Rapid growth kinetics and microstructure modulation of intermetallic compounds within containerlessly solidifying Mo-Co refractory alloys

To explore an innovative approach to prepare high-performance Mo-based refractory alloys, the rapid solidification mechanisms of undercooled Mo-40%Co hyperperitectic alloy were investigated by both electromagnetic levitation (EML) and drop tube (DT) techniques. During EML experiments, typical peritectic solidification still prevailed even at the maximum undercooling of 252 K (0.14 T_L). The dendrite growth velocity of primary σ-Mo₃Co₂ compound varied with alloy undercooling by a power relation, which attained 24.3 mm s⁻¹. As the bulk undercooling increased, the conspicuous dendrite fragmentation of primary phase took place, resulting in its grain refinement and volume fraction reduction. Meanwhile, the second recalescence degree exhibited a decreasing tendency, which indicated that the peritectic reaction was remarkably suppressed in the undercooled state. Under free fall condition, two critical diameters were determined for tiny alloy droplets and the corresponding undercooling thresholds were calculated as 361 and 443 K, respectively. When the alloy droplet was larger than 707 μm, the typical peritectic solidification proceeded and the final microstructure was composed of primary σ-Mo₃Co₂ dendrites and interdendritic ε-Mo₆Co₇ peritectic phase. With the reduction of droplet diameter, the direct nucleation of peritectic phase was induced. In this case, the ε-Mo₆Co₇ intermetallic compound regions appeared preferentially at the periphery of alloy droplets, whereas the typical peritectic microstructure filled in the center area. Once the droplet diameter decreased to 93 μm, the peritectic solidification characteristics disappeared completely and the metastable ε-Mo₆Co₇ single phase became the unique growth morphology.