Tungsten is a metal known for its extreme properties, including the highest melting point of any metal at 3,422 degrees Celsius and a density comparable to gold. This robust nature makes it valuable for high-temperature applications. However, in its pure, unalloyed form, tungsten is highly brittle at room temperature, which is a major constraint for engineering and manufacturing. This brittleness means that while it is exceptionally hard and strong, it can fracture suddenly with minimal bending or stretching.
Defining Brittleness in Metals
Brittleness describes a material’s tendency to fracture with little or no plastic deformation when subjected to stress. This contrasts with ductility, which is the ability of a material to deform plastically, or change shape permanently, before breaking. A ductile metal, such as copper, absorbs significant energy before failure. A brittle material, like glass or cast iron, fails abruptly once its elastic limit is exceeded, absorbing very little energy.
Brittleness is distinct from hardness, which measures a material’s resistance to localized plastic deformation, such as scratching or indentation. Tungsten is one of the hardest pure metals, but this does not prevent its brittleness. Brittleness characterizes fracture behavior, while hardness relates to surface resistance. A brittle material exhibits a steep curve on a stress-strain graph, indicating it resists deformation strongly but breaks suddenly without the “necking” or stretching seen in ductile materials.
The Structural Reason for Tungsten’s Brittleness
Tungsten’s intrinsic brittleness stems from its atomic arrangement, specifically its Body-Centered Cubic (BCC) crystal structure. The BCC lattice has atoms at the corners of a cube and one atom in the center. In metals, plastic deformation occurs through the movement of crystalline defects called dislocations, which slide along specific atomic planes known as slip systems.
The BCC structure offers fewer easy slip systems compared to the Face-Centered Cubic (FCC) structure found in ductile metals like gold or aluminum. At low temperatures, the energy required to initiate movement of screw dislocations in tungsten’s BCC lattice is very high. This resistance prevents the material from deforming plastically, forcing the applied stress to be relieved by cleavage, the sudden breaking of atomic bonds.
This behavior is related to the Ductile-to-Brittle Transition Temperature (DBTT). Tungsten’s DBTT is relatively high, often cited in the range of 180 to 400 degrees Celsius, depending on its purity and microstructure. Since room temperature is significantly below this threshold, tungsten exists in its brittle state. The lack of sufficient thermal energy means the material cannot activate the dislocation movement necessary for ductile behavior, resulting in sudden fracture.
Managing Brittleness Through Processing
Engineers must employ specialized methods to overcome tungsten’s low-temperature brittleness for practical applications. One primary method is temperature control through hot working, which involves shaping the metal above its DBTT. Techniques like hot forging, hot drawing, or hot rolling are performed at temperatures typically between 750 and 1100 degrees Celsius.
Heating the tungsten provides the necessary thermal energy to increase the mobility of dislocations within the BCC structure, temporarily shifting the material into its ductile regime. This allows tungsten to be shaped into wires, rods, or sheets without fracturing. Another strategy is alloying, where tungsten is mixed with other elements to create a composite material with improved mechanical properties.
Alloys often include metals like nickel, iron, or copper, forming dense tungsten heavy alloys. These secondary components act as a ductile matrix that surrounds the tungsten particles, absorbing stress and preventing rapid crack propagation. While alloying decreases brittleness and makes the material easier to machine, it often compromises the extreme high-temperature resistance characteristic of pure tungsten.
Real-World Applications and Limitations
Despite its brittleness, tungsten is indispensable in applications where its extreme hardness and temperature resistance are paramount. Its most famous application is as the filament in incandescent light bulbs, where it withstands temperatures up to 2,500 degrees Celsius without melting. It is also used in industrial heating elements and furnace components due to its high melting point and low vapor pressure.
The brittleness of pure tungsten dictates that it is often processed using powder metallurgy, where the metal powder is pressed and sintered into the final shape rather than being cast and machined. This method is preferred because machining solid, cold tungsten is extremely difficult and energy-intensive due to its resistance to deformation.
The most widespread use of tungsten’s compounds is in cemented carbides, often called tungsten carbide. This compound, formed with carbon, is used for cutting tools, drill bits, and armor-piercing ammunition because of its superior hardness. However, the inherent brittleness of tungsten limits its use in applications requiring high impact resistance or shock loading. For this reason, it is frequently used as a component in a composite structure rather than as a standalone structural material.