The self-diffusion of titanium-44 in TiCx (0.67≤x≤0.97) was studied in the temperature range 1920°–2215°C. The diffusion coefficients are composition independent and can be described by the expression DTi*=(4.36−2.34+5.02)×104 exp[−(176 400±3600)/RT] cm2/sec; titanium diffuses slower than the non-metal atom by a factor of approximately 104 in the temperature range investigated. The results support the concept of independent diffusion along the respective sublattice of each component.
The diffusion coefficients of carbon in single and polycrystalline ZrC have been measured in the temperature range 1350°–2150°C, using radioactive tracer techniques. Volume and short-circuit enhanced tracer diffusion coefficients are represented by the expressions D*=1.32×102 exp (−113 200/RT) cm2/sec and D*=1.6 exp (−90 000/RT) cm2/sec, respectively. The results are compared with apparent diffusion coefficients determined from existing carburization and high-temperature creep data.
The theory of particle coarsening kinetics under control by diffusion through the matrix and by phase-boundary reaction is reviewed, and extended to the case of diffusion control when the particles are closely spaced and for impurity control at the interface. For the first case, d̄=f(T,φ)t⅓, Nv=f(T,φ)t−1, where d̄ is the mean particle diameter, φ is the volume fraction solid, T is temperature, and Nv is the number of particles per unit volume. For the latter two cases, d̄=f(T)t12,Nv=f(T)t−32, the temperature-dependent functions being different for the two cases. The models are used to discuss existing kinetic data on the iron-liquid copper and oxide-liquid silicate systems. New data on the NbC-liquid iron system reveal the absence of the expected diffusion-field interaction effect when particles are close together. The role of surface-active impurities, particularly boron, on the mechanism of growth is discussed and it is shown that, while growth may be diffusion-controlled in the absence of impurities, it may become phase-boundary reaction controlled when appreciable quantities of impurities are present. The apparent coefficient of diffusion of iron in copper is calculated to be about 6.06×10−4 cm2/sec at 1120°C.
The self-diffusion coefficients of carbon-14 in TiC have been measured as a function of composition in the temperature range 1450° to 2280°C, and can be represented by the expressions: D*0.970=6.98±1.24 exp (−95 300±700/RT) and D*0.887=45.44±5.12 exp (−106 800±400/RT) cm2/sec. The effect of composition on the self-diffusion coefficients is discussed and the tracer coefficients are compared with apparent diffusion coefficients calculated from steady-state creep measurements and the kinetics of TiC compound formation.
The self-diffusion of 14C in TiC o .67 was studied in the temperature range 1745°-2720°C. The diffusivity, which exhibits a discontinuity at 2080°C, can be represented by two expressions: D*O.67= (2.85±0.20) X 10-4 exp[ -(49 600±300)/ RTJ cm 2 /sec above 2080°C and D*O.67= (1.14±0.66) X10 2 exp[ -(109,900±2000) / RTJ cm 2 /sec below this temperature. No discontinuity was found in carbon saturated TiCo.97 and the self-diffusion coefficient for this composition can be described to a temperature of 2720 c C by the single expression D*O.97= (6.98±1.24) exp[ -(95 300±700) / RTJ cm 2 jsec.
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