on high-entropy concepts are evaluated to identify pathways beyond conventional single-phase systems. This review therefore provides not only a consolidation of existing knowledge but also a critical framework to guide future alloy development and industrial translation.