1. What Nickel Content Is Required to Classify an Alloy as a Nickel - Based Alloy?
According to the classification standard of the American Iron and Steel Institute (AISI), an alloy with a nickel content ≥ 50% by mass can be defined as a nickel - based alloy. This type of alloy takes nickel as the matrix and relies on solid - solution strengthening elements (such as Cr, Fe) and precipitation strengthening elements (such as Al, Ti) to improve performance.
For some special nickel - based alloys, the nickel content can be slightly lower (40%–50% by mass), but nickel must be the dominant element in the alloy composition (higher than the content of other single elements). For example, some nickel - iron - chromium alloys are classified as nickel - based alloys because nickel is the primary matrix element, even if the nickel content is between 40% and 50%.
In addition, the definition of nickel - based alloys also considers performance characteristics: the alloy must have excellent high - temperature oxidation resistance, corrosion resistance and creep resistance, which are the core properties that distinguish nickel - based alloys from stainless steel and other alloy systems.
2. What Effects Does Adding Tungsten (W) to Nickel - Based Alloys Have on Their Properties?
Solid - solution strengthening effect
Tungsten has a large atomic radius, which is quite different from that of nickel atoms. When tungsten dissolves into the nickel matrix, it will cause severe lattice distortion. This distortion can effectively hinder the movement of dislocations inside the alloy, thereby improving the room - temperature strength, high - temperature strength and hardness of the nickel - based alloy. The higher the tungsten content, the more obvious the solid - solution strengthening effect, but excessive addition will lead to a decrease in the alloy's ductility.
Improvement of high - temperature creep resistance
At high temperatures (above 650℃), tungsten atoms can slow down the diffusion rate of matrix atoms and inhibit the occurrence of grain boundary sliding. This enables the nickel - based alloy to maintain a stable structure and mechanical properties under long - term high - temperature and stress conditions, thus significantly enhancing the creep limit and service life of the alloy. This is particularly critical for nickel - based superalloys used in aero - engine turbine blades and gas turbine components.
Influence on corrosion resistance
Tungsten can improve the pitting corrosion resistance of nickel - based alloys in chloride - containing environments. It can promote the formation of a denser passivation film on the alloy surface and prevent the passivation film from being corroded and broken by chloride ions.
However, excessive tungsten will increase the tendency of the alloy to form intermetallic compounds, which may lead to a decrease in the alloy's intergranular corrosion resistance.
Impact on processing performance
Moderate addition of tungsten can improve the hot workability of the nickel - based alloy, making it easier to shape during forting, rolling 这类 hot - processing processes.
Excessive tungsten will increase the hardness and brittleness of the alloy, increasing the difficulty of cold processing and welding. During welding, tungsten may segregate at grain boundaries, leading to the formation of brittle phases and increasing the risk of welding cracks.
