Black tungsten wire exhibits excellent stability at low to medium temperatures, making it suitable for long-term use. At high temperatures, the environment must be strictly controlled to prevent oxidation, and alloying or process optimization can enhance performance.
- Low to Medium Temperature Range (<1000°C) Stability Characteristics:
- High Mechanical Strength: With a melting point of 3422°C, tungsten maintains a stable crystal structure, high tensile strength, and hardness in this temperature range, with minimal plastic deformation.
- Limited Oxidation Resistance: In air, tungsten surfaces slowly oxidize to form tungsten trioxide (WO₃), but the oxidation rate is low, particularly in dry environments. At around 600°C, the oxidation rate may increase, but the material generally retains structural integrity.
- Application Scenarios: Commonly used in light bulb filaments, electron tube cathodes, etc., requiring a vacuum or inert gas environment to prevent oxidation.
- High Temperature Range (1000°C–2500°C) Stability Characteristics:
- Recrystallization and Grain Growth: Above tungsten’s recrystallization temperature (around 1200°C), grains rearrange and grow, leading to increased brittleness and reduced mechanical properties. Further temperature increases exacerbate grain coarsening, potentially causing fracture.
- Intensified Oxidation: In air, the volatilization rate of tungsten oxide increases significantly with temperature. Above 1500°C, the oxide layer may spall, exposing fresh metal surfaces and accelerating oxidation, ultimately leading to material depletion.
- Phase Transition Risk: Solid-state phase transitions (e.g., β→α phase) may occur under extremely high pressure or specific conditions, but these are rare in typical applications.
- Application Scenarios: Must be used in a vacuum, hydrogen, or inert gas environment, such as in high-temperature furnace heating elements or rocket nozzles.
- Extreme High Temperatures (Near Melting Point, 2500°C–3422°C) Stability Characteristics:
- Near Melting Point: As temperatures approach the melting point, tungsten nears a molten state, with a sharp decline in mechanical strength, leading to plastic flow or melting.
- Uncontrolled Oxidation: The volatilization rate of tungsten oxide becomes extremely high, and no protective oxide layer can form, resulting in continuous material loss.
- Application Limitations: Suitable only for short-duration high-temperature pulse scenarios (e.g., arc discharge) or requires coatings (e.g., rhenium, iridium) or composite materials (e.g., tungsten-rhenium alloys) to enhance stability.
Key Influencing Factors:
- Environmental Atmosphere: Oxidation is the primary failure mechanism for tungsten at high temperatures, and an inert environment significantly extends service life.
- Alloying: Adding elements like rhenium or molybdenum increases the recrystallization temperature, suppresses grain growth, and enhances high-temperature strength.
- Processing Techniques: Cold working (e.g., wire drawing) introduces dislocation strengthening, but annealing is necessary to relieve internal stresses and prevent brittle fracture at high temperatures.
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