Jean Debuigne

Interdiffusion in the body cubic centeredβ-phase of Ta–Ti alloys

Abstract Interdiffusion in the β -phase of Ta–Ti alloys is studied in the temperature range 1000–1900°C. Interdiffusion coefficients are calculated with Den Broeder’s method from concentration profiles determined by EPMA. These coefficients depend strongly on the composition: the interdiffusion coefficient for the lower tantalum concentrations is approximately 1000 times greater than the one for the higher concentrations. The activation energy (150–240 kJ/mol) is characteristic of a monovacancy diffusion mechanism. Except for the impurity diffusion of tantalum in titanium, determined by the Vignes and Birchenall method, anomalous behaviour of the diffusion in the β -phase of titanium is not observed. The diffusion in these alloys is characterized by an important Kirkendall effect, materialized by a marker’s shift towards the titanium rich part of the sample. The intrinsic diffusion coefficients have been calculated between 1000°C and 1600°C, at the composition of the Kirkendall plane, by Heumann’s method: the intrinsic diffusion coefficient of titanium is approximately five times greater than that of tantalum.

The ternary system uranium-boron-carbon

Phase equilibria in the ternary system U-B-C have been established by means of X-ray, metallographic and melting point analyses in the temperature range from 1000 ° C to melting. Three ternary compounds were found to exist: besides the well known monoboroncarbide UBC two new uranium boroncarbides, UB2C and “U5B2C7”. Ternary phase equilibria are characterized by the incompatibility of uranium metal with boroncarbide B4C and by the incompatibility of elemental boron and uranium carbides; an isothermal section of the system U-B-C at 1600° C is presented. At high temperatures the crystal structure of UB2C was found to be isotypic with the homologous compound ThB2C; at temperatures below (1675 ± 25)°C h-UB2C transforms into a low temperature modification with a new (unknown) structure type. The crystal structure of “U5B2C7” is closely related to the structure type of Ho5B2C6−7 as a derivative of La52C6 Employing the Pirani-technique, congruent melting was revealed for UBC and UB2C at (2144 ± 25)°C and (2282 ± 30)°C respectively. Using the clear-cross principle in studying possible phase reactions, the thermodynamic stabilities of UBC, UB2C and U5B2C7 were estimated.

On the diffusion in the Mo–Ta refractory system

Abstract Diffusion in the Mo–Ta system is studied as regards interdiffusion and impurity diffusion over the temperature range from 1500 to 1900 ° C . Interdiffusion coefficients, calculated with the den Broeder or Hall methods, are found to be weakly composition-dependent. Their temperature-dependence leads to Arrhenius parameters consistent with (i) literature data obtained at higher temperatures and (ii) a monovacancy mechanism. The composition-dependence of the Arrhenius parameters implies repulsive interactions between unlike atoms. Linear relationships between those parameters are found to exist among composition. Impurity diffusion coefficients DMo(Ta)0 and DTa(Mo)0 are determined by the Vignes and Birchenall method and are consistent with the interdiffusion coefficient limits. By comparison with literature data, anomalous behavior is obvious over the range 1500– 2220 ° C for both coefficients.