This invention relates to a method of growing composites of the type that exhibit the Soret effect so as to improve their structure and homogeneity.
The Soret effect (also referred to as thermotransport) is the difference in concentration of constituents of a liquid solution that occurs in different parts thereof when such parts are at a different temperature. Composites that are influenced by the Soret effect to a relatively large degree are eutectic alloys (binary, ternary or higher) or polyphase alloys with eutectic-like structure. In growing such composites, the so-called Bridgeman method is often used wherein the alloy is melted in a vertically oriented crucible, and then directionally solidified by a process that moves the crucible relative to a furnace. Relative movement is accomplished by lowering the mold containing the alloy into and through the furnace, or by moving the furnace itself in the opposite direction. The crystal may be grown with or without a seed.
In the corresponding horizontal technique, the crucible is horizontally disposed. In both techniques, a temperature gradient is always present at the solid-liquid interface within the crucible. This situation results in the migration of components either toward the interface or away therefrom due to the Soret effect.
The Soret effect is evidenced in composites grown in these ways by non-homogeneity, i.e., variations in the concentration of the constituents from one end of the crystal to the other. Non-homogeneity (also called inhomogeneity) may be expressed as the percent variation in composition along the length of the crystal. Thus, this Soret effect produces a solidification product whose concentration of one constituent in the end of the crystal formed first, (i.e., the end from which growth started) is greater than the concentration of such constituent in the end of the crystal formed last.
In most crystals, this non-uniformity or non-homogeniety in concentration along the length of the crystal is a second order effect that is usually masked by variations in the constituent concentration along the length of the crystal arising from gravity or thermally induced convection currents in the molten portion of the sample undergoing directional solidification. Reference may be made to U.S. Pat. No. 3,464,812 granted on Sept. 2, 1969 to Utech et al which discloses a system for minimizing convection currents in the molten zone during directional solidification where the materials involved have some degree of electrical conductivity in their liquid states. Such system involves applying to the molten zone, a unidirectional, stationary magnetic field in the region where the transition of the material to its solid state is occurring. By making the magnitude of such field sufficiently high, the effective viscosity of the molten zone is significantly increased thus damping thermally induced laminar and turbulent flow and producing a more uniform solidification product.
While the homogeneity of the resultant solid crystal is significantly improved following the techniques proposed by Utech et al, some residual segregation remains due to the Soret effect. For example, consider a liquid eutectic alloy of aluminum and copper (Al-CuAl.sub.2), which contains 33% copper by weight and 67% of aluminum, held in a temperature gradient such as would exist across the solid-liquid interface of a sample undergoing directional solidification. Copper is observed to migrate toward the cooler region thereby introducing into the crystal a segregation of copper. The extent of such segregation is dependent upon parameters associated with the directional solidification process. It can be shown that the relative contribution to segregation of copper from the Soret effect, .eta. , is given by the expression: EQU .eta. = (D') (X.sub.2) (G)/(R) (1)
where D' = thermal diffusion coefficient of copper in liquid aluminum at eutectic composition (6.2.times. 10.sup..sup.-8 cm.sup.2 /sec.degree. C.)
X.sub.2 = mole fraction of aluminum in the eutectic alloy (0.826) PA1 G= temperature gradient at the solid-liquid interface PA1 R= rate of solidification.
If the rate of solidification is one centimeter per hour, and the temperature gradient at the solid-liquid interface is 50.degree. C./cm, then .eta. = 1%. Since D' and X.sub.2 are constant for an aluminum-copper eutectic, the amount of segregation will depend upon the temperature gradient G and the rate of solidification R, both of which are parameters associated with the growing of a crystal.
As small as the segregation may be due to the Soret effect, it is sometimes still too large for crystals to be used in applications such as eutectic composites used in structural applications; and it is the object of the present invention to provide a new and improved method for minimizing the inhomogeneities in concentrations of constituents arising from the Soret effect, in composites that exhibit this effect, thereby providing a solidification product with improved structure and homogeneity.