Carbon diffusion in the carbides of niobium

Chemical Diffusion and Phase Equilibria in the Vanadium‐Carbon System Above 1900°C

“…interface, Co the carbon concentration iii the liquid vanadium, and y = X / ( 4 U ( VC)'/' where X is the thickness of the VC a t time t, and I I r V C the concentration-independerit chemical diffusion coefficient of carlmi in VC ThiS diffiisioti coeficicrit is given hy the equation 11~"' = X 2 / ( 4 y 2 t ) ' / 1 = K , / 4 y 2 (2 ) For tcmperaturcs below the V2C decomposition isotherm, the solutions to I'ick's Second Law €or the Iiorindary couditions shown in Fig 9 ( B ) 2~2 3 are and Suhstituting the appropriate boundary compositions (Fig. 6) and the parabolic growth constants (Table 111) into Eqs.…”

Interdiffusion in the carbides of the Nb-C system

“…Brizes et al [12] reported a similar phenomenon, and Huang [10] attributed it to widening of the NbC 1Ϫx phase field in the direction of decreasing carbon concentration with increasing temperature. This interpretation is used in the present study-the lamellar layer is included in the Despite the low number of couples, parabolic plots of the square of layer thickness vs time were taken for those couples made using high-density graphite.…”

“…The carbon concentration profiles ( Figure 12) were [13] , and the lower dotted straight line is extrapolated from the layer growth data of Brizes et al [12] nearly linear in the carbon-deficient NbC 1Ϫx and increasing in slope toward the carbon-rich end of the profile. Carbon density had no effect on the overall shape of the profile in that the NbC 1Ϫx profiles in all Nb|C couples shared these features.…”

“…Brizes et al [12] obtained rather higher activation energy terms for layer growth between 1700 ЊC and 2300 ЊC.…”

“…[11,12,13] All of these were determined by parabolic analyses of NbC 1Ϫx layer growth in Nb|C bulk diffusion couples and, therefore, represent average values for the interdiffusion coefficient D across the width of the NbC 1Ϫx phase field. Brizes et al [12] were able to obtain a partial carbon concentration profile in the Numbers denote the reference for the given data. Data are included from the present study at the extremes of concentration.…”