Sulfur has extremely low solubility in the nickel matrix and tends to react with alloying elements (such as Ni, Fe, Cr) to form low-melting-point sulfides (e.g., Ni₃S₂, CrS, FeS). The melting point of these sulfides is generally between 600–900°C, which is much lower than the hot working temperature of nickel-based alloys (1000–1200°C).
During hot processing (e.g., forging, rolling, extrusion), these sulfides will melt first, forming a liquid film along the grain boundaries. This film weakens the bonding force between grains, leading to intergranular cracking when the alloy is subjected to external stress. This phenomenon is called hot brittleness, which seriously reduces the hot working plasticity of the alloy and causes processing defects such as cracks and splits.
When the sulfur content exceeds the limit, fine sulfide particles will precipitate along the grain boundaries or within the matrix. These particles act as stress concentration sources. When the alloy is subjected to impact or low-temperature loading, microcracks are prone to initiate and propagate around the sulfide particles, resulting in a sharp drop in the alloy's Charpy impact toughness and fracture toughness.
For nickel-based alloys used in low-temperature or cryogenic environments (e.g., liquefied natural gas equipment), the increase in sulfur content will significantly increase the risk of brittle fracture.
Sulfides are electrochemically heterogeneous with the nickel matrix. In corrosive media (such as chloride-containing solutions, acidic environments), a galvanic cell is formed between sulfide particles and the matrix. The sulfides act as the anode and corrode preferentially, leading to the formation of pitting corrosion.
In addition, sulfide precipitation at grain boundaries will destroy the continuity of the passive film (Cr₂O₃) on the alloy surface, reducing the alloy's resistance to intergranular corrosion and stress corrosion cracking (SCC). This effect is particularly obvious in high-temperature and high-pressure water environments (e.g., nuclear reactor coolant systems).
Phosphorus is a typical grain boundary segregation element. Even at trace levels, it tends to segregate at the grain boundaries of nickel-based alloys during solidification or heat treatment, rather than dissolving uniformly in the matrix.
The segregation of phosphorus at grain boundaries reduces the cohesion of the grain boundary atoms, making the grain boundaries become the weak link of the alloy. At room temperature or low temperatures, when the alloy is subjected to tensile or bending stress, cracks are prone to initiate and propagate along the phosphorus-segregated grain boundaries, resulting in intergranular brittle fracture. This phenomenon is called cold brittleness, which leads to a significant decrease in the alloy's elongation and reduction of area.
During the welding process of nickel-based alloys, phosphorus will segregate at the grain boundaries of the weld metal and heat-affected zone (HAZ) due to rapid cooling. The segregated phosphorus reduces the grain boundary strength of the weld joint and increases the sensitivity to weld solidification cracks and liquation cracks.
For nickel-based alloy welds used in critical equipment (e.g., chemical reactors, nuclear steam generators), phosphorus-induced weld cracks can severely reduce the service life and safety of the equipment, even leading to catastrophic accidents.
For nickel-based alloys applied in high-temperature environments (e.g., aero-engine turbine blades, industrial furnace components), phosphorus segregation at grain boundaries will accelerate the grain boundary sliding under long-term high-temperature stress. This accelerates the creep deformation of the alloy and shortens the creep rupture life.
In addition, phosphorus can promote the growth of grain boundary carbides, destroying the pinning effect of carbides on grain boundaries and further reducing the high-temperature structural stability of the alloy.
In industrial production, the content of sulfur and phosphorus in nickel-based alloys is strictly controlled according to application scenarios:
For general corrosion-resistant nickel-based alloys (e.g., Hastelloy C276, Inconel 625), the S and P content is usually limited to ≤0.01%.
For high-performance nickel-based alloys used in aerospace, nuclear energy, and other fields (e.g., Inconel 718, Waspaloy), the S and P content is required to be ≤0.005% to ensure extreme mechanical properties and service safety.
Strict control of sulfur and phosphorus impurities is a key link in the smelting and processing of nickel-based alloys, which directly determines whether the alloy can meet the performance requirements of critical engineering applications.
