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In this paper, a laser-based directed energy deposition (DED) technique is used to fabricate FeCoNiCr and CrMnFeCoNi multi-principal-element alloys (MPEAs). Comparing the above samples, the FeCoNiCr samples with coarse columnar grains cracked, while the CrMnFeCoNi samples with equiaxed grain were crack-free. The strategy that removes cracks is to induce a columnar-grain-to-equiaxed-grain transition (CET) with Mn addition to offer more grain boundaries to withstand residual stress in the process of DED-fabricated FeCoNiCr and to help minimize hot cracking. Furthermore, the yield strength, tensile strength, and tensile ductility of the DED-fabricated CrMnFeCoNi obviously improved compared with the DED-fabricated CoCrFeNi and exhibited better isotropic mechanical properties. The present work provides a novel strategy to utilize CET for resisting crack propagation in the process DED-fabricated MPEAs and improvement in mechanical properties of MPEAs. [ABSTRACT FROM AUTHOR]

Ultra-high-speed laser cladding (UHSLC) not only offers an efficient solution to the limitations of conventional laser cladding but also aligns with the contemporary objectives of material conservation and efficiency enhancement in metal protective coatings. This study investigates the fabrication of CrFeCoNiMo 0.2 high-entropy alloy (HEA) coatings using UHSLC and traditional laser cladding (TLC) methods. Key aspects such as macroscopic forming quality, element distribution, dilution rate, grain morphology, hardness, and electrochemical corrosion properties of both UHSLC and TLC coatings are thoroughly examined. Notably, UHSLC-produced coatings, with their significantly reduced thickness ranging from 100 to 300 μm, demonstrate an impressively low substrate dilution rate compared to the 2–3 mm thickness of TLC coatings. The rapid solidification inherent in UHSLC leads to smaller grain sizes in the coating, resulting in enhanced hardness. Moreover, the UHSLC coating exhibits superior corrosion resistance, characterized by a lower maintaining-passivity current density in 1 mol/L H 2 SO 4 and a higher self-corrosion potential relative to TLC coatings. This work presents a novel approach, employing high-efficiency and low-cost laser-cladding technology, to minimize dilution in the matrix and improve the coating properties of HEAs. • There are no macroscopic and microscopic defects in HEAs prepared by UHSLC. • Uniform distribution of elements in ultra-thin coating of CrFeCoNiMo 0.2 HEA. • The CrFeCoNiMo 0.2 HEA nanocrystalline coating was synthesized by UHSLC. • Nanocrystalline CrFeCoNiMo 0.2 HEA coating exhibit excellent corrosion resistance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mg promotes the atomic diffusion of Al-Cu-Mg alloy. • The addition of Mg causes the alloy to generate more large-size Cu clusters. • The introduction of Mg lead to the formation of more FCC phases. • The enrichment of Cu intensified with the increase of Mg content. • Verified the consistency of simulation and experimental results. To analyze the mechanism of the Mg content on Cu precipitation in Al-Cu-Mg ternary alloy, the microstructure evolution of Al-5%Cu, Al-5%Cu-1%Mg and Al-5%Cu-2%Mg alloys during the solidification process was investigated by both the molecular dynamics simulation and solidification experiment methods. The simulated results demonstrate that the introduction of Mg accelerates atomic diffusion. And as the Mg content increases, more large-sized Cu agglomerates are formed in the solidified tissue, due to the Mg plays a connecting role in the process of Cu agglomerate formation. It is also found that the introduction of Mg results in the development of more FCC phases in the final tissue. The experimental results show that Cu elements are concentrated in the Al crystal boundaries, and the degree of Cu enrichment increases with the level of Mg content. The physical phase analysis shows that the final organization is primarily composed of the Al phase and Al 2 Cu phase, and both phases increase slightly with a higher Mg rate. The consistency between experimental and simulation results was verified by a comparative discussion of the Cu agglomerated and precipitated phases. It provides a theoretical basis for improving Al-Cu-Mg alloys' performance enhancement and structural optimization. [ABSTRACT FROM AUTHOR]

