Near-Eutectic Mo-Si-B Refractory Silicides for Molten Salt Reactor Heat Exchangers

Executive summary

Near-eutectic molybdenum-silicon-boron (Mo-Si-B) refractory silicide alloys are a compelling advanced-materials candidate for next-generation molten salt reactor (MSR) heat exchangers because they combine very high temperature capability with strong oxidation and creep resistance, but remain difficult to manufacture and qualify with conventional methods [3][5][6].

• MSR heat exchangers operate in a harsh regime: corrosive salts, elevated temperatures, welding/joining constraints, and compact geometry requirements all compound the materials problem [1][2].
• Existing nickel alloys (e.g., Hastelloy-class materials) are the practical baseline in many fluoride-salt discussions, but the temperature margin and long-term materials qualification envelope remain a core limitation for future systems [1][2].
• Near-eutectic Mo-Si-B alloys are attractive because multiphase Mo/Mo₃Si/Mo₅SiB₂ microstructures can be tuned for a practical balance of toughness, oxidation resistance, and creep strength [3][4][5].
• The catch is manufacturability: brittleness, high brittle-to-ductile transition temperature, and oxygen sensitivity make powder quality and process control central to success in AM and beyond [4][5][7].

Context

Molten salt reactor heat exchangers are a materials-intensive problem, not just a thermal design problem. INL work on molten-salt heat exchangers emphasizes that molten salts are often corrosive (especially as temperature rises), and that materials must simultaneously be corrosion resistant and creep resistant while supporting weldability and compact heat exchanger fabrication [1]. ORNL program summaries likewise note that primary containment and salt-exposed systems face corrosive environments above ~650°C and that long-term corrosion control depends strongly on salt chemistry and redox control [2].

This is why heat exchanger material selection becomes a strategic bottleneck for MSRs. The baseline engineering path often stays close to codified Ni-based alloys because they are familiar and testable, even if they do not fully solve the long-term temperature and lifetime ambition for advanced concepts [1]. For NMI’s Applications section, this makes “hard-to-manufacture, high-upside materials” the right lens: the value is not in relabeling existing alloys, but in highlighting credible alloy systems whose performance case is strong while manufacturability remains the barrier.

Near-eutectic Mo-Si-B refractory silicides fit that profile.

Technical analysis

Alloy class and composition concept

Mo-Si-B alloys are refractory multiphase silicide systems built around a ductile molybdenum solid solution (Mo_ss) plus intermetallic silicide/borosilicide phases such as Mo₃Si and Mo₅SiB₂ (often called the T2 phase) [3][4][5]. Their design logic is straightforward but nontrivial in practice:

• Mo_ss contributes fracture tolerance and crack bridging/trapping behavior.
• Mo₅SiB₂ (T2) contributes high-temperature strength and creep/oxidation resistance.
• Mo₃Si helps stabilize oxidation behavior by influencing silica/borosilicate scale chemistry and viscosity balance [3][4].

The “near-eutectic” framing matters because eutectic or eutectic-adjacent compositions tend to enable fine, regular multiphase microstructures that can improve processability and property uniformity relative to coarse cast structures. The Mo-Si-B literature explicitly documents eutectic and three-phase eutectic microstructures in this system, including directionally solidified eutectic variants [3]. In additive manufacturing, Fichtner et al. specifically reported processing a near-eutectic Mo-16.5Si-7.5B (at.%) powder composition, which makes this a real, published alloy path rather than a hypothetical one [6].

Why this alloy system is attractive for MSR heat exchangers

For compact heat exchangers in advanced reactor systems, the materials challenge is not one property; it is a portfolio of constraints:

1. Temperature margin
2. Creep resistance
3. Environmental resistance (oxidation during transients/fabrication, and salt compatibility in service)
4. Manufacturability into compact geometries
5. Joinability / qualification path

Mo-Si-B is attractive because it directly targets the first three items at a higher temperature class than conventional Ni-based alloys. Recent AM-focused Mo-Si-B work (Mo-9Si-8B) describes the alloy as exhibiting high oxidation, creep, and fracture resistance at high temperature, with a melting point around 2360°C [5]. The same paper also summarizes the core materials science rationale: Si and B additions improve oxidation resistance by enabling a protective silica-based scale, while the intermetallic phases provide high-temperature strengthening [5].

