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[Featured Article] A Review on Microstructures and Properties of High Entropy Alloys Manufactured by Selective Laser Melting

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Release Date: 2020-07-27 Visited: 

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Chen Zhang, Junkai Zhu, Huai Zheng, Hui Li, Sheng Liu and Gary J. Cheng

1.Introduction

High entropy alloys (HEAs) with multi-component solid solution microstructures have the potential for large-scale industrial applications due to their excellent mechanical and functional properties. However, the mechanical properties of HEAs limit the selection of processing technologies. Additive manufacturing technology possesses strong processing adaptability, making it the best candidate method to overcome this issue.

Nowadays, research on SLM of HEA is gradually increasing, and it is becoming a hot spot for researchers in the two fields of HEA and additive manufacturing. Summarizing existing research results, sorting out common scientific problems, and discovering new scientific problems will help researchers better grasp the research focus. Recently, the team of Professor Chen Zhang and Liu Sheng of the Institute of Industrial Science of Wuhan University published the "A Review on Microstructures and Properties of High Entropy Alloys Manufactured by Selective Laser Melting" in the International Journal of Extreme Manufacturing (IJEM). The microstructure characteristics, mechanical properties, tissue-performance relationships and theoretical models of SLM printed HEA are introduced, and new directions are pointed out for future research.

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Fig.1 (a) Schematic setup of SLM processing; (b) procedure of SLM processing.

 2. Powder material for SLM of HEA

At present, the methods of preparing HEA powder mainly include mechanical mixing and pre-alloying. Mechanical mixing is convenient to obtain free proportion of HEA powder, but it is easy to cause element segregation. The pre-alloyed powder is more uniformly distributed than the mechanical mixed powder, which can prevent the components from segregation, thus making the sample structure uniform and stable performance. Therefore, pre-alloyed powder is widely used in the current research on SLM of HEA. The shape and size of the powder also affect the processing quality. Due to the better fluidity of spherical powders, the densest SLM-processed samples were obtained from spherical particles rather than other shaped particles. The use of spherical powder can also reduce splashing, thereby reducing microstructural defects. In addition, the span of particle size distribution also has a significant effect on powder fluidity. When the span is small, the fluidity is better and the relative density of the sample is higher. Therefore, similar to other alloys, the use of spherical and uniform size HEA powder is more beneficial to improve the processing quality.

3. Macro and micro structure characteristics

The structural characteristics of SLM printed HEA are mainly reflected in two aspects: macro defects and micro structure. Cavity defects are the most common defects in SLM, such as unsolidified particles and porosities. As a new material with complex composition, HEAs is prone to pore defects, and it is the main cause of density loss. Increasing the volume energy density by changing the printing parameters can effectively suppress the generation of porosities, as shown in Figure 2. Cracking is another common defect, which is closely related to alloy composition. For example, increasing the composition of Ni in AlCrCuFeNi can effectively suppress cracking, as shown in Figure 3. Since there are many types of HEA elements, there will be a lot of research on controlling element composition to control cracks. In addition, typical additive manufacturing defects such as element segregation and slag inclusion also appeared in the HEA printed by SLM.

Due to the complex thermal cycle and extremely high cooling rate (106 K/s) of the SLM process, the grain size of the SLM printed HEA is as low as a few microns, and it can produce very high density of dislocations, sub-grain boundaries, nano twins, as shown in Figure 4, this will greatly improve the mechanical performance of HEA. Another distinguishing feature is that SLM printed HEAs are more likely to produce precipitates. The traditional manufacturing process of HEA requires a long time heat treatment to produce precipitated phases, and the multi-interface or multi-defect characteristics of the SLM printed HEA lattice provide an ideal growth carrier for the precipitated phases, requiring only a short time of heat treatment or no heat treatment. A precipitated phase can be obtained. The alloy composition is critical to the performance. In addition, subsequent heat treatment of SLM printed HEA can also produce microstructures such as nano twins (as shown in Figure 5).

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Fig.2 (a) Density with different laser energy density; (b–e) XCT images of dimensional morphology of pores in sample with volumetric energy density 123 J/mm 3.

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Fig.3 The effect of different Ni content on SLM printed AlCrCuFeNix cracks: (a) x = 2.0 (obvious crack); (b) x = 2.5; (c) x = 2.75; (d) x = 3.0 (almost no cracks)

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Fig.4 Substructures, dislocation and precipitates of SLM-processed CoCrFeMnNi. (a) High magnification back scattered electron (BSE) imaging of substructures. (b) and (c) Electron channeling contrast (ECC) images of dislocation networks and precipitates (b) in cellular structure and (c) in columnar structure

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Fig.5 (a)~(c) Twinning distribution (Σ3 boundaries of EBSD) of SLM-processed FeCoCrNi after 2h of annealing at (a) 1173K, (b) 1373K, and (c) 1573K, images in upper right corner is recrystallization distribution maps. (d) HRTEM showing the nanotwin of the SLM as-processed CoCrFeMnNi.(e) Nanotwin-HCP lamella composite structure of SLM-processed FeMnCoCrC0.5 after 12% strain.

