Precipitation in a CrCr 2Nb alloy

Abstract

The microstructure of a CrCr 2Nb 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 2Nb coexist. In the Cr matrix, adjacent to the CrCr 2Nb 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 2Nb, also exhibits a high dislocation density and intense deformation bands; no solid-state precipitation of Cr 2Nb in the matrix had occurred. Upon annealing for as little as 3 h at 1200°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 2Nb. 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 ) p and [ 1 01] m// 1 01] p. Exposure at 1200°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 ≅ 11.5 A ̊ was occasionally observed, persisted upon extended annealing, and was identified as an M 6C-type carbide (M = Cr, Nb) that may be stabilized by other interstitial contaminants such as oxygen.