Compatible (with molten FLiBe) Structural Materials

[BS-Fusion]

This topic initiates a focused discussion on candidate materials for a containment sphere intended to interface with molten Yb:FLiBe subjected to cyclic thermomechanical loading and intense pressure surges.

The baseline candidate is Alloy 4917, selected for its high-temperature creep resistance. Alloy 4917 is a γ′-strengthened nickel-based alloy developed by Oak Ridge National Laboratory (ORNL) for high-temperature service in molten fluoride salt environments, particularly FLiBe (LiF-BeF₂). It was designed to outperform traditional materials like Hastelloy-N in both creep resistance and corrosion stability, making it a strong candidate for structural components in advanced nuclear reactors and concentrated solar power systems.

Key Properties of Alloy 4917

Property	Value / Behavior
Base composition	Ni-rich with Mo, low Cr, γ′ precipitates
Max service temp	~850 °C (tested)
Creep rupture life	>2× that of Hastelloy-N at 850 °C
Corrosion resistance	Excellent in FLiBe and molten chlorides
Microstructure	Fine spherical γ′ precipitates (~30 nm)
Fabrication	Scaled up successfully by Haynes Intl
Weldability	Still under evaluation

Performance Highlights

• Creep strength: Alloy 4917 showed superior creep rupture life compared to other ORNL-developed alloys (e.g. 4817, 5017, 5217) at 850 °C and 12 ksi (82.7 MPa).
• Corrosion testing: After 500-hour exposures in FLiBe at 704 °C, 4917 had low normalized corrosion rates, outperforming legacy alloys.
• Microstructure stability: Aging treatments (e.g. 760 °C for 16 hours) produced stable γ′ precipitates that enhance strength without compromising ductility.

Applications

• Fusion reactor components: Structural parts exposed to FLiBe coolant and high neutron flux.
• Molten salt reactors (MSRs): Vessel walls, heat exchangers, and piping.
• Concentrated solar power (CSP): High-temperature salt containment and transport systems.

Design Considerations

• Low chromium content: Avoids CrF₃/CrF₄ formation and leaching in FLiBe.
• γ′ strengthening: Offers high-temperature mechanical stability, but may complicate weldability.
• Not yet ASME code-qualified: Requires further testing for commercial deployment.

[BS-Fusion]

Iridium-Coated TZM Mirror Tiles: Balancing Survivability and Reflectivity in Molten FLiBe

To ensure reliable performance in direct contact with molten FLiBe at approximately 850 °C, the mirror tile design requires precise engineering. The proposed configuration consists of:

• Geometry: Hexagonal tiles, 20–25 cm across
• Thickness: 3–5 mm
• Edge treatment: Beveled edges to mitigate localized stress concentrations
• Interface design: Graded transition layer (e.g., Mo–Ir alloy) to reduce thermal expansion mismatch between Iridium and TZM

Structural Geometry Justification

Hexagonal tiling minimizes interfacial seams. The selected size range optimizes area coverage while limiting the likelihood of tile warping, delamination, or fatigue under thermal cycling and vibration.

Iridium Coating Selection

Iridium offers superior reflectivity at the target wavelength of 1050 nm (~96–98%), with stability at operating temperatures. It demonstrates exceptional chemical resistance in molten fluoride environments, owing to its low solubility and noble character. Thin-film deposition (~15 µm) is preferred to balance material cost and manage differential expansion stresses.

TZM Substrate Suitability

TZM (Titanium-Zirconium-Molybdenum alloy) provides high-temperature mechanical strength and creep resistance up to ~1000 °C. Its thermal expansion coefficient (~6.4 µm/m·K) is well-aligned with that of Iridium (~5.2 µm/m·K), reducing interfacial strain. The material is machinable and more cost-effective than pure Re or Ir substrates, with proven compatibility for Mo-based bonding layers.

Incorporating a graded interface and maintaining modest tile dimensions further reduces risk of structural failure under combined thermal and mechanical loads.