The older but still foundational Mo-Si-B review literature reinforces the same point: the best-performing alloys are not single-phase, but deliberately multiphase, and oxidation resistance depends strongly on phase balance and the Si:B chemistry available to the surface scale [3]. In particular, the literature highlights why a three-phase architecture can outperform simpler combinations in oxidation behavior [3].

For MSR heat exchangers, that combination is strategically relevant because the system-level requirement is often long-duration structural stability in a high-temperature, chemically aggressive environment. Even if direct salt compatibility must still be proven for a specific salt chemistry and redox condition (see limitations below), the temperature/creep margin case is strong enough to justify serious interest.

Limitations and engineering risks

This is not a drop-in material. The key limitations are exactly why it is an NMI-style applications topic.

1) Brittleness and high BDTT

Mo-Si-B alloys remain difficult to process because the intermetallic phases are brittle. The recent PBF-EB study explicitly notes that cracking is a central challenge and points out that the brittle-to-ductile transition temperature (BDTT) exceeds ~1000°C, which is why high preheat/process temperatures are required to avoid crack formation [5]. Broader Mo-Si-B reviews also reiterate brittleness and toughness tradeoffs as a defining constraint [4].

2) Conventional manufacturing limitations

Traditional processing routes (arc melting, powder metallurgy, hot pressing, repeated high-temperature treatments) can produce useful material, but they are costly, slow, and often still struggle with porosity, cracking, or microstructure control [4][5]. This is particularly problematic for heat exchanger architectures, where thin walls, internal passages, and repeatable dimensional accuracy matter.

3) Molten salt compatibility is not yet a solved problem for this alloy class

This is the most important caveat. INL and ORNL reports make clear that molten salt materials behavior is chemistry-dependent, impurity-dependent, and heavily influenced by redox control; even for well-studied alloys, qualification remains a major effort [1][2]. Mo-Si-B may be compelling from a temperature/creep perspective, but salt-specific corrosion performance must be demonstrated (fluoride vs chloride, impurity levels, redox control strategy, welds/joints, thermal cycling, and real service stresses).

In other words: this is a promising candidate class, not a pre-qualified solution.

Manufacturing considerations and advanced manufacturing relevance

Mo-Si-B is exactly the kind of alloy system where processing route determines viability.

Recent AM literature shows a credible path forward:

• LPBF (PBF-LB) feasibility has been demonstrated for near-eutectic Mo-Si-B when very high preheating is used (reported substrate temperatures around 1200°C) [5][6].
• PBF-EB is especially attractive for Mo-Si-B because high powder-bed temperature and vacuum conditions reduce thermal gradients and cracking risk in high-BDTT refractory systems [5].
• Complex geometry potential matters: the PBF-EB paper explicitly highlights PBF’s ability to make hollow structures and complex features, which is directly relevant to compact heat exchanger cores and manifolds [5].

Powder quality is a second major bottleneck. A powder production study focused on refractory AM alloys (including Mo-based silicide systems) highlights issues that matter directly for Mo-Si-B class materials: particle density, oxygen pickup, and impurity/moisture control [7]. It also cites Mo-Si-B LPBF work showing substantial oxygen pickup during thermal exposure, with crack-free builds requiring extensive impurity and moisture control [7]. That is a critical practical insight for any future deployment path.

Implications for LPBF

For LPBF specifically, near-eutectic Mo-Si-B is best understood as a materials-process co-design problem, not just a parameter optimization problem.

