additive manufacturing technology (AM) in high-entropy alloy is increasingly attracting attention. Its extremely fast cold speed (103 –106 K/s) is conducive to the formation of supersaturated solid solution, thereby stabilizing the phase structure of HEAs. In addition, the faster cooling speed can achieve grain refinement, significantly improving the alloy strength and hardness. Directional energy deposition technology (DED) can directly synthesize high-entropy alloys with variable components in situ.
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Laser Manufacturing Center of the School of Mechanical Engineering, Purdue University, synthesized three CoCrFeNiTi high-entropy alloys with different microstructures in situ through DED technology. Through microstructure analysis, it was found that the antioxidant properties of the prepared HEAs were comparable to those of Inconel 625. This work conducted a detailed evaluation of the composition, tissue, hardness, oxidation resistance and wear resistance of HEA, providing reference significance for actual production and application.
related research paper link:
https://doi.org/10.1016/j.jallcom.2022.165469
research points
research team used the DED process to prepare CoCrFeNiTi HEA alloys with different atomic ratios. Due to the different flowability and density of each metal powder, there is a deviation between its actual components and the premixed components, and three different HEA components H1, H2 and H3 were finally designed.
Figure 1 (a) SEM diagram of CoCrFeNiTi powder; (b) EDX diagram of CoCrFeNiTi powder.
To solve incomplete melting and improve chemical heterogeneity, the research team finally selected 45-75 μm sheet-shaped Cr powder to form unmelted nanoparticles and about 200 nm clusters . The proportion and size of unmelted particles in the print are very small, and the impact on mechanical properties and oxidation properties is small. By increasing the energy density by increasing the laser power, reducing the layer thickness and reducing the scanning speed, the incomplete melting problem will be reduced, while reducing the porosity of .
Figure 2 STEM diagram and EDX results of H1 component fusion zone.
Due to the tracking characteristics of the AM process, different fusion zones (FZ) and heat-affected zones (HAZ) can be observed. The size of the heat-affected zones depends on the AM parameters, namely the scanning spacing and layer thickness. Among them, HAZ shows a deeper contrast than FZ after etching, and uses EDX to display the light and dark phases, and measures and labels H1, H2, and H3 respectively.
Figure 3 SEM diagrams and EDX results for different phase structures in H1 components.
Figure 4 SEM secondary electronic images and EDX results of different phase structures of H2 components.
Figure 5 SEM secondary electronic images and EDX results of different phase structures of H3 components.
characterizes the phase structure of different HEAs through the XRD test. Due to the large amount of heterogeneous components, a small amount of Ni4Ti3, Ni3Ti and NiTi may be formed, but there are no obvious peaks. From H1 to H3, the peak intensity of these secondary phases gradually decreases, meaning a decrease in volume percentage, because the relative intensities of different phases can quantify their proportions, and all three XRD maps are dominated by FCC-γ peaks.
Figure 6 XRD spectrum of three HEA alloys synthesized by the DED method.
Due to the unevenness of the tissues of the fusion zone and the heat-affected zone, in order to understand the hardness of different regions on a small scale, a Vickers microhardness test was performed. Since H3 has similar actual components in the fusion and heat-affected zones, the standard deviation of hardness of H3 is reduced compared to H1 and H2. At the same time, the antioxidant properties of the three designed alloys were compared, and the weight gain value comparison was used to verify the potential of HEA alloys in high temperature applications.
Figure 7 The thermal oxidation results of HEA alloy synthesized using DED technology.
summary
This study provides a method to develop new HEA alloys with low-cost premixed elemental powder as raw materials using metal additive manufacturing technology. All three HEAs designed exhibit oxidation resistance comparable to Inconel 625 high temperature alloy, which is about 20 times higher than AISI M2 and 30 times higher than AISI 52100 bearing steel. Because H1 and H2 have higher hardness or, this hardness is reported to have better wear resistance than AISI M2 and AISI 52100 wear-resistant steels, H1 and H2 are considered to have great wear-resistant application potential.
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