F. Weinberg

The columnar to equiaxed transition in tin-lead alloys

Tin-lead alloys were solidified directionally and the position of the columnar to equiaxed transition determined on vertical sections of the ingots. The columnar length was found to increase with decreasing lead concentration and increasing heat transfer coefficient. A mathematical model of the heat flow in the system was used to determine local temperatures, temperature gradients, and velocities in the solidifying alloy. Comparing the position of the columnar to equiaxed transition and local thermal conditions, it was found that the transition occurred when the temperature gradient in the melt at the liquidus temperature was 0.11°C/mm for Sn 10 wt pct Pb, 0.10°C/mm for Sn 5 wt pct Pb, and 0.13°C/mm for Sn 15 wt pct Pb. The position of the transformation was found to be independent of melt superheat for the conditions considered.

The columnar-to-equiaxed transition in Al 3 Pct Cu

A columnar-to-equiaxed transition is observed in Al 3 pct Cu solidified directionally from a chill face. The transition occurs when the temperature gradient in the melt ahead of the columnar dendrites decreases to 0.6 ‡C/cm at dendrite growth rates of about 5 x 10-3 cm/s. Increasing the nuclei density by adding 171 ppm of TiB2 to the melt produces a fine-grained structure without columnar growth. Adding 100 ppm TiB2 has no effect on the cast structure or columnar-to-equiaxed transition. The results are considered in relation to the model for the columnar-to-equiaxed transition proposed by Hunt.[2]

Spangle formation in galvanized sheet steel coatings

Very large grains, termed “spangles,” are produced on galvanized sheet steel coatings when lead is added to the zinc bath. The spangles have been attributed to melt undercooling prior to solidification. The present results indicate this is not the case, undercooling being less than 1 °C. The spangle diameter is shown to be dependent on the alloy addition to the bath, large spangles being obtained with Bi and Sb as well as Pb. The spangle size is related to the surface tension of the alloying addition, the size decreasing as the melt vapor surface tension of the alloying element increases. It is proposed that spangles form dendritically from a nucleus in the melt. Alloy additions with low interfacial energies and very limited solid solubility are highly concentrated ahead of the dendrite tip. This decreases the tip radius and increases the dendrite velocity, producing large grains. The basal plane orientation of the samples varies between 17 and 80 deg with respect to the steel sheet surface, which is inconsistent with basal plane dendritic growth in Zn along (1010) directions. It is proposed that solute additions to the melt and growth in a thin liquid layer can modify the dendrite growth direction, accounting for the spangle orientation.

Interaction of iron particles with a solid-liquid interface in lead and lead alloys

The interaction of iron particles with an advancing solid-liquid interface has been examined in lead and lead alloys. In the case of pure lead solidified with a planar interface, the particle distribution in the solid and quenched liquid was uniform, indicating particles werenot rejected at the interface. For interfaces with a cellular and dendritic structure the particles were concentrated in the cell walls and interdendritic regions. This concentration is accounted for on the basis of the particle velocity and convective flow in the melt during the particle interface interaction. A water model was examined which simulated the metal system, using nylon spheres for particles and a lucite cellular surface for the interface. The results confirmed that particles are concentrated at intercellular regions as a result of the particle velocity and fluid flow.

The pitting corrosion of p-type GaAs single crystals

Abstract Semiconducting p-type GaAs was prepared by thermally diffusing an evaporated layer of Cu into either (001) or (001) surfaces of GaAs wafers. An ohmic contact was constructed on the coated surface, the assembly mounted in epoxy, and the uncoated surface polished metallographically to provide the working electrode surface. Anodic linear sweep and pitting scan polarization studies were conducted in 1 M NaCl (pH 2, 7 and 12) and 1 M NaNO 3 (pH 2 and 7). Passivation and pitting were observed only at pH 7 and the pitting potential ( E pit ) was independent of anion species. The polarization behaviour correlated reasonably well with predictions arising from a theoretical E-pH diagram constructed for the GaAs-H 2 O system. Scanning electron microscopy showed that pits were crystallographic, with walls composed of {111} planes. Pits were narrow, elongated, and exhibited undercutting effects in 1 M NaCl. Pits in 1 M NaNO 3 were open. The long axis of the pits was always parallel to one specific (110) variant in the wafer surface and changed direction by 90° when the wafer surface changed from (001) to (001). Pitting was considered to arise at strain induced flaws in the film caused by selective leaching of the As component and localized changes in solution chemistry. The detailed morphology of the pits was consistent with the layer structure of the compound semiconductor.

