Studies have been made of the microstructures developed in directionally solidified monotectic Al-In, Al-Bi and Zn-Bi alloys, as a function of growth velocity and temperature gradient. With increasing growth velocity and decreasing gradient the microstructures show transitions from regular rod-like arrangements of the lower melting point phase, through arrays of aligned droplets to coarse irregular droplet dispersions. Intermediate stages show rods with longitudinal shape perturbations of a classic Rayleigh-type instability. The changes are discussed in terms of oscillatory instabilities at the solid-liquid interface (enhanced by increasing growth velocity and decreasing temperature gradient) coupled with ripening effects in the solid + liquid region behind the interface.

An alternate in situ technique is presented for preparation of aligned microstructures which has been successfully applied to the preparation of copper alloys. In this technique, a two phase alloy is first prepared by conventional casting techniques to produce a random array of dendrites of phase 1 in a matrix of phase 2. The casting is then reduced to wire size by mechanical reductions. It is demonstrated that the alignment of the dendritic phase produced by the mechanical reduction is surprisingly good, and that one can achieve a wide range of microstructural control over the filament size and morphology by utilization of controlled coarsening anneals followed by additional wire drawing. Examples are presented of Cu-Nb alloys which are converted to Nb3 Sn-Cu superconducting wires possessing excellent critical current values, and of Fe-Cu alloys which produce wires having quite high strength to resistivity ratios.

Cu-Nb composite wires were produced from powder, electrolytically coated with tin and annealed to convert the Nb fibres to Nb 3Sn. The content was varied between 10 wt % and 40 wt %. The superconducting properties of the wires were determined. The mechanical properties, tensile strength, yield strength and ductility were measured as a function of volume fraction and deformation over a wide temperature range. The results are compared with those for wires produced by different techniques.

It is shown that marked microstructural coarsening can occur when eutectic composites are exposed for relatively short periods to temperatures approaching 0.9 TM (where TM is the eutectic melting temperature in °K). Less well known and, perhaps, of more practical importance, are the microstructural changes and consequent changes in properties which result from prolonged exposure at 'safe' operating temperatures, e.g. 0.5 Tm.

Cd-Zn and Pb-Cd eutectic specimens of various thermal histories were examined immediately on preparation and subsequently re-examined after storage at ambient laboratory temperatures for up to 1000 days. In general, microstructural changes from the as-grown and heat-treated conditions were observed. In some cases for the Pb-Cd alloy this consisted of only slight thickening of the already coarse larnellae terminations. However, in the more rapidly frozen, and therefore microstructurally finer, specimens marked discontinuous coarsening was observed, but confined to eutectic-cell-wall regions. There the coarse precipitates grew at many angles, including being perpendicular to the eutectic habit plane but were not separated by any well-defined large angle grain boundaries.

It was concluded that high temperature degradation occurs mainly by continuous coarsening, whereas at lowtemperatures, microstructural changes are effected mainly by discontinuous coarsening.

Various thermal treatments have been applied to Cd-Zn and Pb-Cd eutectic alloys in an attempt to better understand the thermal stability of eutectics close to their melting temperatures. Specimens of each system were grown at 40, 400 and 4000 mm/hr to obtain microstructures which were either lamellar, lamellar/cellular, or cellular. These specimens were thermally treated by either: (1) isothermal annealing at 0.95 Tm (up to 192 hrs), (2) thermal cycling between room temperature and 0.95 Tm (up to 64 cycles), (3) exposing to thermal gradients parallel to the growth direction and maintaining one end of the specimen at 0.95 Tm and the opposite end at room temperature (up to 48 hrs). It was found in all cases that some microstructural changes occurred but that the two alloy systems displayed marked differences in degradation sequence. An explanation and interpretation of these decay sequences is attempted. The mechanical properties of the variouslytreated alloy specimens were determined and are correlated with the observed microstructural features. The Cd-Zn specimens coarsened more rapidly than equivalent Pb-Cd specimens, as might be expected since the interphase boundary energies have been reported as 86 and 67 erg/cm2 respectively.

The solid-liquid interfacial free energies of each of the individual phases comprising the eutectic system, Carbon Tetrabromide-Hexachloroethane, were measured as a function of composition using a “grain boundary groove” technique. Thermodynamic data were combined with groove shape measurements made from high resolution optical photomicrographs of the solid-liquid interfaces to give the interfacial free energy data. An interfacial free energy balance at the eutectic trijunction was performed to obtain all the forces acting on that point. The three interphase interfacial free energies at the eutectic trijunctions as well as a solid-solid phase boundary torque were evaluated.

It was found that the solid-liquid interfacial free energies of the two phases of the eutectic could be evaluated from photomicrographs of growing or stationary eutectic interfaces. In addition, it was found that for a substantial range of freezing conditions the eutectic interface shape can be predicted from a knowledge of the interfacial free energies alone.

An eutectic alloy of antimony and manganese was directionally solidified to produce a composite of cylindrical fibers of MnSb aligned in a Sb matrix. The fibers were about 3 microns in diameter by 1 millimeter long and occupied 30% of the composite volume. Isothermal aging caused the fibers to develop undulations that had an average wavelength of 1.74 + 0.45 times the fiber circumference. The Nichols' and Mullins' model of fiber spheroidization predicts an undulation wavelength of √2 times the fiber circumference, for interface diffusion control. The Coine model, for volume diffusion control, predicts a wavelength that approaches infinity for this fiber volume fraction. Our modeling efforts have shown that the predicted wavelength is quite sensitive to the assumed spatial correlation between nodes on adjacent fibers. We also found that the undulation growth rate achieves a broad maximum as a function of wavelength, which may account for the large standard deviation associated with our measurements.

Ingots having a cross-sectional area of 1.27 cm × 10.8 cm were directionally solidified for testing off-axis tensile properties. To avoid macroscopic banding due to inductive melting and thermal convection, growth rates of 2.2 cm/min were required. The resulting solidification structures were cellular.

The macroscopic tensile behavior of the eutectic alloy was similar to that observed for mechanically formed fiberreinforced composite materials; in particular, three distinct modes of fracture occurred. In Region I, where the angle(ϕ) between the growth direction and the tensile axis was small, the tensile strength of the composite was inversely proportional to cos2 ϕ. In Region II (20° ≤ ϕ ≤ 55°) and Region III (60° ≤ ϕ ≤ 90°), the tensile strength of the composite was inversely proportional to sin ϕ, cosϕ, and sin 2ϕ, respectively.

For all regions, failure was initiated by the fracture of large Al3Ni intercellular blades. In Region I, fracture was by a combination of tensile overload on a plane perpendicular to the tensile axis or shear on a plane inclined to the tensile axis at approximately 45°. In Region II, failure was by shear in a plane containing the growth direction and in a direction parallel to the fibers. For Region III, failure was by plane strain constraint with shear perpendicular to the longitudinal axis of the fibers.

The fracture of large intercellular Al3Ni blades can be explained by the variation of S13 about the tensile axis, and the corresponding intercellular tensile strains which result. In Region II, metallographic examination of tensile specimens shows a Poisson's ratio effect to be contributing to the strength of the composite; occasionally shear between adjacent cells is observed. By using the yield strengths of specimens having fibers oriented parallel to and perpendicular to the tensile axis, an approximate yield surface can be constructed. This yield surface accurately predicts the transition in failure mode from Region II to Region III.