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Refractory Alloys & Necessity

Writer's picture: Dr. C. V. S. KiranDr. C. V. S. Kiran

A focus on materials with superior high-temperature strength, oxidation resistance and wear resistance has always been the focus of World's materials experts. Be it either for Ultra-High Temperatures (Above 1200°C) or for Aerospace and Turbine Applications or for Wear and Abrasion Resistance. Each alternative has unique advantages tailored to specific operational requirements, making material selection dependent on the specific application.

Advanced metallurgy, extreme heat resistance testing and precision engineering
Advanced metallurgy, extreme heat resistance testing and precision engineering

E.g., Operating in high-temperature oxidizer-rich environments without protective coatings is a significant challenge due to the aggressive nature of such environments, which can cause oxidation, hot corrosion and erosion. Most alloys require coatings to enhance their performance, but some materials have intrinsic properties that make them better suited for these conditions.

Refractory metals are Tungsten, Rhenium, Tantalum, Molybdenum and Niobium. These pure metals have the following

  • Tungsten (W) 3380°C, BCC

  • Rhenium (Re) 3180°C, HCP

  • Tantalum (Ta) 3014°C, BCC

  • Molybdenum (Mo) 2617°C, BCC

  • Niobium (Nb) 2468°C, BCC

Further on come the list of metals in the periodic table like

  • Osmium (Os) 3027°C, HCP

  • Iridium (Ir) 2447°C, FCC

  • Ruthenium (Ru) 2250°C, HCP

  • Hafnium (Hf) 2227°C, HCP

  • Technetium (Tc) 2200°C, HCP (Radioactive)

  • Rhodium (Rh) 1963°C, FCC

  • Vanadium (V) 1902°C, BCC

  • Chromium (Cr) 1857°C, BCC

  • Zirconium (Zr) 1852°C, HCP

  • Titanium (Ti) 1670°C, HCP

Osmium has a high melting point, but not always considered a refractory metal, although it has the 3rd highest melting point, as it is rarely used at high temperatures, due to its toxic oxide.

Don't forget

Refractory metals

  • have a very high melting point

  • have a close-packed or nearly close packed crystal structures

  • tend to have a high density

  • are also brittle

  • oxidize easily

  • relatively creep resistant

Since they often have low diffusion rates, they are especially useful for alloying with other elements to improve properties of the base alloy. Specifically, they are often added for creep resistance in alloys. Rhenium is famously slow in nickel-based superalloys and gives rise to the “rhenium effect” of excellent creep resistance in nickel-based superalloys.


Rhenium contributes significantly to the mechanical and thermal properties of these alloys, making them suitable for extreme environments. The Rhenium Effect refers to the unique influence of rhenium (Re) as an alloying element in various metal systems, particularly in high-temperature and high-performance alloys like nickel-based superalloys, molybdenum alloys, and tungsten alloys. Alloys like Ni-based superalloys with small amounts of rhenium exhibit superior creep and tensile strength. Due to Atomic Size Mismatch, strong metallic bonds and Low Diffusivity, Rhenium Addition leads to:

  • Improved High-Temperature Strength resulting from Solid Solution Strengthening as a result of large atomic size and high melting point which enable it to impede dislocation motion in the alloy matrix.

  • Enhanced Creep Resistance resulting from Grain Boundary Stabilization, as a result of reduced grain boundary sliding at high temperatures. This is particularly notable in alloys like Rene N6 and CMSX-4, used in single-crystal turbine blades.

  • Increased Phase Stability resulting from stabilization of the γ' strengthening phase and delaying coarsening during prolonged high-temperature exposure

  • Oxidation and Corrosion Resistance

  • Improved Ductility

  • Thermal Fatigue Resistance

Although advantageous, limitations like High Cost, Oxidation Vulnerability, Processing Challenges due to high hardness exist which need significant alloy design, simulations and engineering.

In alloy design, the following steps depict the design process of refractory high-temperature and high-strength alloys:

  • Defining Application Requirements

    • Operating Temperature,

    • Mechanical Properties

      • strength

      • creep resistance

    • Environmental Conditions

      • Oxidizer-rich environments,

      • Corrosion factors

    • Density Constraints

  • Selection of Base Refractory Metals

  • Alloying Element Additions

    • For Oxidation Resistance

      • Elements: Silicon (Si), Boron (B), Chromium (Cr), Aluminum (Al) - forming protective oxide layers (e.g., SiO₂, Al₂O₃).

    • For Creep and Strength Enhancement

      • Elements: Rhenium (Re), Hafnium (Hf), Titanium (Ti).

    • For Density Reduction

      • Elements: Titanium (Ti), Aluminum (Al)

  • Microstructural Control

    • Grain Size Refinement

    • Precipitation Hardening

  • Computational Modeling and Simulation

    • CALPHAD Method

    • Machine Learning

  • Testing and Optimization

    • Laboratory Equipment

    • Mechanical Testing Machines

      • Thermal Testing

      • Mechanical Testing

  • Final Alloy Selection

  • Balanced Properties

  • Cost Considerations

  • Application Ready


Refractory high-temperature alloys are essential for applications requiring durability in extreme environments. By incorporating elements that improve oxidation resistance, creep strength and thermal stability, alloy design can achieve superior performance with/without reliance on coatings. Here is a list of some of the existing alloys which can cater to high temperature applications


Nickel-Based Superalloys (Advanced Versions of Inconel 718)

  • Rene 41

    • Excellent creep strength and oxidation resistance up to 980°C.

    • Commonly used in jet engine components and gas turbines.

  • Hastelloy X

    • Exceptional strength and oxidation resistance up to 1200°C.

    • Highly resistant to carburization and thermal fatigue.

  • Haynes 282

    • Designed for high-temperature structural applications.

