In superalloys, phosphorus has a dual role. For some high-temperature alloys, an appropriate amount of phosphorus can improve the durability and creep properties, especially for deformed high-temperature alloys. However, for other high-temperature alloys, especially cast high-temperature alloys, phosphorus will have an adverse effect on the mechanical properties and should be controlled at a low level.
The mechanism of action of phosphorus mainly has two aspects: first, phosphorus atoms will segregate at the grain boundaries, enhance the bonding force of the grain boundaries, increase the strength of the grain boundaries, and change the morphology of the grain boundary phases; second, phosphorus will aggravate solidification segregation and affect The solidification process promotes element segregation and the generation of harmful phases. For deformed high-temperature alloys, hot working and heat treatment can basically eliminate or reduce solidification segregation, so the first mechanism of action of phosphorus dominates. For cast high-temperature alloys, solidification segregation is more serious, so the secondary action mechanism of phosphorus plays a major role, resulting in a reduction in mechanical properties.
Research shows that the increase in phosphorus content will promote the formation of Laves phase in cast superalloy K4169, and the Laves phase content increases with the increase in phosphorus content. The Laves phase is massive, and as the phosphorus content increases, the shape of the Laves phase changes from small pieces to large pieces. At the same time, the dendrite structure of the alloy also becomes coarser. All Laves phases exist between dendrites and are the final solidification products. Electron probe analysis shows that phosphorus is severely segregated in the Laves phase or γ/Laves eutectic phase in the final condensation zone, while promoting the segregation of niobium and molybdenum in these regions. In high-phosphorus alloys, the phosphorus content in the Laves phase is as high as 0.982wt%, which is far more than 30 times the total phosphorus in the alloy.
The increase in phosphorus content will reduce the instant tensile properties and durable properties of K4169 alloy. When the phosphorus content is ≤0.008wt%, it has little effect on the room temperature tensile properties. But when the phosphorus content is >0.008wt%, the tensile strength and elongation of the alloy will decrease. At 650°C, when the phosphorus content is ≤0.008wt%, the strength of the alloy increases slightly but the plasticity decreases slightly; when the phosphorus content is >0.008wt%, the strength and plasticity of the alloy decrease significantly. Under the conditions of 650°C/620MPa, as the phosphorus content increases, the durable life and durable plasticity of the alloy will also decrease.
The impact of phosphorus on performance is closely related to its impact on tissue. As the phosphorus content increases, the dendrite structure of K4169 alloy becomes coarser, element segregation intensifies, and Laves phase increases. The Laves phase not only consumes a large amount of niobium and reduces the amount of strengthening phase γ′′ (Ni3Nb) precipitated during the aging process of the alloy, but is also a brittle phase that is also the source of crack nucleation and expansion at high temperatures, reducing the strength and plasticity of the alloy. , leading to premature breakage. The more Laves phases there are, the greater the possibility of cracks breaking or expanding along the Laves phase interface, and the greater the damage to the alloy.
An appropriate amount of δ phase can improve the notch sensitivity of K4169 alloy. The generation of δ phase consumes a certain amount of niobium, causing a softening zone to be formed between the δ phase and the matrix γ. During the crack expansion process, it develops along the softening zone and penetrates deep into the grains to slow down the crack tip, which is beneficial to improving plasticity. In low-phosphorus alloys, a larger delta phase can improve tensile plasticity and durability.
Transmission electron microscopy observations show that dislocations accumulate in front of the δ phase, indicating that the δ phase makes it difficult for dislocations to penetrate the grains, which is beneficial to the improvement of strength. When dislocations accumulate to a certain extent, they can deform through the softening zone between the δ phase and the matrix, relieve stress concentration, and delay the occurrence and expansion of cracks.
In addition, K4169 alloy shows typical interdendritic fracture characteristics. As the phosphorus content increases, the brittle fracture tendency of the alloy increases and the plasticity becomes worse.
In summary, phosphorus is one of the common impurity elements in high-temperature alloys. For some high-temperature alloys, an appropriate amount of phosphorus can have beneficial effects. However, for most cast high-temperature alloys, excessive phosphorus content will lead to performance degradation and should be strictly controlled. High-temperature alloy manufacturers should use low-phosphorus raw materials, control process parameters, reduce the introduction of phosphorus in subsequent processing, and ensure that the phosphorus content of the final product reaches the standard.
Tianjin Anton Metal Manufacture Co., Ltd. is a company specializing in the production of various nickel-based alloys, Hastelloy alloys and high-temperature alloy materials. The company was established in 1989 with a registered capital of 10.0 million, specializing in the production and sales of alloy materials. Anton Metal’s products are widely used in aerospace, chemical industry, electric power, automobile, nuclear energy and other fields, and can also provide customized alloy material solutions according to customer needs. If you need to know the price consultation of alloy materials or provide customized alloy material solutions, please feel free to contact the sales staff.
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Post time: Nov-18-2023
