Institute of Metals Division - An X-Ray Method to Determine the Liquidus in High-Temperature Binary Phase Diagrams (TN)

In low temperature phase diagrams, liquidus curves below about 1500°C are relatively easily established, e.g., by thermal analysis or separation of coexisting solid and liquid phases, followed by chemical analysis. Experimental difficulties increase considerably at higher temperatures and in systems involving more reactive metals. Although there are now thermocouples available for use from 1600" to 2400°c, as summarized by Caldwell,' the need for frequent calibration, aging, and contamination problems, and unavailability of suitable protection tubes has sharply limited their use at higher temperatures. Many of the high melt- ing metals are found to react with refractory oxides. TO avoid such problems, a frequently used procedure is the diffusion and saturation method, as described by Spedding et a1.' for the system La-C. This technique involves the annealing of one metal or a suitable alloy in a crucible of the other element at a temperature above the melting point, followed by chemical analysis of the center of the melt, after equilibrium is established. Further, if an alloy system is to be investigated employing the electron microbeam probe, and calibration charts have been established, it may be possible to measure the alloy concentration at the center of a cored as-cast structure, and to conclude from this points on the liquidus curve in a manner similar to the one to be described. It is known that in phases with a broad homogeneity range the rapidly cooled, cored, as-cast structure of an alloy presents concentrations having different lattice constants; this spread of lattice constant values contributes to the broadened lines of a powder pattern for the as-cast condition. The region of the crystal which is first to solidify will then correspond to the edge of the lines if the change of the lattice parameters with concentration is monotonous. In the course of the determination of the Ta-Tr phase diagram,3 it became evident that such behavior made it possible to construct the