Cellular growth of Zn-rich Zn–Ag alloys processed by rapid solidification
Introduction
For a positive temperature gradient, with an increase in growth velocity the solid–liquid interface during solidification of a dilute alloy varies from plane front to cells, to dendrites, to cells again and finally back to plane front [1]. The two transitions involving plane front and cells are known to relate with the limit of constitutional supercooling and the limit of absolute stability, respectively.
The morphological instability of the solid–liquid interface caused by the constitutional supercooling has been extensively investigated by experiments on dilute alloys [2], [3], [4], [5], [6]. In these studies, with increasing constitutional supercooling (or growth velocity) the development of cells usually follows the sequence: poxes, plate-like (or elongated) cells and regular (or hexagonal) cells. Whereas, around the absolute stability the microstructure transition between plane front and cells is seldom studied except for recent reports on transparent organic alloys [7], [8], [9] in which regular cells developed into plate-like cells prior to plane front with an increase in growth velocity. Ma et al. [10], [11] studied a Zn–1.52 wt.% Cu alloy at growth velocities up to 4.8 mm/s and observed a transition from regular cells to plate-like cells when the growth velocity exceeded 1.0 mm/s. A similar transition was found in solidified dilute Zn–Ag alloys [12], in which plate-like cells dominated at high-velocity domain as compared with regular cells.
Although the cells are predicted to occur between the limit of constitutional supercooling and the limit of absolute stability [1], currently the formation of cellular morphologies is not well explored in the high-velocity domain up to the limit of absolute stability. In order to gain more insight into the development of cells, rapid solidification experiments were carried out on Zn–0.6 and 1.8 at.% Ag alloys processed by laser remelting and melt-spinning techniques. For comparison, Bridgman solidification was performed as well. The effect of growth velocity on cellular spacings was also discussed.
Section snippets
Experimental procedure
Two Zn–Ag alloys containing 0.6 and 1.8 at.% Ag were prepared by melting pure 99.99% Zn and pure 99.99% Ag in air in an induction furnace. Specimens with mm were prepared for laser remelting experiments. A continuous wave 1.0 kW CO2 laser was employed at beam scanning velocities ranging from 16.7 to 120 mm/s. A normally incident laser beam was focused to a spot diameter of 1.0 mm and had a power density of 3.2×104 W/cm2. During the laser treatments, a continuous flow of argon was blown
Results
The local solidification velocity in the laser remelted specimens was determined by the method given in [13] to be 12.0–54.5 mm/s. For pure Zn and dilute Zn alloys melt-spun at a wheel speed of 20 m/s, the maximum growth velocity was reported to be around 300 mm/s by Akdeniz and Wood [14]. Therefore, the solidification techniques employed, i.e. Bridgman, laser remelting and melt spinning, provide a wide velocity range from 0.02 to 300 mm/s.
Development of cellular structures
Compared with dendrites, the cellular structure is a microstructure pattern formed at growth velocities close to the limit of constitutional supercooling (Vc) and the limit of absolute stability (Va), which can be defined here as low-velocity and high-velocity cells, respectively. Vc was predicted as [15]where G is the temperature gradient, DL the diffusion coefficient in the melt and ΔT0 the temperature difference between solidus and liquidus at the composition under consideration. Va
Conclusions
Three solidification velocity-dependent morphologies were mainly identified with increasing growth velocity from 0.02 to 300 mm/s: regular cells or dendrites of η, plate-like cells of η, and plane front of η. At low-velocity range such as V<1.0 mm/s for Zn–0.6 at.% Ag and V<2.39 mm/s for Zn–1.8 at.% Ag, regular cells or dendrites of η occurred. On the other hand, plate-like cells were dominant at higher velocity range such as 1.0<V<54.5 mm/s for Zn–0.6 at.% Ag and 2.39<V<54.5 mm/s for Zn–1.8 at.% Ag.
Acknowledgements
One of the authors, W. Xu, would like to thank Dr. Dong Ma (Institute of Materials Research and Engineering, Singapore) for the valuable discussions and Ms. Zhao-Min Deng for the careful preparation of the manuscript.
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