Precipitation in a Cr-Cr2Nb alloy

The microstructure of a Cr-Cr2Nb 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 Cr2Nb coexist. In the Cr matrix, adjacent to the Cr-Cr2Nb 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 Cr2Nb, also exhibits a high dislocation density and intense deformation bands; no solid-state precipitation of Cr2Nb in the matrix had occurred. Upon annealing for as little as 3 h at 1200 degrees 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 Cr2Nb. 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)(m)//(1 (1) over bar 1), and [(1) over bar 01](m)//(1) over bar 01](p). Exposure at 1200 degrees 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 congruent to 11.5 Angstrom was occasionally observed, persisted upon extended annealing, and was identified as an M6C-type carbide (M = Cr, Nb that may be stabilized by other interstitial contaminants such as oxygen. (C) 1997 Acta Metallurgica Inc.