iTSSe TSS ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2025 45 iTSSe TSS FEATURE generates separate powder batches having purities greater than 99% for each of the refractory elements. Then, the elemental powders are combined and undergo planetary or cryo milling, thereby forming a homogeneous, high-quality RHEA powder, as noted on the right-hand side of Fig. 1. cubelike geometry, as shown conceptually on the right side of Fig. 3. The individual BCC lattice is on the order of a few nm and is therefore impacted by short range order (SRO). It is in this region that SRO can be an important synthesis parameter during thermal sprays. For example, SRO lattice nanoparticles can be formed, and later dispersed into the RHEA powder as a strengthening mechanism; the literature indicates that nano RHEA particles can have yield strength that is two to three times higher than at the macro scale. Fig. 3 — Formation of BCC lattices. Fig. 2 — Microstructure of thermal spray RHEA CrNi2TaVW on SS 316 substrates. All three images show Ta (cold spray bond coat) and CrNi2TaVW but with varying topcoats. (a) cold spray topcoat, (b) plasma spray topcoat, and (c) HVOF spray topcoat. (a) (b) (c) Fig. 1 — Formation of the RHEA powder. TESTING AND ANALYSIS The homogeneous powders were used to generate thin and thick RHEA coatings via cold spray, atmospheric plasma spray and high-velocity oxygen fuel (HVOF). Figure 2 shows the microstructures of the cold spray, plasma spray, and HVOF coated RHEA CrNi2TaVW on tantalum cold spray bond coated SS 316 substrates. All these coatings were subsequently subjected to molten salt corrosion testing at Sandia National Laboratories to simulate a nuclear MSR environment. The RHEA powders were synthesized, forming a solid solution coating having myriad complex crystal formations, with the vast majority comprising of body-centered cubic (BCC) lattices. Face-centered cubic (FCC) and hexagonal close-packed (HCP) lattices, as well as laves and other complex geometries, were also formed; the left-hand side of Fig. 3 shows the conceptual formation of a five-element RHEA BCC lattice. A key point is that thermodynamic principles cause a RHEA powder to form a solid or coating as a random mixture of different lattices. It is statistically impossible to generate a large-scale component that is purely BCC or FCC, etc. Instead, RHEAs are comprised as a random mixture of several types of lattices. For example, a RHEA can be comprised of 70% BCCs, 20% FCCs, 5% HCPs, 3% laves, and the remainder 2% of other miscellaneous formations; such distributions are the rule, not the exception. Moreover, even if a micro-RHEA batch hypothetically could be formed purely of BCCs, each BCC lattice would be different in terms of size and composition, as noted in Fig. 3. Because each type of refractory atom has a different diameter, the interatomic forces between the atoms cause the cube to distort throughout the RHEA as a non-repeating 11
RkJQdWJsaXNoZXIy MTYyMzk3NQ==