What matters most

• Powder chemistry control: oxygen and moisture are not minor variables; they materially affect embrittlement and crack risk [7].
• Preheating strategy: high preheat is effectively a prerequisite for dense, crack-limited LPBF in near-eutectic Mo-Si-B compositions because of the high BDTT [5][6].
• Process selection (LB vs EB): LPBF-LB can work, but process windows are narrow; PBF-EB is often the more natural fit for refractory silicides because of hotter powder beds and vacuum processing [5].
• Geometry choice: thin walls and microchannels amplify residual stress sensitivity, so composition, toolpath, and post-processing must be co-optimized.

Why additive manufacturing matters

MSR heat exchangers increasingly push toward compact, high-surface-area designs. INL work already frames compact heat exchanger fabrication as a major engineering and manufacturing problem, with additive manufacturing listed among relevant fabrication routes [1]. For a material like near-eutectic Mo-Si-B, AM is not just a convenience; it is one of the few routes that can plausibly combine complex internal geometry, tailored thermal history, and an eventual qualification-ready digital process record.

Implications for NMI

Near-eutectic Mo-Si-B is a strong NMI applications topic because it highlights a credible gap between material potential and manufacturing reality.

Conservative NMI-relevant takeaways

1. Composition opportunity (not a product claim)
   Near-eutectic and eutectic-adjacent Mo-Si-B compositions are already validated in the literature as technically meaningful [3][5][6]. The open problem is not whether the alloy family is real; it is whether a manufacturer can reliably produce powder and parts with the necessary chemistry, oxygen control, and repeatability.
2. Powder architecture matters
   For brittle refractory silicides, powder purity, particle density, and composition consistency are first-order variables, not secondary details [7]. This is exactly where a novel powder manufacturing approach could create differentiation, especially if it enables rapid composition iteration or tighter impurity control.
3. Qualification pathway is as important as fabrication
   MSR components are qualification-driven. A technically interesting alloy without a traceable evidence package (powder chemistry, process history, microstructure, corrosion testing, property data) is not a practical reactor material [1][2]. This aligns with NMI’s broader vision of pairing materials innovation with stronger qualification and conformance workflows.
4. No hype required
   The defensible position is not “NMI can make this today.” The defensible position is that near-eutectic refractory silicides are an example of the exact kind of high-value alloy system that becomes strategically important when powder manufacturing and qualification infrastructure improve.

Sources

1. Advanced Heat Exchanger Development for Molten Salts - Idaho National Laboratory / ASME - 2014 - https://www.osti.gov/servlets/purl/1162199
2. Advanced Reactor Technology Program Molten Salt Reactor Campaign FY 2018 Accomplishments Report (Pub117394) - Oak Ridge National Laboratory - 2018 - https://info.ornl.gov/sites/publications/Files/Pub117394.pdf
3. MoSiB Alloys for Ultrahigh-Temperature Structural Applications - Advanced Materials (author manuscript hosted by Lawrence Berkeley National Laboratory) - 2012 - https://www2.lbl.gov/ritchie/Library/PDF/2012_Lemberg_AdvMat_MoSiB.pdf
4. Review of Research Progress on Mo-Si-B Alloys - PubMed Central (open-access review) - 2023 - https://pmc.ncbi.nlm.nih.gov/articles/PMC10420173/
5. Rapid processing window development of Mo-Si-B alloy for electron beam powder bed fusion - PubMed Central / Progress in Additive Manufacturing - 2025 - https://pmc.ncbi.nlm.nih.gov/articles/PMC12479609/
6. Additive manufacturing of a near-eutectic Mo-Si-B alloy: Processing and resulting properties - Intermetallics - 2021 - https://doi.org/10.1016/j.intermet.2020.107025
7. Flexible Powder Production for Additive Manufacturing of Refractory Metal-Based Alloys - Metals (MDPI) - 2021 - https://www.mdpi.com/2075-4701/11/11/1723