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Application of CB752 Alloy in Thermal Protection Components for Hypersonic Vehicle Leading Edges and Comparison with Rare Refractory Metals
CB752 alloy (Nb-752; composition: niobium matrix + 10% tungsten + 2.5% zirconium) is a classic solid-solution strengthened niobium-based refractory alloy. With a melting point of 2425°C, it maintains stable mechanical properties during long-term exposure to high temperatures (1000–1400°C) and exhibits excellent room-temperature ductility, making it suitable for stamping and welding. It is a prime candidate material for thermal protection structures such as leading edges, wingtips, and nose cones of hypersonic vehicles. During high-speed flight, aerodynamic heating at the leading edge exceeds 1200°C; materials must therefore offer high-temperature creep resistance while remaining lightweight and vibration-resistant. Ordinary carbon steels and titanium alloys fail rapidly at these temperatures, whereas rare refractory metals like tungsten, molybdenum, and tantalum suffer from excessive weight and significant room-temperature brittleness. CB752 fills this application gap through its balanced performance profile, emerging as a primary material for next-generation reusable hypersonic thermal structures.
Conventional industrial pure metals are entirely unsuitable for the operating conditions of hypersonic leading edges. Pure titanium and Ti6Al4V have a maximum long-term service temperature of only 350°C; their strength drops precipitously above 400°C, and they suffer from severe high-temperature oxidation and scaling, rendering them incapable of withstanding aerodynamic heating exceeding 1000°C. High-temperature nickel-based alloys have a maximum service temperature below 950°C; above 1100°C, they undergo softening and creep, leading to structural deformation and failure. Ordinary structural steels have low melting points—softening significantly at 800°C—and possess high density and extremely poor high-temperature oxidation resistance. Pure aluminum and copper have even weaker high-temperature capabilities, limiting them to auxiliary structures at ambient temperatures and rendering them useless for thermal protection applications. Even pure niobium, while meeting melting point requirements, has a room-temperature tensile strength of only around 300 MPa and insufficient high-temperature creep resistance; it deforms easily under long-term cyclic thermal loads, making it unsuitable for use as a load-bearing thermal structural component. When compared with four mainstream rare refractory metals—tungsten, molybdenum, tantalum, and hafnium—CB752 stands out for its exceptional engineering suitability. Tungsten boasts a melting point of 3420°C and superior high-temperature hardness, but its density (19.3 g/cm³) is 2.1 times that of CB752 (9.03 g/cm³); extensive use would drastically increase an aircraft's dead weight and reduce maneuverability (g-load capacity). Furthermore, tungsten is extremely brittle at room temperature, prone to cracking during bending or stamping, and notoriously difficult to weld, limiting it to small-scale test specimens. Molybdenum has a melting point of 2620°C and excellent high-temperature creep resistance, yet its density (10.2 g/cm³) exceeds that of CB752, and its ductile-to-brittle transition temperature is relatively high, making it susceptible to brittle fracture from impacts in cold, high-altitude environments; additionally, its processing costs are more than double those of CB752. Tantalum offers both corrosion and heat resistance with a melting point of 3017°C, but its density (16.65 g/cm³) makes it excessively heavy; combined with scarce mineral resources and high procurement costs, it is economically unviable for large-scale deployment. Hafnium exhibits excellent high-temperature oxidation resistance but suffers from scarcity, high costs, and moderate plasticity, restricting its use to niche, extreme-temperature components rather than large-scale structural applications. By incorporating tungsten for solid-solution strengthening at high temperatures and zirconium for grain refinement and precipitation strengthening (enhancing creep resistance), CB752 achieves an ideal balance between high-temperature performance and engineering practicality—prioritizing lightweight design, room-temperature plasticity, processability, and cost-effectiveness while sacrificing only a small margin of ultimate melting point.
Compared to other niobium-based alloys, CB752 offers distinct advantages in balanced performance: Nb-1Zr alloy possesses better plasticity but lower high-temperature strength and significantly weaker creep resistance than CB752; Nb-Hf series high-strength niobium alloys offer superior high-temperature performance but are highly sensitive to impurities and require rigorous welding processes, resulting in low mass-production yields; and the aerospace-grade Nb521 alloy entails higher costs, making its overall cost-performance ratio inferior to that of the mature, mass-produced CB752. CB752 exhibits a room-temperature tensile strength of 510–550 MPa and an elongation of 20%–30%, with mature processing capabilities for bending, stamping, and laser welding. It maintains a tensile strength exceeding 120 MPa at 1300°C and demonstrates excellent resistance to high-temperature creep and thermal shock; it does not crack or spall under repeated thermal cycling, making it suitable for leading-edge applications subject to periodic aerodynamic heating.
CB752 is currently undergoing on-vehicle testing for leading-edge components on multiple domestic hypersonic demonstrator aircraft. It replaces original molybdenum alloy test structures and heavy tungsten-based protective tiles, achieving a total vehicle weight reduction of over 25% and a threefold increase in the service life of thermal structures, while effectively lowering the overall weight and thermal-stress-induced deformation of the thermal protection system. The material's current limitation is relatively weak high-temperature oxidation resistance; without a coating, the long-term oxidation rate at 1200°C is high, necessitating the use of a complementary silicide oxidation-resistant coating. Future development directions include: first, using rare-earth doping to enhance intrinsic high-temperature oxidation resistance and reduce reliance on coatings; second, employing additive manufacturing to integrally form complex, non-standard leading-edge structures, thereby eliminating the risks of thermal stress at welded joints; and third, utilizing low-cost vacuum melting processes to reduce costs, facilitating the transition of CB752 from experimental prototypes to a standard thermal structural material for hypersonic vehicles, and continuing to replace various brittle, excessively heavy, rare refractory metals.
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