New Ultrasonic Vibration Technology Ushers in the Future of High-Entropy Alloy Powders—The Innovative Path to Preparing TiZrTa0.7NbMo High-Entropy Alloy Powder.

In recent years, driven by the rapid advancement of materials science, high-entropy alloys (HEAs) have emerged as a new frontier in materials research. Among them, refractory high-entropy alloys (RHEAs) stand out for their exceptional high-temperature performance, opening up vast application prospects in aerospace, gas turbines, nuclear energy, and other critical industries.

However, the preparation of high-quality high-entropy alloy powders, particularly spherical powders, has long been a major bottleneck restricting their industrial applications. Although conventional powder-production methods such as gas atomization and water atomization are well established, they struggle to achieve the desired powder quality when dealing with high-melting-point alloys, complex compositions, and high viscosities.

This paper provides a detailed analysis of an innovative powder-production technique—ultrasonic vibration-assisted melt atomization—proposed by Professor Dong Fuyu and his colleagues at Shenyang University of Technology, with a focus on its application in the preparation of high-entropy alloy powders, particularly the research findings on the TiZrTa0.7NbMo high-entropy refractory alloy. The study offers new insights for the future development of high-temperature, high-strength materials.

Background

Traditional powder-making methods mainly include mechanical grinding, atomization (flame atomization and gas atomization), and electrolysis.

1. Basic Concepts of High-Entropy Alloys

Traditional alloys typically use a small number of elements as the primary constituents, with trace elements added to tailor their properties. In contrast, high-entropy alloys depart from this conventional approach by employing a multi-element composition—usually comprising five or more elements in roughly equal proportions—resulting in exceptionally high configurational entropy and endowing them with outstanding mechanical properties, corrosion resistance, and high-temperature performance.

2. Refractory High-Entropy Alloys (RHEAs)

Due to their excellent high-temperature resistance, oxidation resistance, high hardness, good toughness, and other superior properties, refractory high-entropy alloys have attracted particular attention from the thermal energy and aerospace industries. High-entropy alloys based on elements such as Ti, Zr, Ta, Nb, and Mo exhibit extremely high melting points and outstanding high-temperature performance, making them ideal refractory materials.

3. Challenges in Powder Preparation

Despite the obvious advantages of high-entropy alloys, their powder fabrication remains highly challenging:

• High melting point: Refractory elements have extremely high melting points, making them difficult to melt and atomize rapidly;
• Complex composition: Multi-element systems are prone to segregation and elemental loss;
• Conventional techniques fall short: traditional methods such as gas atomization struggle to produce powders that are spherical, uniformly sized, and free of porosity.

Therefore, it has become an urgent priority to develop a novel, efficient powder-preparation technique that can precisely control powder quality.

Innovative Breakthrough: Ultrasonic Vibration-Assisted Melt Atomization Technology

1. Introduction to the Technical Principle

Ultrasonic vibration-assisted melt atomization utilizes the mechanical vibrations generated by ultrasound to break down molten metal into fine droplets under vibrational action, which are then rapidly cooled and solidified into powder. Compared with conventional gas atomization, it offers the following advantages:

• Controllable particle size: Particle size can be precisely controlled by adjusting the ultrasonic frequency.
• High powder quality: particles are more spherical in shape, with smooth surfaces and few defects;
• Reduces segregation: rapid solidification effectively inhibits elemental segregation;
• Elimination of porosity: The droplets cool rapidly, resulting in few pores and impurities.

2. Methodology and Process

• Alloy melting: High-purity metal raw materials are used and melted in an electric arc furnace under an inert atmosphere to produce a homogeneous alloy ingot.
• Melt vibration: Under ultrasonic vibration, liquid metal is broken into fine droplets;
• Cooling and solidification: The droplets are rapidly cooled in air or a protective atmosphere to form a powder;
• Powder collection: High-quality spherical powders are obtained in large quantities using a specialized collection device.

This technology is particularly well suited for the preparation of high-entropy alloy powders with high melting points and high viscosities, opening up new possibilities for the industry.

Preparation and Characterization of TiZrTa0.7NbMo High-Entropy Refractory Alloy Powder

01 Experimental Materials and Preparation

The research team selected five high-purity metals—Ti, Zr, Ta, Nb, and Mo—and formulated an alloy composition with the molar ratio Ti:Zr:Ta:Nb:Mo = 1:1:0.7:1:1. The alloy ingot was first produced through repeated melting in a high-vacuum furnace. Subsequently, ultrasonic vibration-assisted melt atomization was employed to break the liquid alloy into fine powder.

