Effect of Compositional Changes of Laves Phase Precipitate on Grain Boundary Embrittlement in Long-Term Annealed 9 Pct C
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INTRODUCTION
DUE to their high creep strength and good resistance to oxidation, 9 to 12 pct chromium ferritic steels have been widely used for various high-temperature components of fossil-fired power plants.[1–4] To meet demands to increase the operating temperature, elemental W and Mo were added to ferritic steels.[5,6] The excellent mechanical properties of these steels are mainly achieved by the dispersive distribution of fine precipitates in a tempered martensitic matrix. However, after long-term operation at elevated temperature, microstructural degradation, such as a recovery of the sub-grain structure, evolution of the second phase, and coarsening of precipitates, was observed in ferritic steels; this degradation in turn led to a reduction of creep strength.[7–11] In particular, it was found that the Laves phase, i.e., (Fe,Cr)2(W,Mo), was nucleated at boundaries, and its coarsening rate was much faster than those of other precipitates, such as MX and M23C6.[12–14] In addition, the enlarged Laves phase precipitates of over a critical size at grain boundaries were reported to act as
CHEOLJUN BAE, ROSA KIM, JUNG HO HEO, and JONGRYOUL KIM are with the Department of Material and Chemical Engineering, Hanyang University, 55, Hanyangdaehak-ro, Sangnok-gu, Ansan-si, Gyeonggi-do, 15588, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted April 18, 2018. Article published online July 5, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleation sites for creep cavities that suddenly caused the fracture of the ferritic steels.[12,15,16] In 9 to 12 pct chromium ferritic steels, the formation of cavities by the Laves phase has been explained by the local stress concentration when Laves phase precipitates exceeded a critical size. Goods reported that external stress resulted in inhomogeneous deformation at the interface due to the different elastic moduli and stress flow characteristics between Laves phase precipitates and the matrix.[15] This should lead to a stress concentration at the interface, inducing a constrained zone of plasticity with the size of a Laves phase precipitate. Thus, it is easy to form cavities at the interface of coarse precipitates. However, according to previous reports, the critical size of Laves phase precipitates to form cavities varied from a few hundred nanometers to a few micrometers.[17] This strongly suggests that the formation of cavities results from other effects than the size of the Laves phase precipitates. In particular, the coarsening of Laves phase precipitates is known to be accompanied by the diffusion of alloying elements.[18–20] This compositional change in both the precipitates and adjacent matrix should change the amount of the local stress concentration because of different levels of solid solution hardening. This indicates that the compositional change of Laves phase precipitates during high-temperature operation is an important factor for the formation of cavities. However, few studies have been carried out to investigate the above microstructural changes
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