Tungsten (W) is an element celebrated for its extreme physical properties, making it a material of choice in high-demand industrial and scientific applications. This dense, silvery-white metal boasts the highest melting point of any metal on the periodic table, reaching 3422 °C (6192 °F). Its exceptional rigidity and high-temperature performance result from strong metallic bonding and its body-centered cubic (BCC) crystal structure. While pure tungsten is used for filaments and electrodes, the material most people interact with is often tungsten carbide, a compound used for cutting tools and jewelry. To overcome this material’s resistance, specific and extreme forces—mechanical, thermal, or chemical—must be applied to induce failure.
Brittle Fracture and Mechanical Stress
Despite its reputation for hardness, pure tungsten is inherently brittle, particularly at room temperature, making it vulnerable to sudden, high-energy impact. This behavior is due to its BCC crystal lattice, which limits the number of “slip systems” available for atomic planes to slide past one another. When a metal is struck, a tough material absorbs energy by deforming plastically. Tungsten’s limited capacity for plastic deformation causes it to fracture instead.
The failure mechanism is a process known as brittle fracture, where an applied stress propagates cracks rapidly along cleavage planes within the crystal structure. The material’s high Brittle-to-Ductile Transition Temperature (BDTT), often exceeding 300 °C, means it behaves like a glass-like solid in most ambient conditions. At low temperatures, the internal energy is insufficient to activate the dislocation movement needed to blunt a crack tip, allowing the crack to race through the material.
The presence of micro-cracks or impurities, often introduced during manufacturing processes like sintering, acts as a stress concentration point that initiates the fracture. When the tensile stress at the tip of these microscopic flaws exceeds the material’s ultimate tensile strength, the crack quickly spreads throughout the sample. Therefore, a sudden, sharp impact, such as from a high-speed projectile or a hammer blow, is one of the most effective ways to break tungsten. The failure is instantaneous and catastrophic, differentiating it from the slow deformation seen in tougher, more ductile metals.
Failure Due to Extreme Heat and Thermal Shock
Tungsten’s high melting point means it does not fail by simply melting under normal operating conditions. However, it is susceptible to two distinct failure modes at elevated temperatures: thermal shock and creep.
Thermal Shock
Thermal shock failure occurs when the material is subjected to a rapid change in temperature, such as quick cooling after intense heating. This rapid temperature gradient causes one section of the material to expand or contract much faster than an adjacent section, creating immense internal stresses that can exceed the material’s tensile strength. Because tungsten is not ductile at lower temperatures, it cannot relieve this stress through plastic deformation and instead forms catastrophic internal cracks. This failure mode is a significant concern for high-heat components, such as those used in fusion reactors or rocket nozzles, which experience cyclical heating and cooling.
High-Temperature Creep
The second failure mode is high-temperature creep, which is the slow, permanent deformation of the material under continuous stress. While creep can occur in many materials, it requires temperatures well above the point where traditional metals fail. In tungsten, significant creep deformation is observed in the range of approximately 1200 °C to 2500 °C.
The mechanism for this slow failure involves the movement of dislocations within the crystal lattice at these extremely high temperatures. Under persistent mechanical load, the dislocations slowly rearrange and accumulate, leading to a gradual and irreversible change in the component’s shape. Although the material never melts, the dimensional instability caused by creep ultimately renders the component useless in precision applications.
Chemical Degradation and Oxidation
Tungsten is generally resistant to most common acids and bases at room temperature, but it can be chemically degraded under specific, aggressive conditions. The most common form of chemical failure is rapid oxidation when the metal is exposed to air at elevated temperatures. While a thin, protective oxide layer forms at room temperature, this layer breaks down as the temperature increases.
Oxidation begins to become significant around 400 °C in air and accelerates rapidly above this point. The reaction forms tungsten trioxide (\(\text{WO}_3\)), which can be gaseous or highly volatile above 750 °C. This volatility means the oxide layer does not remain to protect the underlying metal. Instead, it is removed, continuously exposing fresh tungsten to the oxygen and causing the material to thin quickly.
At ambient temperatures, tungsten can be intentionally dissolved using specialized, highly corrosive chemical mixtures in a process called wet chemical etching. These etchants typically combine a strong oxidizing agent, like nitric acid (\(\text{HNO}_3\)), with hydrofluoric acid (\(\text{HF}\)). The nitric acid oxidizes the tungsten surface, while the hydrofluoric acid simultaneously dissolves the resulting tungsten oxide layer by forming soluble fluoride compounds. This combination is highly effective at dissolving tungsten for industrial applications like semiconductor manufacturing, but it requires specialized containment and handling due to the extreme hazards posed by the acids.