The movement of particles in liquid metals due to gravity

The buoyancy of a single crystal copper cube in a lead tin melt was examined. A neutral buoyancy melt density of 8828 Kg/m3 at 250 °C was obtained which corresponds to a copper density of 8930 Kg/m3 at 20 °C. The copper cube was found to change from a floating to sinking position with changes in melt density of 1 Kg/m3. Contact with the crucible walls or meniscus prevented the copper cube from moving under buoyancy forces resulting from density differences as high 12 Kg/m3. This is attributed to surface tension forces at the points of contact of the copper, melt, crucible wall, and meniscus. Very small density variations in the copper due to micro-porosity and other imperfections was found to have a very large effect on the movement of the copper under buoyancy forces. In experiments in which grains of tin in partially solidified pure tin were allowed to settle under controlled conditions, it was found that many grains appeared to be separated from neighboring grains. In addition, regions clear of grains were observed in the settling region. It is suggested that this results from clumping of the grains and bridging of the clumps as they settle in the melt.

The casting of steel

The solidification of metals will be considered as it relates to the casting process. Current theories and mechanisms will be examined with reference to their applicability to situations encountered in industry. The solidification process in static cast steel ingots and continuously cast steel will be considered in some detail, based on experimental observations. These observations were made from in-plant tests in which radioactive gold was added to the liquid pool during solidification. The results will be related to a mathematical model of the system based on a heat flow analysis. Using the model, predictions of middle and center line cracking of continuously cast billets have been made which conform with observed cracking.

The influence of convection on heat transfer in liquid tin

Heat flow through liquid tin has been measured with convective flow present in the melt. The results indicate that heat flow in the melt is enhanced by a factor of 9.6 due to this fluid flow, as compared to heat flow through a stagnant melt. The enhancement factor was found to be essentially constant at temperature differences greater than 4 °C across the melt. This is shown to be due to a constant fluid flow rate at higher temperature differences. It is noted that the thermal flow through a convective melt can be strongly limited by the thermal resistance of the container walls along the heat flow path.

The shear strength of alumina/aluminum alloy interfaces

Metal matrix composites consisting of alumina particles in an aluminum alloy matrix are increasingly finding applications in the automotive industry and elsewhere. The mechanical properties of the composites depend on the properties of the base constituents as well as the size, volume fraction and distribution of the second phase. The present investigation was undertaken to examine the feasibility of directly measuring the shear strength of an alumina/aluminum alloy interface using an alumina cylinder cast in an aluminum alloy and pushing the cylinder through the alloy. The effect on the shear strength of the aluminum alloy composition, test temperature, and surface treatment have been examined.

The behaviour of cadmium monocrystals and polycrystals in acid chloride solution

Abstract The polarisation characteristics of Cd polycrystals and monocrystals, with (0001), (1120), and (1010) surfaces, were studied in 0.1 M NaCl + 0.04 M HCl (pH 1.3) and compared with thermodynamic equilibria. Anodic dissolution rates were independent of surface orientation, exhibiting Tafel slopes ( b a ) near 27 mV that were reasonably consistent with a rate controlled process based on mass transport of CdCl + from the Outer Helmholtz Plane (OHP). Other processes are likely to have played a contributory role in the anodic behaviour, because crystallographic faceting accompanied dissolution. Cathodic evolution of hydrogen exhibited mixed rate control, with a cathodic Tafel slope ( b c ) near −81 mV. Hydrogen evolution appeared to be sensitive to surface orientation, with the (1010) surface showing the largest deviations in behaviour.