The influence of crystalline anisotropy on neck growth during the sintering of zinc

Abstract

Polycrystalline zinc wires of two grain sizes and zinc single crystal wires of four orientations all of them 0.010 in. in diameter were grown. Pairs of wires from each of the six groups were sintered together at 365°C for 8 hr in a pressurized dry hydrogen atmosphere and the widths of the necks formed between pairs were measured. In the single crystal pairs, the neck width depended upon the relative orientation of the basal planes in the wire with the largest width up to 2.7 times as large as the smallest. In the polycrystalline samples those with fine grain size, of the order of neck width, had small necks, while those with coarse grain size, of the order of the wire diameter, had large necks. The observed variations in the width of the neck with relative orientation and with grain size must be caused by the anisotropy of those properties of zinc which determine the rate of material transport during sintering. For sintering by diffusional flow these are specific surface free energy and diffusivity, for sintering by evaporation and condensation they are specific surface free energy and vapor pressure. Since for these mechanisms approximate equations for the rate of neck growth exist, the maximum ratios of neck width due to the anisotropies of specific surface free energies, diffusivities and vapor pressures can be estimated. They are considerably smaller than the observed ratio of up to 2.7. The remaining material transport mechanism for the early stages of sintering is plastic flow which, in zinc, is highly anisotropic, since even at temperatures near the melting point the critical resolved shear stress for slip on the basal slip system is five to ten times lower than for slip on any other system. The measured neck width of the single crystal wire pairs can be directly related to the ease of basal slip. Those pairs which are oriented so that no basal slip is possible have small necks, while those which are favorably oriented for basal slip have large necks. The small neck width in the fine-grained wires must be due to constraint which inhibits basal slip. No such constraint exists in the necks of the coarse grained wires. Material transport by plastic flow must play an important role in the sintering of the zinc wires.