Journal Published Online: 09 March 2020
Volume 9, Issue 1

Carbide Faceting in Alloy 690: HRTEM Study



Alloy 690 (Unified Numbering System [UNS] No. 6690) in thermally treated (TT) condition is an advanced steam generator tubing material that offers high corrosion resistance with tailored carbide morphology and chromium concentration adjacent to the grain boundary. The morphology of the intergranular carbides was found to have strong dependence on the grain boundary character distribution (GBCD) from an electron backscattered diffraction (EBSD) study. The effect of thermal aging on crystallography and faceting of carbides was studied with the aid of high-resolution transmission electron microscopy (HRTEM). Carbides formed at 600°C, 700°C, and 800°C were structurally consistent with face centered cubic (fcc) M23C6. The orientation relationship of the carbides with grain allowing maximum atomic correspondence was found to be (111)C ‖ (111)M and <110>C ‖ <110>M. HRTEM analysis revealed that the carbides were partially faceted with multiple facet initiation at lower temperature and become fully faceted with fewer stable facets at 800°C. The non-faceted parts of carbides had lattice structure discontinuity from the faceted portion creating new energy interface within the carbides. It is suspected that the growth of carbides, formation of facets, and facet transformation takes place via ledge growth and coalescence. Stable, low-energy faceted interfaces of 800°C remain inactive and hold the carbides firmly, preventing dissolution or dislodging in corrosive environments and thus providing superior corrosion resistance.

Author Information

Santra, Sumita
Department of Atomic Energy, Hyderabad, India
Ramana Rao, S. V.
Department of Atomic Energy, Hyderabad, India
Kapoor, K.
Department of Atomic Energy, Hyderabad, India
Srivastava, D.
Department of Atomic Energy, Hyderabad, India
Pages: 19
Price: $25.00
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Stock #: MPC20190130
ISSN: 2379-1365
DOI: 10.1520/MPC20190130