Phase Selection in Undercooled Ni-3.3 Wt Pct B Alloy Melt

The change of eutectic solidification mode in undercooled Ni-3.3 wt pct B melt was studied by fluxing and cyclic superheating. The eutectic structure is mainly controlled by the undercooling for eutectic solidification, Delta T (2), instead of Delta T (1), the undercooling for primary solidification. At a small a dagger T (2) [e.g., 56 K (56 A degrees C)], the stable eutectic reaction (L -> Ni3B + Ni) occurs and the eutectic morphology consists of lamellar and anomalous eutectic; whereas at a larger a dagger T (2) [a parts per thousand yen140 K (140 A degrees C)], the metastable eutectic reaction (L -> Ni23B6 + Ni) occurs and the eutectic morphology consists of matrix, network boundary, and two kinds of dot phases. Further analysis declares that the regularly distributed dot phases with larger size come from the metastable eutectic transformation and are identified as alpha-Ni structure, whereas the irregularly distributed ones with smaller size are a product of the metastable decomposition and tend to have a similar structure to alpha-Ni as it grows. Calculation of the classical nucleation theory shows that the competitive nucleation between Ni23B6 and Ni3B leads to a critical undercooling, Delta T (2) (*) [125 K < Delta T (2) (*) < 157 K (125 A degrees C < a dagger T (2) (*) < 157 A degrees C)], for the metastable/stable eutectic formation.