Tungsten is generally considered brittle at low temperatures, but its behavior at high temperatures is more complex. At elevated temperatures, tungsten can exhibit improved ductility, especially when it is in a pure state or when specific conditions are met. However, it can also become brittle under certain conditions, such as when it is impure or exposed to oxidizing environments.
Summary of Answer:
- Low Temperature Behavior: Tungsten is brittle at low temperatures due to its high ductile-to-brittle transition temperature (DBTT), which is usually above room temperature.
- High Temperature Behavior: At high temperatures, tungsten can become more ductile, particularly in pure forms or when alloyed and treated appropriately. However, it can also become brittle if exposed to oxidizing conditions or if it contains impurities.
Detailed Explanation:
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Low Temperature Brittleness: Tungsten, like other refractory metals, has a high ductile-to-brittle transition temperature. This means that below a certain temperature, it behaves in a brittle manner, lacking the ability to deform plastically. This characteristic makes tungsten difficult to machine or fabricate at room temperature or lower temperatures. The reference mentions that pure tungsten is difficult to machine and cannot be easily drawn or extruded into tubes, primarily due to its brittleness at these temperatures.
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High Temperature Ductility: At higher temperatures, tungsten's behavior changes significantly. Above its DBTT, tungsten becomes more ductile, allowing it to be formed and worked more easily. This is particularly true for tungsten wire, where the application of heat enhances its formability. The reference notes that tungsten retains its strength well at elevated temperatures and is used in heating elements up to very high temperatures due to its high melting point and resistance to oxidation and creep.
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Influence of Impurities and Oxidation: Despite its ductility at high temperatures, tungsten can become brittle if it is not pure or if it is exposed to oxidizing conditions. The reference highlights that tungsten oxidizes rapidly in air above 1200°C, which can lead to embrittlement and failure. This necessitates the use of protective atmospheres or vacuum conditions when operating tungsten at high temperatures to prevent oxidation and maintain its mechanical integrity.
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Alloying and Heat Treatment: The mechanical properties of tungsten, including its ductility and brittleness, can be modified through alloying and heat treatment. Adding elements like cerium or lanthanum can improve its creep strength and overall mechanical properties. Metalworking and alloying can also reduce the DBTT, making tungsten less brittle at lower temperatures.
In conclusion, while tungsten is brittle at low temperatures, its behavior at high temperatures is more nuanced, influenced by factors such as purity, alloying, and exposure to oxidizing environments. Proper handling and processing can enhance its ductility and utility in high-temperature applications.
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