4. Mechanical properties and models

The review summarizes the tensile and compression data of different types of HEA printed by SLM, and compares the performance of HEA prepared by different processing methods. Because the HEA prepared by the traditional process lacks strengthening means, the structure is simple, the grain size is large, and its yield strength and ultimate tensile strength are much lower than the performance of the SLM sample. Annealing the printed HEA will promote the formation of precipitated phases, soften the HEAs, and promote its impact toughness. In addition, SLM printed HEA has improved corrosion resistance and wear resistance.

The HEA strengthening mechanism is different from other metals. The most prominent features are the frictional stress of the crystal lattice and the unusual solid solution strengthening. The multiple characteristics of HEAs make the traditional model unsuitable for predicting the degree of solid solution strengthening of HEAs. At the same time, the extremely high heating and cooling rate of SLM generates a large number of dislocations. The dislocations are very concentrated and can almost be regarded as grain boundaries. The main strengthening mechanisms are summarized as lattice friction stress strengthening, grain boundary strengthening, dislocation strengthening and precipitation strengthening.

5. Summary and Outlook

The existing research efforts of SLM-processed HEAs have focused on analyzing microstructures and static mechanical properties. The majority of samples were simple block parts. The characteristics of the rapid melting and cooling rates in SLM processing improved the microstructures of HEAs, including grain refinment, increased dislocation density, phase precipitation, and nanotwin generation. These characteristics increased the mechanical strength of SLM-processed HEAs beyond that of other commonly used forming methods. Similar to traditional alloys, process optimization, addition of alloying elements, and heat treatment of SLM-processed HEA remained the main methods of regulating microstructure and improving performance.

However, defects such as pores and cracks still exist in the SLM-processed components. These defects may have minimal effects on static strength but are fatal to dynamic fatigue performance. Eliminating these defects may further enhance the mechanical performance of SLM-processed HEAs. Therefore, research on defects deserves future efforts. These future studies should investigate mechanisms of defects generation, effective methods of defect suppression, theoretical models of metallurgy processes, and simulations of heat transfer, flow, and stress in SLM of HEAs.

Present research efforts on mechanical strength focus on tensile strength, compressive strength, and hardness However, limited research focusing on the impacts of experimental fatigue, strength, and life prediction on the final application has been conducted. This research gap hinders the acceptance of this method for broader applications beyond the space industry with one time use and without critical components. An important feature of HEAs is their excellent stability under high- and low-temperature strength. The mechanical properties of SLM-processed HEAs under extreme environments have not been reported in the existing literature. Future research must focus on the fatigue performance, high-temperature strength, low-temperature strength, high-temperature creep, and fatigue properties of SLM-processed HEAs.

The theoretical predictive models of mechanical strength are primarily based on single-component alloys. However, considering the characteristics of HEA microstructures, we believe that the strengthening model of HEAs should have its own features. In order to improve the current understanding of HEA strengthening methods, future research should include advanced simulation methods of various scales, such as density functional theory and molecular dynamics. Modeling validation tools also need to be developed. Ideally, these tools should be able to monitor processing steps in terms of microstructures, environments, stress/deformation evolution, and defect initiation and control.

The functional properties of the SLM-processed HEA parts with complex geometric structures and composition of composite materials possess vital research value for chemical, energy storage materials, electronic functional devices, electromagnetic shielding, and stealth applications. Until now, no studies in this field have been reported. It will become the focus of future research considering the demand for new high-performance materials in aforementioned areas.

Existing research on SLM of HEAs focuses on FeCoNiCrMn and AlFeCoNiCr series alloys. The SLM of lightweight HEAs has not been reported. Using lightweight alloys is an important method of weight reduction and carbon emission reduction. Research on SLM of lightweight HEAs has the potential for significant industrial applications. 

6.  About the Authors

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Chen Zhang, associate professor, Institute of Industrial Sciences of Wuhan University, mainly engaged in laser manufacturing and additive manufacturing research, including cross-scale and cross-material laser cutting and welding, arc additive manufacturing, selective laser melting forming, etc.

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Sheng Liu, Yangtze river scholars Distinguished Professor, Dean of the School of Power and Mechanical Engineering of Wuhan University, Executive Dean of the Institute of Industrial Sciences, Distinguished Professor of Huazhong University of Science and Technology, ASME Fellow, IEEE Fellow, the main research fields are advanced manufacturing (additive manufacturing, laser shock peening, femtosecond laser micromachining), advanced materials and mechanics, microelectronics, optoelectronics, LED, MEMS, automotive electronic system packaging and assembly, rapid reliability evaluation and design, micro-scale inspection and computer-aided design of microelectronic mechanical systems, IC design, advanced materials and mechanics.

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