Merton C. Flemings

Rheological behavior of Sn-15 pct Pb in the crystallization range

Rheological behavior of Sn-15 pct Pb alloy in the solidification range has been investigated using a Couette type viscometer. In samples partially solidified before shearing, deformation is localized and primarily intergranular. Samples containing more than about 0.15 fraction solid exhibit an “apparent yield point” which is on the order of 106 dyne per sq cm and increases with increasing fraction solid. When shearing is conducted continuously while the alloy is cooled from above the liquidus to the desired final fraction solid, shear stresses required for flow are reduced by about three orders of magnitude. The solid-liquid mixture now behaves as a fluid slurry. Structural examination shows that shear takes place throughout the cross section of the specimen and that the solid is present as a fine grained particulate suspension. Flow behavior can be described by a viscosity which depends on fraction solid, decreases with increasing shear rate and exhibits hysteresis when shear rate is changed. For shear rates of 200 sec−1, at 0.40 fraction solid, viscosity is about 5 poise which is equivalent to that of heavy machine oil at room temperature. The fact that the slurry is highly fluid at large fractions solid suggests potential applications in new and existing metal casting processes.

Semisolid solidification of high temperature superconducting oxides

Experiments are reported on two techniques for melt‐texture processing Ba2YCu3O6.5 by directional solidification from a semisolid melt containing particles of BaY2CuO5 and a copper‐rich liquid. One of these employs an electric resistance furnace with ambient or oxygen enriched atmosphere; the other is a laser‐heated furnace operating at 1.3 atm oxygen. Solidification interface morphologies and other structural features were examined in quenched specimens. Depending on growth rate and temperature gradient, three different types of growth morphologies of the growing 123 phase were observed: ‘‘faceted plane front,’’ ‘‘cellular dendritic’’ or ‘‘equiaxed blocky.’’ The interface temperature decreased markedly with increasing growth rate for the faceted plane front specimens. In the remaining specimens, solidification took place over a range of temperatures. The temperature of the ‘‘root’’ of the solidification front dropped, but temperature of the solidification front ‘‘tip’’ did not. A solidification model is ...

Elimination of Solute Banding in Indium Antimonide Crystals by Growth in a Magnetic Field

Samples of tellurium‐doped indium antimonide that exhibited solute banding when solidified in a horizontal boat under ordinary conditions showed no bands when solidified in the presence of a vertical dc magnetic field. The effect of the field was to damp the thermal convection in the molten sample to the point of suppressing turbulence, thereby eliminating the temperature fluctuations in the liquid that cause the bands.

Preparation and casting of metal-particulate non-metal composites

A new process for the preparation and casting of metal-particulate non-metal composites is described. Particulate composites of ceramic oxides and carbides and an Al-5 pet Si-2 pct Fe matrix were successfully prepared. From 10 to 30 wt pct of A12O3, SiC, and up to 21 wt pct glass particles, ranging in size from 14 to 340 ώ were uniformly distributed in the liquid matrix of a 0.4 to 0.45 fraction solid slurry of the alloy. Initially, the non-wetted ceramic particles are mechanically entrapped, dispersed and prevented from settling, floating, or agglomerating by the fact that the alloy is already partially solid. With increasing mixing times, after addition, interaction between the ceramic particles and the liquid matrix promotes bonding. Efforts to mix the non-wetted particles into the liquid alloy above its liquidus temperature were unsuccessful. The composite can then be cast either when the metal alloy is partially solid or after reheating to above the liquidus temperature of the alloy. End-chilled plates and cylindrical slugs of the composites were sand cast from above the liquidus temperature of the alloy. The cylindrical slugs were again reheated and used as starting material for die casting. Some of the reheated composites possessed “thixotropy.” Distribution of the ceramic particles in the alloy matrix was uniform in all the castings except for some settling of the coarse, 340ώ in size, particles in the end-chilled cast plates.

Interdendritic fluid flow and macrosegregation; influence of gravity

An analysis is given of interdendritic flow behavior during solidification of castings and ingots, assuming resistance to flow is as in other types of porous media. Driving forces for the flow are solidification contractions and gravity acting on a fluid of variable density. Detailed flow calculations are given for horizontal, unidirectional, steady-state solidification, using aluminum-copper alloys as examples. Conditions are quantitatively described under which gravity induced convection becomes an important contributory cause of macrosegregation. A critical condition of flow is shown to produce local melting with resulting formation of “channel-type” segregates. Qualitative examples are given of application of the ideas presented to interpretation of macrosegregation in commercial ingots, with specific reference to centerline segregation and “channel-type” segregation, including “V” segregates, “A” segregates and “freckles”.

