Crystallographic features of the structure of cast and quenched cobalt–niobium alloys

By the means of metallography, scanning and transmission electron microscopy, and EBSD-analysis, the features of the structure formation upon $\beta\to\alpha$ (fcc-hcp) – polymorphic transformation in Co–Nb binary alloys were studied. It was shown that upon gradual cooling of the crystallized ingot, in each $\beta$-grain of the alloy nucleation of the $\alpha$-phase crystals of several orientations out of 4 possible in accordance with the Wasserman's orientational relations occurs. The formation of any of the 4 possible orientations of the $\alpha$-phase is equally probable. At room temperature, only $\alpha$ (hcp)-martensite was found in the structure of the studied alloys. The misorientation of the substructure in the martensitic crystals length-wise in cast alloys does not exceed 1 deg. After the homogenization and the subsequent quenching in salted water, the structure of Co–Nb alloys does not undergo neither morphological, nor crystallographic changes, but becomes noticeably more fine-grained. In this case, the misorientation of the substructure elements of the martensitic crystals length-wise increases several times, consequent to the high level of quenching microstresses in martensite. No phases with multilayer lattices such as $NR$ martensite were found.