Effect of carbides on the creep properties of a cast Ni-base superalloy M963 tested at 800 and 900 °C over a broad stress range has been investigated. Correlation between the carbides and creep properties of the alloy is enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). During high temperature creep tests, the primary MC carbide decomposes sluggishly and a large amount of secondary carbides precipitate. The cubic and acicular M 6 C carbide precipitates at the dendritic core region. Extremely fine chromium-rich M 23 C 6 carbide precipitates preferentially at grain boundaries. The M 6 C and M 23 C 6 carbides are found to be beneficial to the creep properties of the alloy. At lower temperature (800 °C), the interface of MC carbide with matrix is one of the principal sites for crack initiation. At higher temperature (900 °C), the oxidation and the precipitation of μ phase are the main factors for significant loss in creep strength of the alloy.

Effect of carbides on the creep properties of a Ni-base superalloy M963

Ni-base superalloy Rene′41 was produced by the laser melting deposition (LMD) manufacturing process. The microstructure was characterized by optical and scanning electron microscopy and X-ray diffraction. Results showed that ultra-fine rapid directionally solidified columnar grains with some fine MC carbide particles in the interdendritic zone were formed in the as-deposited samples along the deposited-direction due to the high thermal gradient and solidification cooling rate. The columnar grains were composed of well oriented cellular dendrites with a primary arm spacing of approximately 35 μm. The size of γ′ precipitates in the dendritic cores was larger than that in the interdendritic zones. Mechanical test data have shown that the LMD material had good high-temperature mechanical properties along longitudinal section.

Microstructure and mechanical properties of rapid directionally solidified Ni-base superalloy Rene′41 by laser melting deposition manufacturing

The microstructure and microhardness of Ni-base superalloy IN-738LC fabricated by laser solid forming (LSF) with the preheating was investigated. The results showed that with the increase of the preheating temperature (T 0 ) (25 °C, 800 °C, 900 °C and 1050 °C), the total crack length decreased significantly, the segregation of Ti and Al worsened, and the volume fraction of equiaxed grain increased, as well as the size of blocky carbides, γ-γ′ eutectic and γ′ particles. The high-angle grain boundaries, at which the continuous liquid film can be easily formed, were found to be susceptible to cracking. Both the concentration of Ti and Al and the increase of dendrite spacing contribute to the growth of carbides and γ-γ′ eutectic in the interdendrite while the size of the γ′ phases was largely affected by preheating. It was interesting to find that γ′ phase exhibited a bimodal distribution in the bottom of the deposit with the preheating at 1050 °C. The average microhardness of the deposits without preheating and with preheating at 800 °C and 900 °C is about at the same level, while the deposit with preheating at 1050 °C has the lowest average value. The crack-free deposit can be obtained when T 0 was up to 1050 °C, and its room temperature tensile properties are superior to cast IN-738LC alloy.

The effect of preheating on microstructure and mechanical properties of laser solid forming IN-738LC alloy

The M23C6 precipitation behavior in a cast Ni-base superalloy M963 was investigated after tensile creep testing at 800°C and strain-controlled low cycle fatigue testing at 700–950°C. During high temperature creep and low cycle fatigue, the primary MC decomposed into M6C continuously, and a great amount of secondary carbide, chromium-rich M23C6, precipitated preferentially in the periphery of MC and γ +γ′ eutectic at grain boundaries. M23C6 was rarely present in grain interior, indicating that grain boundary promoted M23C6 carbide precipitation. The M23C6 precipitation was closely dependent on the stress state and testing temperature, seemed to be independent of the total strain amplitude. M23C6 is unstable during low cycle fatigue testing. The occurrence of M23C6 precipitation was sharply reduced during low cycle fatigue testing at 950°C. Crack was easily initiated at interface between MC and matrix, while fine M23C6 was effective to prevent grain boundary migration.

M23C6 precipitation behavior in a Ni-base superalloy M963

Thermal exposure specimens of K465 superalloy are carried out at 850, 950, 1000, 1050 and 1100 degrees C in order to investigate the stability of carbides. MC carbide is stable from 850 to 950 degrees C; M6C carbide is most stable above 1000 degrees C; M23C6 carbide appears in the 950-1000 degrees C temperature range. The rate of MC decomposition increases with the rise of thermal exposure temperature. Some M6C carbide particles, which are transformed from MC carbide, have irregular morphology and adhere to the original MC carbide; the other M6C carbide particles are precipitated in the forms of needle and tiny block. Furthermore, needle-shaped M6C carbide is the most prevalent and has the coherent relationship with the matrix. M6C and script-like MC carbide particles are detrimental to room temperature tensile and stress rupture properties of the alloy. M23C6 particles are precipitated in grain boundaries, but have little effect on the properties of the alloy because of their small amount. (c) 2006 Elsevier B.V All rights reserved.

Relative stability of carbides and their effects on the properties of K465 superalloy

MC-type carbide formation in MAR-M247 LC superalloy was systematically investigated using directional solidification and quenching methods in sample growth rates between 0.8×10−6 and 15×10−6 m s−1. The results indicate that carbide growth rate, carbide forming element enrichment and surrounding solid geometry determine carbide morphology and carbide composition. The carbide forming element enrichment and interface energy control carbide nucleation. Heterogeneous carbide nucleation can occur above the alloy liquidus temperature. Carbide forming element enrichment and trapping behavior of the solid–liquid interface control carbide growth, which occurs at inter-secondary γ-dendrite arm positions and the mushy zone bottom which are rich in carbide forming elements. At fast sample growth rates, the solid–liquid interface can trap carbide nuclei in front of it. This trapping tends to occur at the inter-secondary γ-dendrite arm region.

MC carbide formation in directionally solidified MAR-M247 LC superalloy

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MC carbide formation in directionally solidified MAR-M247 LC superalloy