ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2025 44 iTSSe TSS iTSSe TSS First manufactured in 2010, refractory high entropy alloys (RHEAs) were conceived for aerospace applications requiring high-strength materials with better structural performance than high-nickel superalloys[1]. Since then, much progress has been made in the field, such as the use of diverse manufacturing processes and specialized implementations (e.g., thick coatings and low-density alloys)[2-4]. This has resulted in a multitude of applications for the energy and aerospace sectors[5-8]. BACKGROUND RHEAs typically consist of equiatomic combinations of four to six refractory elements[1-2]. Note that an equiatomic composition increases the solid solution entropy of the material, which is a measure of the ability of the material to mix. Therefore, the alloy can reach a higher degree of homogeneity, which can remarkably increase the material strength beyond the capacity of any one of its individual components. For example, the yield strength of HfNbTaZr is about 2320 MPa. By contrast, the yield stress of individual components based solely on either Hf, Nb, Ta, or Zr, is approximately 125 to 362, 50.3 to 207, 380, and 230 MPa, respectively; clearly, the RHEA composition is much stronger than the individual components. Moreover, higher entropy generation reduces the magnitude of the Gibbs formation energy, while the random distribution of the refractory elements increases the material performance metrics[9-10]. Not surprisingly, some RHEA combinations are noted for having excellent wear, abrasion, and/or creep resistance, such as HfNbTiZr, MoNbTaVW, and MoNbTaTiZr[11-13]. Based on the RHEA design, manufacturing, and testing experience of the authors’ team, a point has been reached whereby thick-surface layer thermal spray processes generate corrosion-resistant bimetallic RHEAs to effectively resolve the corrosion issue for both chloride- and fluoride- based molten salt reactors (MSRs)[13]. The starting compositions for the RHEA feedstocks used in the present study are typically multicomponent equiatomic mixtures. However, it is the capability of the thermal spray process that enables the formation of a bimetallic alloy via a transient quenching thermal spray mechanism. MANUFACTURING METHODS The RHEA powder is formed by first generating elemental powders for each of the refractory element feedstocks, to form individual batches of element powder. Then, the element powders are milled together, thereby generating a homogeneous RHEA powder. Planetary ball mills have been used for decades to reduce particle size at laboratory and pilot scales, and in the last few years the application of planetary ball mills has extended to mechano- chemical approaches[13]. This milling method involves placing the material to be processed along with grinding balls into rotating jars. The jars rotate on their own axis and also orbit around a central axis, resulting in a unique grinding motion. The collision of the grinding balls with the material leads to size reduction through mechanical impact and attrition, resulting in a homogeneous powder after several hours of milling. Planetary milling is suitable for large-scale production of powders for mass-production of RHEA components. Where increased atomic distribution is desired, such as in defense components and quantum hardware components, cryo milling can produce nanograined metal alloys without the interaction of oxygen. During this milling process, the metal mixtures are placed under liquid nitrogen to protect them from exposure to air and oxidation. Because of its processing complexity, cryo milling results in finer alloy powders, but is about 30% more expensive than planetary milling. Normally, RHEA powder cryo and planetary milling is done under controlled, inert conditions to avoid O2 oxidation. However, small quantities of O2 may be desired to form refractory element oxides. The authors’ research and that of the literature confirm that RHEA oxides can provide significant corrosion resistance[1]. The authors’ team can provide one-of-a-kind optimization of O2 dispersal within the RHEA on a rapid, large-scale basis. An example of a RHEA consisting of five different refractory element powders is shown in Fig. 1. In particular, the individual elemental powders are generated via the team’s integrated, plasma spheroidization approach, which LEADING-EDGE SYNTHESIS OF LARGE-SCALE, COMMERCIAL-GRADE RHEA COATINGS Refractory high entropy alloys can be processed into powders and applied as highperformance coatings offering superior strength and corrosion resistance for aerospace, energy, and nuclear applications. Satish Dixit,* Plasma Technology Inc., Torrance, California Sal Rodriguez, Sandia National Laboratory, Albuquerque, New Mexico *Member of ASM International FEATURE 10
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