Unidirectional solidification of Zn-Rich Zn-Cu peritectic alloys - II. Microstructural length scales

Experimental results are presented of solidification microstructure length scales including η-phase cell spacing, primary ε secondary dendrite arm spacing, size of nonaligned dendrite of primary ε, and volume fraction of primary ε, as functions of alloy concentration (containing up to 7.37 wt% Cu) and growth velocity (ranging from 0.02 to 4.82 mm/s) in the unidirectional solidification of Zn-rich Zn-Cu peritectic alloys. Intercellular spacing (λ) of two-phase cellular structure decreases with increasing growth velocity (V) such that λV ^1/2 is constant at a fixed alloy concentration in parametric agreement with the KGT and Hunt-Lu models. The value of λV ^1/2 varies from 216 ± 10 to 316 ± 55 μm^3/2/s ^1/2 with decrease in alloy concentration from 4.94 to 2.17 wt% Cu. These values are much greater than for normal eutectic systems but comparable with monotectic alloys. Dendritic secondary arm spacing (λ₂) of primary ε decreases with increasing V such that λ₂V^1/3 is constant ranging 14.9 ± 0.9 to 75.6 ± 8.1 μm^4/3/s^1/3 with increase in alloy concentration (C₀) from 2.17 to 7.37 wt% Cu, which is in parametric agreement with predictions of arm-coarsening theory. The volume fraction (f_ε) of primary ε increases with increasing V for Zn-rich Zn-3.37, 4.94 and 7.37 wt% Cu hyperperitectic alloys. Predictions of the Scheil and Sarreal-Abbaschian models show good agreement with the observed f_ε for Zn-4.94 wt% Cu at moderate V from 0.19 to 2.64 mm/s, but fail at low V of less than 0.16 mm/s and at high V of greater than 3.54 mm/s. The measured average size, Λ, of nonaligned dendrites of primary ε decreases with increasing V such that ΛV ^1/2 is constant for a given alloy, increasing from (0.98 ± 0.04) × 10³ to (7.2 ± 0.7) × 10³ μm^3/2/s ^1/2 with increase in alloy concentration from 2.17 to 4.94 wt% Cu.