Abstract:Al-4.95%Zn alloy is directionally solidified in a modified Bridgman apparatus with higher temperature gradient to investigate response of cellular/dendritic microstructures and primary spacing to the variation of growth velocity under near-rapid directional solidification condition. The results show that, with increasing growth rate, there exists a transition from dendrite to fine cell and a wide distribution range in primary cellular/dendritic spacing at the given temperature gradient. The maximum, λmax, minimum ,λmin, and average primary spacing, -λ, as functions of growth velocity,v, can be given by λmax=12 340v-0.8353, λmin=2 953.7v-0.7717,λ=7 820.3v-0.8333, respectively.

Key words: Al-4.95%Zn alloy; near-rapid directional solidification; cellular/dendritic microstructures; primary spacing