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

Experimental results are presented of solidification microstructure length scales including eta-phase cell spacing, primary epsilon secondary dendrite arm spacing, size of nonaligned dendrite of primary epsilon, and volume fraction of primary tr, 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 (lambda) of two-phase cellular structure decreases with increasing growth velocity (V) such that lambda V-1/2 is constant at a fixed alloy concentration in parametric agreement with the KGT and Hunt-Lu models. The value of lambda V-1/2 varies from 216 +/- 10 to 316 +/- 55 mu m(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 (lambda(2)) Of primary epsilon decreases with increasing V such that lambda(2)V(1/3) is constant ranging 14.9 +/- 0.9 to 75.6 +/- 8.1 mu m(4/3)/s(1/3) with increase in alloy concentration (Co) from 2.17 to 7.37 wt% Cu, which is in parametric agreement with predictions of arm-coarsening theory. The volume fraction (f(epsilon)) of primary epsilon 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(epsilon) 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, Lambda, of nonaligned dendrites of primary epsilon decreases with increasing V such that Lambda V-1/2 is constant for a given alloy, increasing from (0.98 +/- 0.04) x 10(3) to (7.2 +/- 0.7) x 10(3) mu m(3/2)/s(1/2) with increase in alloy concentration from 2.17 to 4.94 wt% Cu. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.