Infiltration of fibrous preforms by a pure metal. Part I. Theory

General expressions are derived to describe fluid flow and heat transfer during infiltration of fibrous preforms by a pure metal. Analytical solutions to the problem are given for the case of unidirectional infiltration into a uniform preform of aligned fibers under constant applied pressure. Calculations are carried out for infiltration kinetics (including total infiltrated length) and temperature distribution, using as an example alumina fiber/aluminum composites. Limiting cases leads to very simple expressions. Initial fiber temperatures both above and below the metal melting point are considered. In the case of fibers at a temperature significantly below the metal melting point, it is concluded that the factor most strongly influencing infiltration is the solidification of metal in the interfiber region. In the calculations, it is assumed that this solidification is in the form of a uniform solid metal sheath around the fibers. Metal superheat, when present, serves to progressively remelt the solidified sheath from the upstream end of the preform. Fiber volume fraction and initial temperature are predicted to have a major effect on infiltration kinetics, while metal superheat exerts a relatively minor influence. When no external heat extraction is present and a constant pressure is applied to the metal, flow through the preform continues indefinitely. For the case of external heat extraction, flow ceases when sufficient solidification occurs to block flow.

Deformation of semi-solid Sn-15 Pct Pb alloy

The rheological behavior of semisolid Sn-15 pct Pb alloy was studied using a parallel-plate viscometer. Small nondendritic and dendritic semisolid samples of the alloy were deformed under a constant load at initial pressures up to 232 kPa (33.6 psi) and at fractions solid from 0.15 to 0.60. Strain-time data for the nondendritic material obey the non-Newtonian, two-parameter, Ostwald-de-Waele, power-law model,i.e. μ = mγn−1, where μ is viscosity γ shear rate andm andn are constants. For fractions solid above about 0.30, the following empirical equation relates viscosity, shear rate and fraction solidμ = a exp (bfs) γ(cfs+d) 0.3 <fs < 0.60 wherefs is fraction solid anda, b, c, d are constants. The nondendritic alloy deformed homogeneously without cracking to very large strains (up to 80 pct). Dendritic alloys required much higher loads and cracked easily. For the nondendritic alloys the forging pressures to obtain 50 pct compression were of the order of 7 to 70 kPa (1 to 10 psi) for fractions solid under 0.55 and 172.5 to 207 kPa (25 to 30 psi) for fraction solid of about 0.60. For the dendritic alloys, the forging pressure required to achieve 10 pct compression is about 85 kPa at a fraction solid of 0.35 and increases rapidly with increasing fraction solid.

Macrosegregation in a multicomponent low alloy steel

Macrosegregation theory is extended to predict the formation of channel-type segregation for multicomponent systems. Specifically, calculations are carried out for 0.7 pct C steel, by considering heat, mass and momentum transport in the mushy zone. In the model used for calculations the momentum transport equation and the energy equation were solved simultaneously. It is confirmed, by comparing calculated results with experimental results, that this model successfully predicts the occurrence of channel-type segregation. This analysis is also more rigorous than previous works on macrosegregation because previous analyses were done by solving for convection in the mushy zone with an “uncoupled” temperature field. Using the model, the effects of adjusting the compositions of silicon and molybdenum in steel were quantitatively evaluated in order to show how channel-type segregates can be avoided by adjusting alloy composition. A method of optimizing composition to minimize segregation is presented. It is recommended that this methodology be applied to alloy design so that ingots of alloys amenable to commercial practice can be obtained readily with a minimum amount of “trial-and-error” development work and expense.

Effect of Strain Rate on Deformation Behavior of Semi-Solid Dendritic Alloys

The behavior of semi-solid Sn-Pb alloys was studied in compression between two parallel plates. Small dendritic samples were deformed at cross-head speeds leading to initial strain rates ranging from 1.3 × 10-3 s-1 to 1.2 × 103 s-1 in the semi-solid state at a temperature just above the eutectic. At the lower rates of deformation, breakdown of the dendrite structure occurs, at strains of 0.2 to 0.4, and a high degree of segregation of the liquid phase occurs. For higher rates the segregation no longer occurs to such a great extent and the alloy deforms more homogeneously. Some related experiments involving compression over a filter are presented to obtain stress-strain relations in bulk compression for later analysis. The behavior in compression of alloys in the semi-solid state may be used as a refining process in the low strain-rate range where segregation of the liquid is large. It may also prove useful in the high strain-rate range as a forming method.

Infiltration of fibrous preforms by a pure metal. Part II. Experiment

In a previous paper, a theory was developed to describe the flow of a pure metal into a fibrous preform. This paper presents experimental data to test the results of the theory for pure aluminum flowing into fibrous alumina preforms. An apparatus was designed and built for unidirectional infiltration under constant pressure and carefully controlled temperature parameters. A sensor was also developed to measure the position of the liquid metal in the fibrous preform during the experiment. This technique enabled quantitative comparison of theory and experiment. Experimental data are reported for the infiltration by 99.999 and 99.9 wt pct pure aluminum of SAFFIL alumina fibers fabricated into two-dimensionally random preforms. Fiber volume fraction was varied from 0.22 to 0.26, fiber preheat temperature was varied from approximately 483 to 743 K, and metal superheat was varied from 20 to 185 K. Infiltration pressure was varied from 1 to 4.5. MPa (145 to 650 psi). Agreement between theory and experiment was very good under all the experimental conditions studied for the 99.999 wt pct pure matrix. The impurity level of the metal was found to influence infiltration significantly. The measured perform permeability for 99.9 wt pct aluminum was much lower than that for 99.999 wt pct aluminum.