• Generation of hard phases and grain refinement in enhanced bionic units. • Bionic unit samples have good hard-soft alternating bionic structure. • Quantitative relation between distribution spacing and wear resistance is obtained. • Hard-soft bionic structure improves wear resistance by altering the wear pattern. • Adjusting the distribution spacing has a more positive effect on wear resistance. In this study, the biomimetic unit samples (BUSs) of hard-soft structure with different distribution spacings were successfully prepared by laser quenching biomimetic processing technique (LQBPT). Firstly, the microstructure, microhardness, tribological properties and wear surfaces were investigated. The BUSs had randomly oriented martensite and M 7 C 3 , and the grains were refined, resulting in favorable hard-soft structures. Compared to the untreated samples, the BUS at distribution spacing of 3 mm had excellent wear resistance. Its average friction coefficient and weight loss were reduced by 17.95 % and 40.0 %, respectively. Then, it was investigated the quantitative relationship between the distribution spacing and the wear resistance and its wear resistance mechanism. This improved wear resistance was attributed to hard-soft biomimetic structure. [ABSTRACT FROM AUTHOR]

• La 2 O 3 improved the temperature gradient and reduces crack susceptibility. • La 2 O 3 promoted nucleation and grain refinement and increases dislocation density. • La 2 O 3 doping increased the hardness of the coating and improved the wear pattern. In this study, CoCrFeNiMoSi and CoCrFeNiMoSi + 0.5 % La 2 O 3 High entropy alloy (HEA) coatings were prepared by laser cladding. The microstructure was consisted mainly of body-centered cubic structure, face-centered cubic structure and intermetallic compounds of Si. The addition of rare earth La 2 O 3 resulted in the refinement and uniformity of the CoCrFeNiMoSi + 0.5 % La 2 O 3 HEA coating grains, leading to an increase in microhardness from 430 HV to 520 HV. Additionally, the friction coefficient decreased by 0.15 and the wear rate decreased by 2/3. These findings indicate that the appropriate addition of La 2 O 3 has significant potential in enhancing the friction and wear performance of HEA coatings. [ABSTRACT FROM AUTHOR]

To strengthen face-centered cubic structured high entropy alloys, this paper explores a new method of ceramic reinforced CoCrFeNiMo 0.2 high entropy alloy composite gradient coating. In this study, four sets of gradients were created by first preparing a pristine alloy coating without silicon carbide content on the substrate, followed by preparing composite coatings with varying silicon carbide mass fractions (8 %, 16 %, 24 %) on the pristine coatings. The coatings were then analyzed for their phase composition, microstructure, microstructure evolution process, and friction wear properties. The study reveals that the microstructure of the initial alloy coating consists of a single-phase face-centered cubic solid solution. It is observed that intragranular segregation occurs in the coating, resulting in the boundary enrichment of Mo and Cr elements. As the silicon carbide addition increases, the segregation behavior becomes more pronounced, and the generation of body-centered cubic phase is observed. The second, third, and fourth layers exhibit significantly higher average hardness, measuring at 594 HV, 722 HV, and 788 HV respectively. This can be attributed to the presence of body-centered cubic structure, carbide, and grain boundary strengthening effects. On average, the hardness of these layers is three to four times higher than that of the first layer. With an increase in the number of layers, the wear mechanism changes from adhesive wear to abrasive wear. The wear rates of the second, third, and fourth layers were all reduced by at least 87 % compared to the first layer. However, there were different forms of abrasive wear observed between these layers. The findings of this study have significant engineering implications. • The microstructural evolution of columnar grains to equiaxed dendrites was studied. • The high-density of dislocations enhances the diffusion of Mo and Cr elements. • The tissue shows phase separation act containing the BCC structure. • The shift behavior of the wear mechanism of the gradient structure was studied. [ABSTRACT FROM AUTHOR]

