Introduction: Nanostructured metals and alloys often exhibit high strength, but at the expense of their signature ductility. By exploiting the non-equilibrium processing conditions of laser powder bed fusion (L-PBF) additive manufacturing, we developed biphasic nanolayers consisting of FCC/L12 and BCC/B2 phases in Ni40Co20Fe10Cr10Al18W2 eutectic high-entropy alloy (EHEA) Structure, the alloy has ultra-high yield strength (>1.4 GPa) and large tensile ductility (~17%). The deformation mechanism of the additively prepared EHEA was studied using in-situ synchrotron X-ray diffraction and high-resolution transmission electron microscopy. The high yield strength is mainly due to the high density of layered interfaces that effectively blocks dislocation motion. The fine nanolayered structure and low stacking fault energy (SFE) promote the stacking fault (SF)-mediated deformation in the FCC/L12 nanolayered structure. The accumulation of abundant dislocations and SFs at the BCC/B2 nanosheet interface can effectively increase local stress and promote dislocation proliferation and martensitic transformation. With the assistance of the semi-coherent layered interface, the cooperative deformation of the two phases makes the printed EHEA highly ductile. Furthermore, we demonstrate that post-printing heat treatment allows us to tune the non-equilibrium microstructure and deformation mechanisms. After annealing, the SFE and thickness of FCC nanosheets were significantly reduced, further promoting the formation of a large number of SFs. The dissolution of nanoprecipitates in BCC/B2 nanosheets reduces spatial constraints, further promotes martensite transformation and enhances work hardening. Our work provides fundamental insights into the rich and diverse deformation mechanisms underlying the superior mechanical properties of additively manufactured biphasic nanolayered EHEAs.

Ultra-high strength (in the giga-Pascal range) and high ductility are key properties for developing lightweight components for applications requiring structural reliability and energy efficiency. In order to obtain high strength, grain-refined Hall-Page strengthening has been widely used in many metallic materials. Unfortunately, this strategy often results in a loss of ductility. This well-known strength-ductility trade-off has been a long-standing challenge for structural materials. Over the past few decades, materials with heterogeneous properties including bimodal, lamellar, gradient, nanodomain dispersion, multiphase, and hierarchical microstructures have been developed to overcome the strength-ductility dilemma. However, a key challenge limiting their widespread use in structural applications lies in their processing. To date, heterogeneous materials have mostly been prepared by methods such as thin film deposition, surface mechanical treatment or multi-step thermomechanical treatment, which are not easily applicable to materials with large volumes and complex geometries.

Additive manufacturing (AM) or 3D printing is an emerging technology that can directly produce complex and near-net shape parts. Metal 3D printing by laser powder bed fusion (L-PBF) involves rapid and spatially variable heating, melting, solidification, and cooling cycles and provides ample opportunities for tailoring microstructure and mechanical properties. This has been demonstrated for a wide range of alloys such as stainless steel and aluminum alloys. However, the successful printing of L-PBF depends not only on the processing conditions but also to a large extent on the inherent thermophysical properties of the feedstock. For example, laser absorptivity, vapor pressure, solidification range, and thermal expansion coefficient can determine the presence of various printing defects, including porosity, loss of volatile alloying elements, thermal cracking, accumulation of thermal residual stress, or part deformation and delamination. These defects can deteriorate the mechanical properties of AM metal alloys, thereby being detrimental to the performance and longevity of AM metal alloys. It is worth noting that most of the more than 5,500 existing alloys cannot be 3D printed with L-PBF.

Eutectic high-entropy alloys (EHEAs) are a class of multi-principal element alloys that typically have a dual-phase structure and have good strength and ductility, excellent high-temperature properties, corrosion resistance, and wear resistance. In addition, the narrow solidification range and isothermal eutectic reaction are beneficial to mitigate thermal cracking during solidification, making EHEAs ideal candidates for AM. Extensive research has been conducted on the AM feasibility of EHEAs such as AlCoCrFeNi2.1, Materials Science Network, Ni32Co30Cr10Fe10Al18, Al0.75CrFeNi, and AlCrFe2Ni2. These alloys exhibit excellent mechanical properties, including high strength and large ductility. However, a basic understanding of their deformation mechanisms is essential.

