Metastable coupled growth kinetics between primary and peritectic intermetallic compounds within the liquid Mo-37 wt% Co refractory alloy

The metastable coupled growth between primary σ-Mo₃Co₂ and peritectic ε-Co₇Mo₆ phases for hypoperitectic Mo-37 wt% Co refractory alloy was investigated by electromagnetic levitation (EML) and drop tube (DT) methods. The highest alloy undercooling attained 261 K (0.14 T_L) under EML conditions, which reached up to 311 K (0.17 T_L) in DT experiments. At small undercoolings, the typical peritectic solidification prevailed and the corresponding double recalescence processes were in-situ recorded for levitated alloys. In this case, the primary σ-Mo₃Co₂ phase preferentially nucleated from liquid alloy and its growth velocity monotonously increased as a power function with the rise of undercooling. When the liquid undercooling increased beyond the threshold of 162 K, the competitive nucleation between σ-Mo₃Co₂ and ε-Co₇Mo₆ compounds was facilitated, leading to a novel lamellar eutectic-like microstructure for peritectic-type alloys. Meanwhile, the measured growth velocity exhibited a sudden drop at first and then slightly increased with the bulk undercooling. Once the alloy undercooling exceeded 295 K (D = 226 µm), only the metastable coupled growth mechanism could occur within the liquid alloy. The final microstructure of hypoperitectic Mo-37 wt% Co alloy even transformed from lamellar to anomalous eutectic-like morphology in a sufficiently undercooled state. Furthermore, as the undercooling increased, the random orientation relationship between σ-Mo₃Co₂ and ε-Co₇Mo₆ phases in typical peritectic solidification mode was replaced by the parallel orientation, i.e. {111}_σ//{101̅0}_ε for metastable coupled growth mechanism.