02 Microstructure and Morphological Characteristics of the Powder

Scanning electron microscopy (SEM) observations reveal that the powder exhibits a highly spherical morphology with a smooth surface, free of obvious cracks and impurities, and a narrow particle-size distribution—characteristics that meet the fundamental requirements for high-quality powders. Furthermore, micro-computed tomography (Micro-CT) analysis indicates low internal porosity, with predominantly micropores that are uniformly distributed.

03 Powder Microstructure Analysis

X-ray diffraction (XRD) analysis reveals that the powder consists predominantly of two body-centered cubic (BCC) phases (BCC1 and BCC2), with no significant interstitial phases or phase transformations observed. This indicates that ultrasonic vibration atomization effectively suppresses segregation, thereby maintaining a uniform elemental distribution.

04 High-Temperature and Hardness Performance

Nanoindentation tests reveal that the powder exhibits a significantly higher hardness than conventional as-cast samples (average ~6 GPa), indicating exceptionally high local strength. This advantage stems from the fine grain structure and microalloying resulting from rapid solidification.

Advantages of Ultrasonic Vibration-Assisted Atomization Technology

1. Achieving High-Quality Powder

Conventional atomization techniques struggle to produce pure, spherical, and size-uniform high-melting-point high-entropy alloy powders. In contrast, ultrasonic vibration-assisted atomization effectively overcomes this challenge, enabling the fabrication of powders with smooth surfaces, minimal contamination, and controllable particle sizes.

2. Enhancing Material Properties

High-quality powders provide a solid foundation for additive manufacturing processes such as laser powder bed fusion (LPBF). Powders prepared using ultrasonic technology significantly enhance forming quality and improve mechanical strength and high-temperature performance.

3. Reduce preparation costs and energy consumption

Compared with conventional gas atomization, ultrasonic vibration atomization can produce large quantities of high-quality powders under lower energy consumption, thereby enabling industrial-scale production.

Illustrated Guide

Figure 1. Schematic diagram of RHEA powder preparation by ultrasonic atomization.

Figure 2. SEM images of TiZrTa0.7NbMo RHEA powder: (a–c) powder morphologies corresponding to particle size ranges of 15–50 μm, 50–100 μm, and 100–150 μm, respectively; (d–f) the corresponding particle size distributions for (a–c).

Figure 3. Micro-CT analysis of TiZrTa0.7NbMo RHEA powder: (a–c) 3D reconstructions of powders in different particle-size ranges; (d–f) 3D representations of pores extracted from (a–c); (g–h) particle-size distributions and sphericity values for the powders in (a–c); (j–l) equivalent-sphere-diameter distributions of porosity in (d–f).

Figure 4. Schematic diagram of the ultrasonic atomization mechanism.

Figure 5. XRD pattern of TiZrTa0.7NbMo RHEA powder.

Figure 6. Microstructure and elemental distribution in the cross-section of a single powder particle: (a) BSE image and EDS mapping results; (b) Microstructure and EDS mapping results for the region enclosed by the red box in (a).

Figure 7. EBSD images of powder cross-sections: (a) BC map; (b) IPF map.

Figure 8. Schematic diagram illustrating the mechanism by which ultrasound influences the solidification microstructure of droplets.

Figure 9.0.65: Nanoindentation of the powder cross-section at a loading rate of 5 mN/s and a load of 10 mN: (a) load–depth curve; (b) nanohardness and elastic modulus.

High-entropy alloys, as star contenders in the materials of the future, hold tremendous promise for a wide range of applications. Crucial to unlocking this potential is the production of high-quality powders. Ultrasonic vibration-assisted melt atomization stands out with its unique advantages, providing a robust platform for the high-quality fabrication of high-entropy alloy powders. The successful implementation of this technology not only overcomes the limitations of conventional processes but also ushers in a new era for the industrial-scale production of high-temperature, high-strength materials. Looking ahead, as the process continues to be refined and equipment is scaled up, ultrasonic vibration technology is poised to play an increasingly pivotal role in the manufacturing of a broader array of advanced materials, thereby setting the pace for emerging trends in materials science and engineering.