Ni40Co20Fe10Cr10Al18W2 (at. %) is an EHEA that exhibits excellent mechanical properties in the as-cast state due to its dual-phase B2/FCC micro-layered structure. This study shows that L-PBF can effectively refine the microstructure of Ni40Co20Fe10Cr10Al18W2EHEA to form dual-phase nanosheets composed of FCC/L12 and BCC/B2 phases. Compared with as-cast, the yield strength of printed Ni40Co20Fe10Cr10Al18W2 EHEA doubled without a large loss in ductility. Researchers studied additive manufacturing using state-of-the-art characterization and modeling tools, including in-situ synchrotron high-energy X-ray diffraction (HEXRD), transmission electron microscopy (TEM), and first-principles density functional theory (DFT) calculations Deformation mechanism of Ni40Co20Fe10Cr10Al18W2. The high ductility and sustained work hardening obtained at high flow stresses are caused by several sequentially activated deformation mechanisms. The fine nanolayered structure and low stacking fault energy (SFE) promote abundant stacking faults (SFs) in the FCC/L12 nanolayer. At the BCC/B2 nanosheet interface, the accumulation of defects such as dislocations and SFs can effectively increase local stress, thereby inducing stress-induced martensitic transformation (SIMT) and dislocation nucleation. The cooperative deformation of FCC/L12 and BCC/B2 nanosheets, assisted by the semi-coherent lamellar interface, can effectively enhance work hardening and achieve greater plasticity. Furthermore, we demonstrate that post-printing heat treatment allows us to further tune the non-equilibrium microstructure and deformation mechanisms. After annealing at 900°C for 1 hour, the SFE and thickness of FCC nanosheets were significantly reduced, further promoting the formation of a large number of SFs; the dissolution of nanoprecipitates in BCC/B2 nanosheets reduced spatial constraints and further promoted martensitic transformation. Enhanced work hardening. Our results not only demonstrate that laser additive manufacturing can be used to design metal alloys with excellent mechanical properties, but also provide a fundamental understanding of the deformation mechanisms underlying the special mechanical properties of additively manufactured biphasic nanolayered EHEAs.

Seven top research institutes including the University of Massachusetts, Lawrence Livermore National Laboratory, and Argonne National Laboratory conducted research on this. The relevant research results are titled Deformation mechanisms in an additively manufactured dual-phase eutectic high-entropy alloy. Published in the journal Acta Materialia.

Link:https://doi.org/10.1016/j.actamat.2023.119179

We further characterized the structure and orientation of the layered interface using PED. Figure 2a shows the continuously alternating FCC/L12 and BCC/B2 nanolayered structures. There is a Kurdjumov-Sachs (K-S) orientation relationship between <011>FCC // <111>BCC and {111}FCC // {011}BCC (Fig. 2b). Figure 2c shows the semi-coherent interface corresponding to high-resolution transmission electron microscopy (HRTEM). The fast Fourier transform (FFT) diffraction pattern in Figure 2 confirms the K-S orientation relationship. As shown in Figure 2c and e, this type of interface is energetically more favorable and the lattice mismatch can be compensated periodically by misfit dislocations or SFs. Notably, Figure 2f shows that lattice coherence can be maintained over long distances (>10 nm) in the absence of misfit dislocations. The HEXRD results show that the lattice parameters of the FCC/L12 phase and BCC/B2 phase are 3.59082±0.00004 Å and 2.87126±0.00010 Å respectively, and the lattice mismatch rate is 2.09%. The semicoherent interface with small mismatch strains can effectively improve the strength and ductility of additively manufactured biphasic nanolayered EHEA, which will be discussed later.

In order to elucidate the influence of non-equilibrium solidification characteristics on the deformation mechanism of additively manufactured Ni40Co20Fe10Cr10Al18W2 EHEA, we also studied the structure and mechanical behavior of the material after post-printing heat treatment as a comparison. As shown in Figure 3a and b, annealing at 900 °C for 1 h does not significantly change the colony size but can eliminate most printing-induced dislocations. The average thickness of the fcc substrate layer decreased by about 20% (from 119 nm to 95 nm), while the average thickness of the bcc substrate layer increased by about 46% (from 43 nm to 63 nm), which may be due to the increase in temperature. The result of interface migration. Interestingly, HEXRD measurements show that the annealed Ni40Co20Fe10Cr10Al18W2 is mainly composed of disordered FCC (56 vol. %) and a mixture of disordered and ordered BCC phases (44 vol. %) (Fig. 1e-f). The completely disordered nature of the FCC nanosheets is accompanied by the annihilation of the ordered L12 nanostructures after annealing, which is also reflected in the absence of additional superlattice points in the selected area electron diffraction (SAED) pattern in Figure 3b. Compared with the printed samples, the volume fraction of the FCC phase decreased from 72% to 56%, and the volume fraction of the BCC/B2 phase increased from 28% to 44%. Furthermore, in the annealed Ni40Co20Fe10Cr10Al18W2, the element partitioning between the two phases is stronger due to the acceleration of atomic diffusion at high temperature (Fig. 3c and Table 1). We also noticed that W-rich precipitates were enriched in Ni and Al at the colony boundaries after annealing (Fig. 3a and c). Due to the low volume fraction and large average spacing of these precipitates, their contribution to strengthening is not significant.

