Dendrite growth and Vickers microhardness of Co7Mo6 intermetallic compound under large undercooling condition

The dendritic growth process and Vickers microhardness enhancement of primary Co7Mo6 phase in undercooled liquid Co-50%Mo hypereutectic alloy are systematically investigated by using electromagnetic levitation and drop tube. It is found that the rapid solidification microstructures are mainly characterized by primary Co7Mo6 dendrites plus interdendritic (Co7Mo6+Co) eutectic irrespective of experimental conditions. In electromagnetic levitation experiment, the obtained maximum undercooling reaches 203 K (0.12TL). With the rise in bulk undercooling, primary Co7Mo6 dendrite growth velocity monotonically increases according to a power function and reaches 22.5 mm-1 at the highest undercooling. The secondary dendrite spacing decreases from 45.8 to 13.6 m, while Co content in primary dendrites shows an increasing trend. This indicates that an evident grain refinement and solute trapping take place for primary Co7Mo6 dendrites during rapid solidification. The dependence of Vickers microhardness on Co content follows an exponential function. Moreover, the variation of Vickers microhardness with the grain size also satisfies an exponential relationship. In addition, Lipton-Kurz-Trivedi/Boettinger-Coriel-Trivedi model is used to analyze the growth kinetics of primary Co7Mo6 dendrites. In the experimental undercooling range, the growth process of primary Co7Mo6 dendrites is controlled mainly by solute diffusion and they grow sluggishly. Under free fall condition, liquid Co-50%Mo alloy is subdivided into many droplets inside a drop tube and their diameters range from 1379 to 139 m. With alloy droplet size decreasing, both droplet undercooling and cooling rate increase rapidly. In a large droplet-diameter regime above 392 m, primary Co7Mo6 phase displays faceted-growth characteristics. Furthermore, primary Co7Mo6 dendrites are refined greatly and their solute solubility is significantly extended as droplet size becomes smaller. Once the alloy droplet diameter decreases to a value below this threshold value, the faceted-growth characteristics start to disappear gradually, which is accompanied with a conspicuous grain refinement and a solute solubility extension. Both the solute solubility enhancement and grain size refinement contribute significantly to the exponential improvement in microhardness if primary Co7Mo6 phase grows in a faceted way. Otherwise, the solute solubility enhancement and grain size refinement result in the linear increase of Vickers microhardness. Theoretical analyses demonstrate that the primary phase microhardness is strongly dependent on its solute content and morphology characteristic.