R.P. Agarwala

Diffusion of carbon in stainless steels

Abstract Using residual activity technique, diffusion of carbon-14 in 304, 347 and 316 steels has been studied in the temperature range of 450–1200°C. The temperature dependence of diffusivity could be expressed as Dc/304s. steel = 6.18 exp (−44 610/RT); Dc/347s. steel = 0.35 exp (−40 140/RT); Dc/316s. steel = 0.19 exp (−37 400/RT). The activation energy for the diffusion of carbon in steels has been explained on the basis of alloying effect of the constituents in steels. Furthermore, thermodynamic calculations for the free energy of formation of various carbides have been carried out to predict the carbon pick up by stainless steels as Cr23C6 from uranium carbide in a uranium carbidestainless steels system. In order to study the segregation of carbon along the grain boundaries of stainless steels, in the present investigation autoradiographie technique has been employed. The results indicate that the segregation of carbon is due to the preferential precipitation of Cr23C6 along the grain boundaries of stainless steels.

Diffusion of chromium in Inconel-600

Abstract The lattice and grain boundary diffusion of 51Cr in Inconel-600 (nickel base alloy) has been studied using Gruzins residual activity technique in the temperature range of 673–1523K. The lattice and grain boundary diffusivities can be expressed as D(1073–1523K) = (1.60 ± 0.30) × 10 −4 exp ( −q RT )m 2/s , q = 277.67 ± 4.21 and D gb σ(673–1073K) = (4.23 ± 0.80) × 10 −14 exp ( −q RT ) m3/s, q = 179.54 ± 3.60.δ is the grain boundary width and The activation energies are given in kJ/mol and R in joule per degree per mole. The effect of chromium and iron on the diffusivity in the alloy has been discussed on the basis of bond strength.

Diffusion of vanadium in niobium, zirconium and vanadium

Abstract Diffusion of vanadium has been studied in niobium, zirconium and vanadium crystals using radioactive tracer and residual activity technique. In niobium and alpha zirconium, the diffusion coefficients (cm2/sec) have been given: Dv/Nb(1000°–1400°C) = 2.21 exp (−85,000/RT) and Dv/α-zr(600°–850°C) = 1.12 × 10−8 exp(−22,900/RT) In beta zirconium and vanadium, little deviation from linearity in the plots of log D vs. 1 T have been detected and the diffusion coefficients (cm2/sec) have, therefore, been described in two temperature ranges as: D v β-zr (870°–1200°C) = 7.59 × 10 −3 exp (−45,800/RT) D v β-zr (1200°–1400°C) = .32 exp (−57,200/RT) and D v v (700°–1300°C) = 0.107 exp (−64,600/RT) D v v (1050°–1400°C) = 10.45 exp (−76,800/RT) All these data are discussed in light of Kidsons model and the impurity content of the host metals.

Diffusion of vanadium in aluminium and nickel

Abstract Diffusion studies of vanadium in aluminium and nickel were carried out using residual activity method. The diffusion coefficients (in cm2/sec) are described as D V A1 = 6.05 × 10 −8 exp (−19,600/RT) D V Ni = 0.87 exp (−66,500/RT) The lower diffusivities associated with transition elements in aluminium are compared and discussed with the results of diffusion studies of the same elements in nickel.

Diffusion of chromium in aluminium

Abstract Diffusion coefficient of chromium in aluminium has been determined over temperature range of 250–605°C using radioactive tracer and residual activity technique. Diffusion coefficient (in units of cm2/sec) is given by D Cr Al = 3.01 × 10 −7 exp (−15,400/RT) The frequency factor and activation energy are quite small as expected for low solid solubility of chromium in aluminium. In the present case diffusion is supposed to occur through dislocation short circuiting paths.

SELF AND IMPURITY DIFFUSION IN ALPHA-ZIRCONIUM.

Abstract The values of frequency factor ( D 0 ) and activation energy ( Q ) for self and impurity diffusion in alpha zirconium have been reported to be extremely low compared to the predicted theoretical values. The extremely low value of D 0 and Q both for self and impurity diffusion in alpha zirconium suggests a possibility of transport of material through dislocation pipes. In this present paper, random walk analysis and the Hart relation have been used to analyse quantitatively the contribution of randomly oriented dislocations towards apparent volume diffusivity for self and impurity (tin, chromium, silver, vanadium and molybdenum) in alpha zirconium. Quantitative analysis indicates the enhancement of apparent volume diffusivity due to the presence of randomly distributed dislocations (even of the order of 10 6 –10 7 dislocation lines/cm 2 ).

