Crystallographic Features of the Structure of Cast and Quenched Cobalt–Niobium Alloys

Specific features of structure formation at the β → α (fcc → hcp) polymorphic transformation in Co–Nb binary alloys have been investigated by metallography, scanning and transmission electron microscopies, and electron backscattering diffraction analysis. It is shown that gradual cooling of a crystallized ingot induces nucleation of α-phase crystals of several orientations (of the four ones that are possible in correspondence with the Wassermann orientation relationships) in each β grain of the alloy. Only α(hcp) martensite has been found in the structure of investigated alloys at room temperature. Misorientation of the substructure along the martensite-crystal length in the cast alloys does not exceed 1°. After homogenization and subsequent quenching in salty water, the structure of Co–Nb alloys does not undergo morphological and crystallographic changes but becomes significantly refined. In addition, misorientation of substructure elements along the martensite-crystal length increases several times, which is a consequence of the high level of quenching microstress in martensite. No phases with multilayer lattices of the NR-martensite type are revealed.