Key Takeaways & Executive Findings
- •• Addition of trace lanthanum (5.0×10-5 to 3.4×10-4 wt.%) to a second-generation nickel-based single crystal superalloy reduces elemental segregation and increases γ/γ′ eutectic content in the as-cast state. • Lanthanum promotes the formation of strip-shaped clusters in interdendritic regions, composed of Ni5La and Ni3Ta, with the Ni5La proportion increasing with La content. • Heat treatment induces incipient melting in cluster regions, increasing microporosity, and transforms cluster morphology from strip to elliptical while altering composition to include MC carbides. • The study provides a metallurgical basis for optimizing La content and heat-treatment parameters to enhance high-temperature oxidation resistance and mechanical properties of single crystal superalloys.
Abstract
To enhance the high-temperature oxidation resistance and mechanical properties of a second-generation nickel-based superalloy, various concentrations of lanthanum (La) ranging from 5.0×10-5wt.% to 3.4×10-4wt.% are added to the alloy. The microstructure of the nickel-based single crystal superalloy with trace of La was examined by means of SEM, EDS and TEM. Results show the addition of La decreases the segregation of elements and increases the amount of γ/γ′ eutectics of the as-cast alloy, and in the interdendritic region, the growth of eutectics is accompanied by the growth of strip clusters composed of Ni5La and Ni3Ta. As the La content in the alloy increases, the proportion of Ni5La in the cluster increases. After heat treatment, incipient melting occurs in the cluster regions, leading to an increase in microporosity compared to the original as-cast samples. Furthermore, the heat treatment alters the shape of the clusters from a strip morphology to an elliptical one, and it changes their composition from Ni5La and Ni3Ta to a combination of Ni5La, Ni3Ta, and MC carbides.
1. Introduction
The nickel-based single crystal superalloy is a distinguished structural material known for its exceptional high-temperature strength, resistance to oxidation, and ability to withstand hot corrosion. It is extensively used in the manufacture of high-temperature components for aerospace engines [1,2], and its importance has grown in recent years. The pursuit of enhanced thermodynamic efficiency in aircraft engine turbine blades has led to an increase in operating temperatures [3,4]. However, the higher temperatures also subject single crystal superalloys to more intense oxidation, which could shorten their service life. To counteract this issue, a common approach involves adding rare earth (RE) elements to the alloy [5-7], with a special focus on the use of lanthanum (La).
However, the excessive addition of rare earth (RE) elements can lead to a reduction in the initial melting temperature of nickel-based superalloys, imposing limitations on the temperature range suitable for the solution heat treatment of the alloy [8]. Therefore, precise control of RE element addition is crucial to prevent adverse effects on the microstructure and mechanical properties of superalloys. Guan et al. [9] observed that the addition of yttrium (Y) could increase the eutectic content in a second-generation single crystal superalloy. However, this addition resulted in a decrease in stress rupture life at 1,093 °C/158 MPa from 49.5 h to 27.1 h as Y increases from 0 to 6.6×10-4wt.%, and the tensile strength at 870 °C from 1,028 MPa to 931 MPa as the Y content increases from 0 to 7.0×10-5wt.%. In another study, Pang et al. [10] incorporated both La and Y into a single crystal superalloy, CMSX-4. Their findings indicated that CMSX-4 with a Y+La level of up to 6.5×10-5wt.% exhibited a lower initial melting degree, with the total area fraction of melting being less than or equal to 0.1%. However, at a Y+La level of 3.5×10-4wt.%, significant incipient melting was observed, which affected approximately 0.5% of the total area. After exposure at 1,000 °C and 1,100 °C for 1,000 h, the addition of Y and La increased the formation tendency of TCP (topologically close-packed) phase in CMSX-4 with a Y+La level of 6.5×10-5wt.%, but seemed to decrease at a higher Y+La level of 3.5 ×10-4wt.%.
The element La has a significant effect on high-temperature oxidation resistance of single crystal superalloys, offering a more beneficial impact compared to Y [11]. While previous investigations have focused on the effects of Y or the combination of Y and La on single crystal superalloys, the individual impact of La on the microstructure, as well as its effect during heat treatment, has been neglected. Hence, it is crucial to investigate the specific influence of La on the microstructure of single crystal superalloys.
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Hong Gao, Kai Guan, Ren-jie Cui, Jian-chao Qin, Zi-han Zhao, Zhao-hui Huang (2025). Effect of lanthanum on microstructure of a nickel-based single crystal superalloy. China Foundry. https://doi.org/10.1007/s41230-025-4006-4
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Frequently Asked Questions
What is the effect of lanthanum addition on the microstructure of nickel-based single crystal superalloys?
The addition of lanthanum (La) in trace amounts (5.0×10-5 to 3.4×10-4 wt.%) reduces elemental segregation and increases the amount of γ/γ′ eutectics in the as-cast alloy. It also promotes the formation of strip-shaped clusters composed of Ni5La and Ni3Ta in interdendritic regions, with the proportion of Ni5La increasing with La content.
How does heat treatment affect lanthanum-containing single crystal superalloys?
Heat treatment causes incipient melting in the cluster regions, leading to increased microporosity compared to as-cast samples. It also changes the cluster morphology from strip to elliptical and alters their composition to include Ni5La, Ni3Ta, and MC carbides.
Why is lanthanum added to nickel-based single crystal superalloys?
Lanthanum is added to enhance high-temperature oxidation resistance and mechanical properties. It is a rare earth element that can improve oxidation resistance, but excessive addition can lower the initial melting temperature, so precise control is necessary.
What are the optimal lanthanum concentrations for improving superalloy properties?
The study investigates La concentrations from 5.0×10-5 to 3.4×10-4 wt.%. The optimal range is not explicitly stated, but the findings suggest that controlling La within this range can reduce segregation and increase eutectics, while excessive amounts may lead to incipient melting during heat treatment.
What techniques were used to examine the microstructure in this study?
The microstructure was examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM).
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