Figure 4a shows the representative engineering tensile stress-strain curve after printing and annealing of Ni40Co20Fe10Cr10Al18W2EHEAs. The yield strength (σYS) of the printed sample is 1.42±0.01 GPa, which is approximately twice that of the as-cast sample (0.75 GPa), and the ultimate tensile strength (σUTS) is 1.64±0.01 GPa. The printed sample also showed a large uniform elongation of ~16.5%. After annealing at 900°C for 1 h, the uniform elongation of L-PBF Ni40Co20Fe10Cr10Al18W2EHEA increased to ~ 23%, and σYS and σUTS decreased to ~ 1.08±0.01 GPa and 1.46±0.01 GPa, respectively. Figure 4b shows the work hardening rate as a function of true strain (εT) after printing and annealing of Ni40Co20Fe10Cr10Al18W2 EHEAs. The work hardening rate of both specimens decreased sharply in the first stage, indicating that dislocation slip plays a prominent role in the early stage of plastic deformation. As εT further increases, the work hardening rate of the printed sample gradually decreases, but still remains above 1.5 GPa, indicating plastic instability. For high-strength printed samples, the continuous work hardening ability under high flow stress (σUTS - σYS=0.23 GPa, σYS/σUTS=0.86) leads to a large uniform elongation. It is worth noting that the annealed sample exhibits a small hump after stage i. In stage II, the work hardening rate gradually increases until εT ≈ 12.5%, which can be attributed to the general effect of additional deformation carriers such as SFs and martensite transformation (see later discussion). The work hardening rate of the annealed sample gradually decreases in the third stage, and the phase transformation tends to be saturated.

In this study, we used L-PBF to refine the lamellar thickness of dual-phase Ni40Co20Fe10Cr10Al18W2 EHEA to the nanometer level, thereby improving its mechanical properties. The ultra-high yield strength reaches 1.4 GPa and the tensile ductility reaches 17%. This doubles the yield strength as cast without a large loss in ductility. Using in-situ synchrotron HEXRD and advanced electron microscopy techniques, the microstructure and deformation mechanism of additively manufactured Ni40Co20Fe10Cr10Al18W2 EHEA with significantly enhanced mechanical properties were revealed. The main opinions are summarized as follows: First, the inherent strong thermal gradient and high cooling rate of L-PBF provide a unique way to achieve non-equilibrium, highly refined nano-layered structures in dual-phase Ni40Co20Fe10Cr10Al18W2 EHEA. Secondly, the high strength of Ni40Co20Fe10Cr10Al18W2 EHEA mainly comes from the refined nano-layered structure, and the dislocations caused by the rapid solidification of L-PBF and ordered L12 structures are further enhanced. FCC/L12 and BCC/B2 nanosheets have strong ductility, which is mainly due to the continuous activation of multiple deformation mechanisms by FCC/L12 and BCC/B2 nanosheets. The deformation mechanism of FCC/L12 nanosheets at around 5% strain is: Complete dislocation slip evolves into SF nucleation. At the same time, the semicoherent layered interface serves as the preferred location for defect accumulation, effectively increasing local stress and promoting dislocation proliferation and martensitic transformation in adjacent BCC/B2 nanosheets. Post-print heat treatment allows us to further tune the non-equilibrium microstructure and deformation mechanisms. The stronger element distribution between the two phases can significantly reduce SFE and promote the formation of SFs in FCC nanosheets. The dissolution of BCC nanoprecipitates in BCC/B2 nanosheets can relieve spatial restrictions, further promote martensitic transformation, and improve work hardening ability.