Dislocation Etch Pits and Phase Relationships in Dimolybdenum Carbide Single Crystals

Abstract

Hexahedral, triangular, and spiral etch pits were brought out on Mo2C single crystals by etching with dilute nitric acid. These etch pits were correlated with the hexagonal Mo2C structure and the veining substructure as found on all planes of the crystals. The total dislocation density on as-grown crystals was calculated to be 5×106 dislocations/cm2. Annealing at temperatures ranging from 1600° to 2200°C resulted in a concomitant increase in the number of etch pits present. This increase is associated with the polygonization of the veining substructure during annealing, the increasing dispersion of the veining and impurity precipitate particles within the Mo2C matrix, and the formation of Mo2C-MoC within the veins. After annealing at 2200°C, the total dislocation density increased to 1×108 dislocations/cm2. The transition from the veining to the band-type substructure during annealing at 2200°C and above is associated with the formation of the Mo-Mo2C eutectic and the Mo-MoC within the bands, as determined by electron microprobe and electron diffraction analyses.