Diffusion of carbon in zirconium and some of its alloys

Abstract Diffusion of carbon in zirconium, zircaloy-2 and Zr- 2.5% Nb has been studied in the temperature range 873–1523K for zirconium and zircaloy-2 and 753–1523K for Zr-2.5% Nb alloy, using the residual activity technique. The diffusivities (in m2/s) in the α and β phases could be represented by D C/α-Zr (873–1123K) = (2.00 ± 0.37) × 10 −7 exp [ −(151.59 ± 2.51) RT ] D C/α-Zircaloy-2 (873–1043K) = (1.41 ± 0.32) × 10 −7 exp [ −(158.99 ± 3.14) RT ] D C/α-Zr-Nb-alloy (753–873K) = (4.68 ± 0.88) × 10 −7 exp [ −(159.98 ± 2.91) RT ] D C/β Zr ((1143–1523K) = (8.90 ± 1.60) × 10 −6 exp [ −(133.05 ± 1.46) RT ] D C/β Zircaloy-2 (1263–1523K) = (2.45 ± 0.61) × 10 −5 exp [ −(150.29 ± 1.72) RT ] D C/β Zr-Nb alloy (1143–1523) = (1.70 ± 0.42) × 10 −5 exp [ −(158.20 ± 2.09) RT ] The activation energies are given in kJ/mole. In the phase transition region, the diffusivities could be represented by the empirical relation: D = DαCα · DβCβ, where Cα, Cβ are the concentrations of the two phases in the alloy and Dα, Dβ are the extrapolated values of diffusion co-efficients in the α and β phases respectively. The results have been explained in terms of the interstitial mechanism of diffusion.

Self- and impurity diffusion in zirconium-manganese alloys

Abstract Self- and impurity diffusion in Zr-Mn alloys (Mn∼0–2 at. %) have been studied using the sectioning method in the temperature range 1173–1473 K. With an increase of the Mn content in the alloys, the diffusion parameters (D 0 and Q) for 95Zr diffusion in these alloys varies from 0·31 × 10−8 to 3·36 × 10−8 m2/s and 105·25 to 125·34 KJ/mole, respectively, while for 54Mn diffusion, they vary from 5·38 × 10−7 to 0·08 × 10−7 m2/s and 140·65 to 104·62 KJ/mole. Thus the addition of manganese to zirconium results in the enhancement of self-diffusivities while retarding impurity diffusion. The impurity correlation factor calculated using Le Claires model for body-centred-cubic crystal is 0·42. The results are consistent with a vacancy mechanism of diffusion.

Diffusion of nickel and tin in zircaloy-2

Abstract The diffusion of nickel and tin in alpha and beta zircaloy-2 has been studied by the residual activity technique in the temperature range 650–1250 °C. The temperature dependence of diffusivities (cm 2 /sec) can be expressed as D Sn /α-zircaloy-2 (650–780 °C) = (3.31 ±0.56) × 10 −6 exp( −36.630±377 RT) D Sn /β-zircaloy-2 (980–1250 °C) = (2.80 ±0.56) × 10 −1 exp( −55.905±809 RT) and D Ni /α-zircaloy-2 (650–780 °C) = (1.00 ±0.17) × 10 −7 exp( −29.339±296 RT) D Ni /β-zircaloy-2 (980–1250 °C) = (7.30 ±1.16) × 10 −3 exp( −47.960±587 RT) . The results have been explained on the basis of alloying characteristics of zircaloy-2 and it is noticed that with the exception of tin, the small percentage of other constituents in zircaloy-2 does not affect the diffusion properties.

Recovery in neutron irradiated tungsten

Abstract The recovery of defects in partially and well annealed tungsten has been studied using electrical resistivity technique. The irradiation was carried out at reactor ambient temperature (∼343 K) to a fluence of 1022 n/m2. Isochronal annealing studies of the defects show two stages centred around 0.17Tm and 0.31Tm referred to as Stage III and Stage V respectively. Stage III shows a peak temperature shift to lower temperature for increasing amount of recovery. Isothermal annealing of the defects in Stage III and Stage V has been carried out and activation energy calculated using Meechan-Brinkman analysis. A single value of activation energy (1.69 ± 0.20 eV) and essentially a second order kinetics has been found in Stage III. However, no single value of activation energy could be obtained for Stage V (the activation energy varies from 2.5 to 3.6 eV). On the basis of the activation energy, reaction order and the negative shift in the peak temperature, Stage III has been attributed to arise due to migra...