Research on the Microstructure and Performance of AlCoCrFeNi_2.1 Eutectic High-entropy Alloy Manufactured by Selective Laser Melting

Abstract

The growing need for advanced structural materials with exceptional durability under harsh operating conditions, particularly in fields like aerospace engineering, nuclear power systems, and biomedical implants, has spurred significant research into multi-component high-entropy alloys (HEAs). HEAs exhibit remarkable potential because of their outstanding mechanical and thermal characteristics, including superior tensile strength, hardness, thermal stability, corrosion resistance, and wear resistance. However, conventional production techniques like casting or forging encounter considerable challenges, including complex forming processes, low material efficiency, high costs, and restricted design flexibility. These challenges have significantly constrained the broad implementation and practical utilization of HEAs across engineering fields. As an innovative approach, selective laser melting (SLM), a prominent approach within the metal additive manufacturing (AM) domain, offers a transformative solution. By leveraging computer-aided design (CAD) models, SLM enables producing high-performance materials with unparalleled design freedom, driving innovation across industries. In this sense, the synergy between HEAs and AM represents a cutting-edge frontier in materials engineering, meriting in-depth exploration.

This research focuses on the AlCoCrFeNi_2.1 EHEA system, chosen for investigation due to its great printability and hierarchical microstructure of dual-phase colonies for excellent mechanical properties. EHEAs exhibit resistance to hot and cold cracking due to their isothermal eutectic reaction and narrow solidification range, enabling the formation of fine microstructures. AM demonstrates particular compatibility with eutectic alloy systems while simultaneously enhancing the material's combined strength and ductility characteristics through precise control of microstructural features. The current research aims to systematically study microstructural evolution and performance regulation mechanisms of AM-fabricated EHEAs, thus developing HEA design theories and process optimization methods applicable to AM.

Firstly, our investigation reveals the B2→FCC phase transition phenomenon occurring in the rapidly solidified AlCoCrFeNi_2.1 EHEA produced by SLM. We discover novel nanotwinned (NT) nanoprecipitates and thus develop an ultrastrong EHEA with adequate ductility through the SLM and subsequent post-annealing processes. These NT precipitates, unprecedented in traditionally processed EHEAs, display a face-centered cubic (FCC) structure, featuring the ultrafine twin thickness of approximately 2.4 nm and form from the ordered body-centered cubic phase (B2) during annealing at 600 °C. Our systematic microstructural analysis reveals a sequential two-step precipitation mechanism of NT precipitates. Initially, needle-like hexagonal close-packed phases with dense stacking faults (SFs) precipitate, subsequently transforming into FCC precipitates with an NT structure. Such sequential precipitation mechanisms in EHEAs result from the low stacking fault energy of precipitates and the elevated dislocation density characteristic of additively manufactured specimens. These densely dispersed NT nanoprecipitates enhance mechanical strength by roughly 560 MPa relative to the initial state, ultimately achieving remarkable ultimate tensile strength near 2.2 GPa while maintaining 4% uniform elongation.

Secondly, we clarify the “process–microstructure–property” internal relationship of AlCoCrFeNi_2.1 EHEA produced by SLM. This study presents a comprehensive investigation of how annealing temperature variations influence both microstructural development and mechanical characteristics in AlCoCrFeNi_2.1 EHEA produced by SLM. As elevating annealing temperature, the far-from-equilibrium AlCoCrFeNi_2.1 EHEA with B2-phase dominant gradually transforms into the FCC phase-dominated equilibrium state. Following the initial 960 °C/1 h annealing treatment, a secondary aging process was conducted at 600 °C for 8 h. This two-step heat treatment facilitates the homogeneous distribution of nanoscale L1₂ particles within the FCC matrix, ultimately inducing an exceptional combination of mechanical properties: 1628 MPa ultimate tensile strength coupled with 19.5% elongation, exceeding most prepared by conventional casting with thermo-mechanical treatment. Systematic characterizations reveal that such excellent strength-ductility originates from complementary deformation behaviors. The FCC(L1₂) phase undergoes plastic deformation primarily through dislocation movement SFs along {111} slip planes, while the B2 phase exhibits diverse dislocation substructures and activates dislocation cross-slip along {112} slip planes.

Thirdly, we examine the relationship between microstructure and corrosion behavior in the SLM-fabricated AlCoCrFeNi_2.1 EHEA. By exploiting thermal cycling and rapid solidification inherent in SLM, we create a corrosion-resistant AlCoCrFeNi_2.1 EHEA with a homogeneous elemental distribution between FCC and B2 phases, which may be directly implemented in service. Both phases are Al-Cr-enriched, enabling effective surface passivation and making the as-built alloy exhibit superior corrosion resistance compared with post-annealed and as-cast specimens in saline environments. Detailed microstructure characterization reveals the spontaneous formation of Cr/Al-dominated amorphous oxide layers on both phase surfaces, serving as an effective barrier against the Cl-ion attack. After annealing, the composition partition between both phases is increased significantly, which in turn leads to a compact Cr-enriched protective film on the FCC region and a porous Al-dominated layer on the B2 phase, respectively. In this case, when attacked by Cl-ions, passive films are more susceptible to breakdown in the B2-phase region, ultimately compromising the alloy's electrochemical stability in saline media.

The present findings provide foundational knowledge elucidating the intrinsic connections between phase transformations and functional characteristics in additively manufactured multiphase alloy systems with hierarchical microstructural complexity. Further, this work paves the way for broader industrial adoption of AM technologies, enabling the production of advanced materials tailored to specific applications.

Date of Award	28 Aug 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jian LU (Supervisor)