A study by researchers at the University of Utah in Salt Lake City, Utah, describes ways to improve the ductility of tungsten. It is generally believed that pure tungsten and tungsten alloys with a small amount of alloy are brittle at room temperature and have high ductile to brittle transition temperatures (DBTT). Improving the ductility of tungsten is of great significance to the production and application of tungsten.
Although numerous studies have been reported over the decades to improve the ductility of tungsten, this remains a challenge, due in part to a poor understanding of the mechanical properties of tungsten and its dependence on microstructure.
Tungsten - rhenium alloying is almost the only known method to improve the ductility of tungsten by alloying. Although a large number of studies have been reported in recent years on the effects of additives, including oxides, carbides and others, the effect of these additives on the ductility of tungsten has so far been inconclusive or not obvious under the influence of thermal machining. Using the microstructure of ultrafine particles or nanocrystals to improve the ductility of tungsten is another approach that looks promising.
Tungsten is a refractory metal with unique properties. It has the highest melting point among all elements, high elastic modulus, high density, high thermal conductivity and excellent mechanical properties at high temperature. These special properties make tungsten the material of choice for many applications. In recent years, tungsten has also been identified as one of the materials for plasma surface components in fusion reactors due to its high melting point, low sputtering rate and high ionosputtering corrosion resistance.
However, a major disadvantage of tungsten is that it has little ductility at room temperature, and its ductility to brittleness transition temperature (DBTT) is very high. The poor ductility of tungsten poses great challenges both to its machinability and its performance in harsh applications.
To improve ductility, the researchers suggest that there are two main contributing factors: the inherent lack of tightly packed planes and poor cohesion of grain boundaries. Among various methods, thermal machining has been found to be the most efficient. The DBTT of tungsten can be reduced from more than 700℃ to less than 300℃ by rolling at a temperature lower than recrystallization temperature. Several major factors contribute to the improvement of the ductility of deformed tungsten, including lamellar microstructure and high dislocation density after rolling.
In order to minimize recrystallization during high-temperature processing, cold processing based on traditional deformation techniques is also used to improve the ductility of tungsten. Due to the very high recrystallization temperature of tungsten, "cold" processing can be carried out up to about 1400℃. In this way, recrystallization and grain growth of tungsten during deformation can be prevented, resulting in finer lamellar microstructure and higher dislocation density in the material.
Cold-rolled tungsten at 400℃ shows increased dislocation density, more low-angle grain boundaries, and a significant improvement in strength, as well as a lower DBTT, compared to the high-temperature rolled material.
Another well-known method of improving the ductility of tungsten is alloying with rhenium. It has been reported that Peierls stress of tungsten can be reduced and additional slip surfaces can be facilitated by the formation of a solid solution of tungsten and rhenium through so-called solution softening. However, rhenium is a rare element with high cost, making these alloys too expensive for many applications. Considerable research work has been directed at replacing rhenium with tantalum, vanadium, titanium or other elements to achieve similar results.
So far, however, there is little experimental evidence for the effectiveness of these alloying elements. In recent years, based on the research progress of metals and ceramics, nanocrystalline or ultrafine structure has been explored as a method to improve the ductility of tungsten. In order to produce nanocrystalline or ultrafine tungsten particles, top-down and bottom-up methods have been studied.
Tungsten is a refractory metal with unique properties. It has the highest melting point among all elements, high elastic modulus, high density, high thermal conductivity and excellent mechanical properties at high temperature. These special properties make tungsten the material of choice for many applications. In recent years, tungsten has also been identified as one of the materials for plasma surface components in fusion reactors due to its high melting point, low sputtering rate and high ionosputtering corrosion resistance.
However, a major disadvantage of tungsten is that it has little ductility at room temperature, and its ductility to brittleness transition temperature (DBTT) is very high. The poor ductility of tungsten poses great challenges both to its machinability and its performance in harsh applications.
To improve ductility, the researchers suggest that there are two main contributing factors: the inherent lack of tightly packed planes and poor cohesion of grain boundaries. Among various methods, thermal machining has been found to be the most efficient. The DBTT of tungsten can be reduced from more than 700℃ to less than 300℃ by rolling at a temperature lower than recrystallization temperature. Several major factors contribute to the improvement of the ductility of deformed tungsten, including lamellar microstructure and high dislocation density after rolling.
In order to minimize recrystallization during high-temperature processing, cold processing based on traditional deformation techniques is also used to improve the ductility of tungsten. Due to the very high recrystallization temperature of tungsten, "cold" processing can be carried out up to about 1400℃. In this way, recrystallization and grain growth of tungsten during deformation can be prevented, resulting in finer lamellar microstructure and higher dislocation density in the material.
Cold-rolled tungsten at 400℃ shows increased dislocation density, more low-angle grain boundaries, and a significant improvement in strength, as well as a lower DBTT, compared to the high-temperature rolled material.
Another well-known method of improving the ductility of tungsten is alloying with rhenium. It has been reported that Peierls stress of tungsten can be reduced and additional slip surfaces can be facilitated by the formation of a solid solution of tungsten and rhenium through so-called solution softening. However, rhenium is a rare element with high cost, making these alloys too expensive for many applications. Considerable research work has been directed at replacing rhenium with tantalum, vanadium, titanium or other elements to achieve similar results.
So far, however, there is little experimental evidence for the effectiveness of these alloying elements. In recent years, based on the research progress of metals and ceramics, nanocrystalline or ultrafine structure has been explored as a method to improve the ductility of tungsten. In order to produce nanocrystalline or ultrafine tungsten particles, top-down and bottom-up methods have been studied.
