Formation Mechanism of Lamellar M 23C6 Carbide in a Cobalt-Base Superalloy During Thermal Exposure at 1000 °C

doi:10.1007/s40195-017-0596-1

Abstract

Abstract:

The precipitation of the lamellar-shaped M ₂₃C₆ carbide within the dendritic matrix of a cobalt-base superalloy during thermal exposure at 1000 °C has been investigated. Such a precipitation is not commonly observed in cobalt-base superalloys. It is found that M ₂₃C₆ particles nucleate preferentially at stacking faults (SFs) in the dendritic matrix and grow along the SFs to develop a lamellar character. Additionally, a Cr depletion zone is observed in the vicinity of the lamellar M ₂₃C₆ carbide, which strongly supports the presence of Suzuki segregation.

Key words: Cobalt-base superalloy, Carbide precipitation, Thermal exposure, Scanning/transmission electron microscopy

Wei-Min Gui, Hong-Yu Zhang, Hai-Bo Long, Tao Jin, Xiao-Feng Sun, Qi Zheng. Formation Mechanism of Lamellar M ₂₃C₆ Carbide in a Cobalt-Base Superalloy During Thermal Exposure at 1000 °C[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(1): 27-32.

Figures/Tables 5

Fig. 1 BSE micrographs of the test alloy heated at 1000 °C for: a 0.5 h, b 1 h, c 4 h, d 50 h, e 100 h f 300 h

Fig. 2 Evolution of primary carbides with various heat treatment time at 1000 °C: a BSE image of carbide morphology for 0.5 h, b EDS pattern of M 6C carbide, c BSE image of carbides morphology

Fig. 3 TEM micrographs of the test alloy a in the as-cast condition and after heated at 1000 °C for: b 0.5 h, c 1 h, d 4 h, e 50 h f 100 h

Fig. 4 EDS analysis intended to detect Suzuki segregation around the SFs in the test alloy: a STEM image of the alloy heated for 300 h, bprofile of EDS analyses from each solute element along the green line in a. The red circles in the EDS profile of Cr show that a Cr depletion zone exists around the lamellar M 23C6 carbide

Table 1 EDS compositions of M 23C6 and the matrix (wt%)

	Cr	Ni	W	Al	Co
Matrix	25.41	10.97	3.93	0.44	59.24
M ₂₃C₆	77.11	2.02	5.2	-	15.67

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Formation Mechanism of Lamellar M ₂₃C₆ Carbide in a Cobalt-Base Superalloy During Thermal Exposure at 1000 °C

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