Effect of LaB6 Particles on the Microstructure and High-Temperature Oxidation Resistance of Plasma-Cladded Inconel 625 Coatings

Literature Overview

This study investigates the influence of lanthanum hexaboride (LaB₆) particle addition on the microstructure and high-temperature oxidation resistance of plasma transferred arc (PTA) cladded Inconel 625 (IN625) alloy coatings. The work addresses a critical challenge in high-temperature component protection: enhancing the oxidation resistance of nickel-base alloy coatings through rare earth element (REE) modification. LaB₆ is selected as a REE source due to its low melting point (845°C), chemical stability, and ability to modify grain boundaries and oxide scale morphology in metallic coatings.

Core Technical Content

Material System and Modification Strategy

Inconel 625 is a Ni-Cr-Mo-Nb alloy (Ni-21Cr-9Mo-3.5Nb) widely used for cladding in high-temperature, corrosive environments. Its primary limitation is moderate oxidation resistance above 800°C, where the protective Cr₂O₃ scale becomes unstable and spallation occurs. The addition of LaB₆ particles aims to:

1. Refine grain structure: LaB₆ particles act as heterogeneous nucleation sites, reducing grain size and increasing grain boundary density.
2. Modify oxide scale morphology: La³⁺ ions segregate to oxide scale grain boundaries, inhibiting cation diffusion and promoting a denser, more adherent oxide scale.
3. Introduce reactive element effects: Similar to Y and Ce, La modifies the thermodynamic stability of oxide phases and reduces scale spallation tendency.

Microstructural Analysis

The study reveals significant microstructural modifications with LaB₆ addition:

Grain structure:

• Unmodified IN625: Columnar grains with average width 50–80 μm
• 2% LaB₆: Columnar grains refined to 35–55 μm
• 5% LaB₆: Columnar grains refined to 25–45 μm
• 10% LaB₆: Equiaxed grains of 15–30 μm, indicating transition to equiaxed morphology

Phase composition:

• Matrix: γ-Ni solid solution (FCC)
• Precipitates: Nb-rich Laves phase (Ni₃Nb), δ-ferrite (in dilution zone)
• LaB₆-derived phases: La₂O₃, La₂O₂CO₃ (at grain boundaries and oxide scale)
• Carbides: TiC, NbC (in dilution zone)

Dilution zone:

• LaB₆ particles concentrate at the substrate-overlay interface, creating a La-enriched zone
• Dilution rate increases slightly with LaB₆ content (from 12% to 18%) due to altered melting behavior
• δ-ferrite content increases with LaB₆ addition, requiring careful control to avoid cracking

High-Temperature Oxidation Behavior

The oxidation testing is conducted at 900°C in air, with weight gain measured over 100 hours:

Oxidation kinetics:

• Unmodified IN625: Linear kinetics (W ∝ t), indicating non-protective scale
• 2% LaB₆: Parabolic-linear mixed kinetics
• 5% LaB₆: Parabolic kinetics (W² ∝ t), indicating protective Cr₂O₃ scale
• 10% LaB₆: Parabolic kinetics with slight deviation due to La₂O₃ scale formation

Oxide scale composition (XRD analysis):

• Unmodified: Cr₂O₃, NiO, Fe₂O₃ (in dilution zone)
• 5% LaB₆: Cr₂O₃ (dominant), minor NiO, La₂O₃ at scale surface
• 10% LaB₆: Cr₂O₃, La₂O₃, LaCrO₃ (chromite-type oxide)

Mechanical Properties

The increase in hardness with LaB₆ addition is attributed to grain refinement and precipitation of La-containing phases. However, the decrease in bond strength at 10% LaB₆ suggests that excessive LaB₆ content promotes brittle phase formation at the interface.

Engineering Practice Insights

Process Optimization

The study provides clear guidance for optimizing LaB₆ content in IN625 cladding:

1. Optimal LaB₆ content: 5 wt% provides the best balance of oxidation resistance, mechanical properties, and processing stability.
2. Powder preparation: LaB₆ particles must be thoroughly mixed with IN625 powder to ensure uniform distribution. Sieving to 45–75 μm and mixing for 30 minutes in a planetary mixer is recommended.
3. Arc stability: LaB₆ addition slightly increases arc stability due to its low melting point, but excessive content (>8%) can cause spatter and uneven deposition.
4. Interpass temperature: Must be controlled below 200°C to prevent La segregation at grain boundaries and subsequent cracking.

Common Defects and Countermeasures

Standards and Qualification

The qualification of La-modified IN625 coatings requires custom testing beyond standard overlay qualification, including:

• Extended oxidation testing at service temperature
• Thermal cycling testing (RT to 900°C, 100 cycles)
• Creep testing at 800–900°C
• Metallographic examination for La phase distribution

Key Questions and Reflections

The study raises several important questions for practical application:

• Long-term oxidation behavior: The 100-hour oxidation test provides useful data, but practical components may experience thousands of hours of service. How does the LaB₆ modification perform under prolonged exposure?
• Thermal cycling resistance: The study focuses on static oxidation, but practical components experience thermal cycling. Does LaB₆ modification improve or degrade thermal cycling resistance?
• Creep behavior: The addition of LaB₆ may affect creep strength at 800–900°C. Is the improvement in oxidation resistance worth a potential reduction in creep life?
• Cost implications: LaB₆ is a relatively expensive material. What is the economic justification for its addition, considering the improved oxidation resistance?
• Environmental concerns: La is a rare earth element with environmental and supply chain considerations. Are there alternative REE sources (e.g., CeB₆, YB₆) that provide similar benefits?

Study Insights and Implications

This study demonstrates that LaB₆ modification is a viable strategy for enhancing the high-temperature oxidation resistance of IN625 cladding coatings. The optimal 5% LaB₆ content provides a significant improvement in oxidation resistance (60% reduction in weight gain) without compromising mechanical properties or bond strength.

The practical implications for high-temperature component protection are significant. For applications such as gas turbine components, furnace parts, and heat exchangers operating at 800–900°C, LaB₆-modified IN625 cladding could extend service life and reduce maintenance intervals. However, the study also highlights the need for careful process control and qualification to ensure consistent performance.

From a metallurgical perspective, the study provides insight into the mechanisms of reactive element effects in metallic coatings. The segregation of La to oxide scale grain boundaries inhibits cation diffusion and promotes a dense, adherent Cr₂O₃ scale. This understanding can be applied to the design of other REE-modified coatings for specific service conditions.

The most valuable takeaway is the demonstration that minor additions of REE-containing compounds can significantly improve the performance of conventional overlay alloys. This opens avenues for further research into other REE sources, modification strategies, and application-specific optimization. Engineers working in high-temperature component protection should consider LaB₆ modification as a viable option for IN625 cladding, subject to proper qualification and process control.