    • Superior creep resistance compared to Inconel 718, with good weldability.

Nickel-Cobalt-Based Superalloys

  • MAR-M 509

    • Contains cobalt, tungsten and chromium, offering excellent wear resistance and high-temperature strength up to 1100°C.

  • Haynes 188

    • Cobalt-based with outstanding oxidation resistance and thermal stability up to 1200°C.

    • Used in combustors, liners and transition ducts in gas turbines.

Molybdenum-Based Alloys

  • TZM Alloy (Titanium-Zirconium-Molybdenum)

    • Excellent strength and creep resistance at temperatures above 1200°C.

    • Commonly used in aerospace and nuclear applications.

  • Mo-Si-B Alloys

    • Resistant to oxidation and creep at temperatures >1400°C.

    • Used in turbine blades and heat shields.

  • Molybdenum-Rhenium Alloys (Mo-Re)

    • High-temperature strength and ductility, with enhanced resistance to cracking.

Tungsten-Based Alloys

  • W-Re Alloys (Tungsten-Rhenium)

    • Used in rocket nozzles and aerospace components for extreme heat applications.

  • W-Re-Hf Alloys

    • Excellent for extreme environments like rocket nozzles and space thrusters.

  • Tungsten Heavy Alloys (W-Ni-Fe)

    • Exceptional wear resistance and density, suitable for radiation shielding and high-impact environments.

Ceramic-Metal Composites (Cermets)

  • TiC-Co (Titanium Carbide-Cobalt)

    • Exceptional wear resistance and thermal conductivity.

    • Commonly used in cutting tools and high-wear aerospace components.

  • WC-Co (Tungsten Carbide-Cobalt)

    • Excellent for wear and abrasion resistance at moderate to high temperatures.

Niobium-Based Alloys

  • High melting point and good oxidation resistance, used in aerospace applications like rocket engines.

  • C-103(NbSi2, Cr3Si, Fe3Si2; Interface: Nb5Si3; Oxidation Temperature: 1300◦C)

    • Excellent for oxidizer-rich environments due to its ability to resist oxidation and maintain strength at elevated temperatures.

    • Commonly used in aerospace engines and rocket nozzles.

  • Nb521(NbSi2,MoSi2; Interface: Nb5Si3; Oxidation Temperature: 1700◦C)

  • Nb-Si(NbSi2,Nb4Si5CrFe3, Fe4Nb4Si7; Interface: Nb5Si3; Oxidation Temperature: 1400◦C)

  • Nb-Si-Ti(NbSi2,(Fe, Cr)3Si2; Interface: Nb5Si3; Oxidation Temperature: 1400◦C)

Tantalum-Based Alloys

Tantalum Tungsten Alloys (Ta-W)

  • Intrinsically resistant to oxidation and corrosion in aggressive oxidizing atmospheres.

  • Tantalum forms a protective oxide layer that self-heals under high temperatures.

  • Applications: Aerospace, nuclear reactors and chemical reactors.

  • Superior corrosion resistance and stability in extreme environments.

  • R512E (MoSi2, M5Si3; Interface: Nb5Si3; Oxidation Temperature: 1200◦C)

Rhenium Alloys (Co-Re, W-Re, Mo-Re)

Rhenium enhances the oxidation resistance and creep strength of alloys like tungsten and molybdenum. They are highly resistant to oxidation and have excellent thermal stability, making them suitable for oxidizer-rich environments. Applications include Rocket nozzles, turbine blades and combustion chambers.

Co-Re Alloys

Co-Re alloys (Cobalt-Rhenium alloys) are high-performance materials primarily developed for applications requiring high-temperature strength, corrosion resistance and excellent wear properties. These alloys combine the characteristics of cobalt (Co) and rhenium (Re) to achieve remarkable mechanical and physical properties. They have superior high-temperature performance compared to other cobalt-based or nickel-based alloys. Also, enhanced ductility which reduces the risk of mechanical failure in critical applications in addition to excellent fatigue and creep resistance under cyclic loading conditions.

Properties

  • Co-Re alloys retain their mechanical strength and resistance to deformation at elevated temperatures, often exceeding 1200°C.

  • These alloys resist oxidation and corrosion in harsh environments, making them suitable for aerospace and turbine applications.

  • High wear resistance due to the formation of strong carbide phases when combined with other elements (e.g., chromium or tungsten).

  • Rhenium enhances the ductility of cobalt alloys, making them more resistant to cracking under stress.

Composition

  • Cobalt (Co): Base element, providing strength, wear resistance and magnetic properties.

  • Rhenium (Re): Added in varying amounts (typically 3–10%), improving high-temperature performance and ductility.

  • Additional elements like:

    • Chromium (Cr): Enhances corrosion resistance.

    • Tungsten (W) or Tantalum (Ta): Improves hardness and strength.

    • Carbon (C): Forms carbides for increased wear resistance.

Difficulties

Rhenium is an expensive and rare metal, increasing the overall cost of Co-Re alloys. Difficult to manufacture and process due to high melting points and complex metallurgy. Rhenium’s scarcity limits the widespread adoption of Co-Re alloys.

High-Entropy Alloys (HEAs)

  • CoCrFeNiMn Alloy

  • AlCoCrFeNiTi Alloy


Designing refractory high-temperature and high-strength alloys involves leveraging elements like tungsten (W), molybdenum (Mo), tantalum (Ta), niobium (Nb) and rhenium (Re), known for their high melting points, creep resistance, and oxidation resistance. These alloys are pivotal in extreme environments, such as aerospace, energy and nuclear industries.


Highly complex products can be developed using cutting-edge metal printing technologies, such as LASER powder bed fusion. Can this HELP the Refractory alloy sector?

References are in the process of Updation. Will be updated soon....





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