Superior mechanical properties and multiple strengthening mechanisms of additively manufactured face-centered cubic high entropy alloys

Abstract

The high-entropy alloys (HEAs) concept has opened up a new avenue for alloy design, which encompasses broad-spectrum microstructures and properties. Among various HEA systems, CoCrNi-based HEAs with a single face-centered cubic (FCC) structure have received considerable attention in regard to their exceptional ductility. Nevertheless, the low strength of FCC-type HEAs has restricted their widespread applications as structural materials. Since solid-solution strengthening has a limited enhancement effect, introducing multiple strengthening mechanisms during deformation offers new possibilities to address this challenge, e.g., second-phase strengthening, metastable phase transformation, heterogeneous-induced hardening, etc. Since cast alloys retain few dislocations, complex thermo-mechanical processing (TMP) is necessary for synergistic strengthening. Nevertheless, TMP-processed alloys still suffer from grain coarsening and undesirable dislocation density. Because of continuous demand for more complexity in design and efficient use of materials, additive manufacturing (AM) has emerged as a new technology in fabricating parts in a layer-wise manner. Besides, recent research has demonstrated the ability of AM to produce HEAs with superior mechanical properties compared to their counterparts obtained by conventional techniques. The ultra-high cooling rate inherent in AM facilitates the formation of refined grain structures and abundant dislocations within the alloys. This results in enhanced properties and advantageous microstructures for further strengthening through post-processing treatments. Despite the great efforts dedicated to AMed alloys, the dilemma remains in synergizing FCC HEAs with superior ultra-high strength and ductility. Moreover, in-depth studies are urgently needed to advance the understanding of the formation mechanisms and strengthening effects of the microstructural heterogeneity by AM. This research incorporated alloy compositional design strategies and AM techniques for tailoring microstructure and multiple strengthening mechanisms. First, ceramic nanoparticles reinforced CoCrFeMnNi HEAs were successfully fabricated by AM for additional secondary phase strengthening. The as-AMed sample showed a significant increase in yield strength, exceeding that of its original CoCrFeMnNi counterpart by 42%. Yield strengthening also resulted from the refined grain sizes, high dislocation density, and load transfer effects. We also conducted a thorough analysis of defects in the samples to reveal the feasibility of AM in producing near-fully dense FCC HEAs. Second, the research demonstrated the success of engineering phase metastability into a non-equiatomic Co-rich FCC HEA through AM. We established the relationship between the hierarchical micro-features, the tensile properties, and the high-cycle fatigue resistance. Multiple mechanisms were activated simultaneously to attain a cumulative effect on the resultant performance. The activation of the additional phase transformation effect ensured substantial work hardening capacity. Third, to further strengthened FCC HEAs, we incorporated coherent nano-precipitations into Co-rich HEAs through strategic compositional design. One-step cold rolling was adopted for the AMed specimen to regulate the hierarchical features, i.e., grain morphology and precipitation behavior. Detailed analysis revealed the impact of heterogeneous microstructures and alternative strengthening mechanisms. The refined grains and facilitated discontinuous precipitation resulted in an excellent yield strength of 1430 MPa and a tensile strength of 1800 MPa with 16% ductility. This research achieved superior mechanical properties of FCC HEAs through the AM technique and compositional design strategy, established relationships between their microstructure evolution and the underlying deformation mechanisms, and revealed their multiple strengthening mechanisms. These findings are expected to open a broader processing window for achieving a wide spectrum of mechanical properties for HEAs.