Precipitation in a Cr-Cr{sub 2}Nb alloy
- Oak Ridge National Lab., TN (United States). Metals and Ceramics Div.
The microstructure of a Cr-Cr{sub 2}Nb two-phase alloy containing 12 at.% Nb was examined in the as-cast and annealed conditions using optical and electron microscopy techniques. In the as-cast condition, a Cr solid-solution matrix and a eutectic mixture of Cr solid solution and C15 Cr{sub 2}Nb coexist. In the Cr matrix, adjacent to the Cr-Cr{sub 2}Nb interface, dislocations are observed and are likely a consequence of the thermal mismatch between the two phases. The matrix solid solution, away from the eutectic Cr{sub 2}Nb, also exhibits a high dislocation density and intense deformation bands; no solid-state precipitation of Cr{sub 2}Nb in the matrix had occurred. Upon annealing for as little as 3 h at 1,200 C, however, precipitation occurs; these precipitates, with the C15 crystal structure, have a rod morphology with a hexagonal cross-section and faceted end-caps. These precipitates are twinned, the twinning system being the same as in the eutectic Cr{sub 2}Nb. Mismatch dislocations are observed in the precipitate/matrix interface and dislocations arising from thermal contraction mismatch are also present in the Cr matrix adjacent to the precipitate matrix interface. The precipitate-matrix orientation relationship was (101){sub m}{parallel}(1{bar 1}1){sub p} and [{bar 1}01]{sub m}{parallel}[{bar 1}01]{sub p}. Exposure at 1,200 C for extended times results in some precipitate coarsening. A second type of precipitate with a cube-on-cube orientation with the matrix and a lattice parameter of a {approx_equal} 11.5 {angstrom} was occasionally observed, persisted upon extended annealing, and was identified as an M{sub 6}C-type carbide (M = Cr, Nb) that may be stabilized by other interstitial contaminants such as oxygen.
- OSTI ID:
- 540950
- Journal Information:
- Acta Materialia, Journal Name: Acta Materialia Journal Issue: 9 Vol. 45; ISSN 1359-6454; ISSN ACMAFD
- Country of Publication:
- United States
- Language:
- English
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