The negative effects of thermal cycles in the process of additive manufacture present a challenge for the control of microstructure so as to fabricate the products with improved properties compared with conventional casting technique. In this work, AlCoCrFeNi 2.1 eutectic high-entropy alloy (EHEA) was prepared by laser metal deposition (LMD). Compared with conventionally cast EHEA samples, the LMD-fabricated EHEA samples showed significantly enhanced tensile strength (by 19.7%) and increased tensile ductility (by 56.4%). Such enhancement in tensile properties was attributed to the refinement of the uniformly distributed eutectic-structure, which improved the strain hardening/dislocation accumulation capability of the EHEA. The present work provides a new strategy to utilize both the high cooling rates of LMD and the eutectic-structure characteristics for forming refined homogeneous structures and thus achieving superior mechanical properties to those prepared by traditional processing techniques. [ABSTRACT FROM AUTHOR]

Al x CoCrFeNi (x = 0, 0.2, 0.45, 0.7 and 1) high entropy alloys (HEAs) have been successfully synthesized by laser melting deposition (LMD). It is found that the microstructure of LMD-CoCrFeNi HEA with increasing the content of Al has been transformed from a single face-centered cubic (FCC) phase to mixture of FCC + BCC duplex phases, and then to body-centered cubic (BCC/B2) phases. The addition of Al (Al 0.2 , Al 0.45 and Al 0.7) remarkably improved the yield strength σ 0.2 (from 138 MPa to 185 MPa, 262 MPa and 771 MPa) and the tensile strength σ t (from 634 MPa to 675 MPa, 863 MPa and 1171 MPa) of the samples, with the prize of plasticity decrease from 46% to 45%, 41% and 11% in tension. This work provides an approach of LMD-fabricated CoCrFeNi-based HEAs with different Al addition to accommodate the microstructure and mechanical properties, and insights into the correlation of mechanical properties with the solid solution strengthening or/and the precipitation of hardening phase. • Al x CoCrFeNi (x = 0, 0.2, 0.45, 0.7 and 1) HEAs have been prepared by LMD method. • The ΔH mix and VEC are used to predict the phase stability for LMD-fabricated HEAs. • The LMD HEA has been changed from a single FCC phase to BCC phase via Al addition. • With the increase of Al amount, σ 0.2 and σ t of the HEAs under the tensile and compression test have been obvious improved. [ABSTRACT FROM AUTHOR]

Homemade nano-agglomerated powders 8YSZ powders for PS-PVD were prepared by the spray drying, then calcination processes at four different temperatures (500 °C, 700 °C, 900 °C and 1100 °C) were carried out on the spray-dried powders. Checked by laser particle sizer, scanning electron microscope (SEM) and X-ray diffraction (XRD), the physical properties, microstructure and phase constitutions of the calcined powders were investigated. The results show that the size of powders calcined at 500 °C is increased relative to the spray-dried powder, whereas the powders calcined at 700 °C, 900 °C and 1100 °C possess smaller size. The binding force of the primary particles tend to rise with the increase of calcination temperature. When the temperature was up to 900 °C and above, it was found that the sintering neck indicating with strong binding was formed between the primary particles. In parallel, the powders underwent an m-ZrO 2 to t-ZrO 2 transition as the calcination temperature rose. It is also found that the PS-PVD prepared coatings which were obtained by using the above powders undergo a transformation from a feather-like to a dense laminate structure as the calcination temperature rises. It is noteworthy that the coating obtained by the powders calcined at 700 °C have a special three-layer composite structure of near dense surface layer, columnar intermediate layer and dense sub-layer. The composite structural coating has excellent adhesion and thermal shock resistance, with a bonding strength of 81MPa and no major spalling when water quenched 100 cycles at 1100 °C. [ABSTRACT FROM AUTHOR]