Anomalous Diffusion of 14C in TiC0.67

Abstract

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.

Diffusion of carbon in TiC1−y and ZrC1−y

“…This plot indeed fits more or less into the range of data found by Sarian [ 7 ]: D~ (calc) =0.026 exp[ -(40000/T) ] cm2/s, leading to an apparent activation energy of 340 kJ/ mole. For higher values of y, however, we calculate much higher values for D~ than found by Sarian [7,8] and Eremeev et al [10].…”

“…The tracer diffusion measurements of Sarian gave the somewhat surprising result that the tracer diffusion coefficient of carbon D~ in nearly stoichiometric TiCo.97 was larger than in TiCoss7 (despite the larger amount of carbon vacancies in the latter), whereas D"~, for TiCo 66 was found to have an intermediate value [ 8,9 ].…”

“…[7][8][9][10] and our values for the intrinsic diffusion coefficient De. Obviously the thermodynamic factor plays an important role in this respect.…”

“…With respect to titanium carbide, Adelsberg et al [ 1 ], Vansant et al [ 2 ], Shcherbedinskaya et al [3], KOyama et al [4] and Quinn et al [5] have reported on chemical diffusion in a reaction-grown layer. Chemical diffusion of carbon or titanium into a single crystal was studied by Kohlstedt et al [6], whereas Sarian [7][8][9] studied the tracer diffusion of C 14 and Ti 44 in titanium carbide. Eremeev et al [10] investigated the C 14 tracer diffusion coefficient in polycrystalline samples.…”

Diffusion of Carbon in TiC

“…The increasing carbon content. According to Darken(12), the relation among the chemical diffusivity (D), intrinsic diffusivity (D), and self-diffusivity (D*) is expressed as follows: (6) For the transition metal monocarbide which has the NaCl-type structure, the diffusivity of metal tracer is smaller than that of carbon. For TiC, D*Ti is smaller than D*c by a factor of 104 according to Sarian's measurement(13) Therefore, when carbon diffuses in a carbide crystal, the titanium sublattice can be assumed to be fixed.…”