Effect of Withdrawal Rate Transition on Convergent Competitive Growth of Bi-Crystals in a Nickel-Based Superalloy

Directional solidification experiments with the seeding method were conducted to investigate the convergent competitive growth process of bi-crystals in a nickel-based superalloy under withdrawal rate transition conditions, aiming to clarify the behavior and mechanism of non-steady-state convergent grain competition. The results show that competitive overgrowth after a withdrawal-rate transition is influenced by the pre-transition steady-state withdrawal rate. Following the transition, the primary dendrite arm spacing (PDAS) does not adjust instantaneously; instead, it evolves over a finite solidification distance toward a new steady state that retains a dependence on the prior growth history. The competitive advantage of the favorably oriented (FO) grain progressively increases as solidification proceeds after the transition. By correlating the transverse and longitudinal microstructural evolution, the role of this history-dependent transient response, termed “growth inertia” in this work, in convergent bi-crystal competition is discussed. These findings indicate that transient solidification history should be considered when designing withdrawal-rate schedules for single-crystal blade casting.