Grain-boundary Segregation Factor Research Articles

Emission Mössbauer Spectroscopy of Grain Boundaries in Ni–34%Cu Alloy

The grain boundaries in Ni–34%Cu alloy have been analyzed by emission Mossbauer spectroscopy. The mechanism of grain-boundary Co diffusion in the alloy and the grain-boundary segregation factor of this element are determined. The grain-boundary segregation of interstitial impurities is observed at low annealing temperatures.

Grain Boundary Diffusion of 57Co in Nickel

Grain boundary diffusion of cobalt in Ni of purity level 99.98 wt% was measured over a wide temperature interval using the radiotracer technique and applying the 57Co radioisotope. The diffusion experiments were performed in Harrison’s type B and C kinetic regimes. Temperature dependences of triple product and grain boundary diffusivity of Co in Ni have been determined. The value of grain-boundary segregation factor of cobalt in nickel was determined to be about unity in the temperature interval under investigation. It is demonstrated that the purer the material, the higher is the grain-boundary diffusivity and the lower the corresponding activation enthalpy of diffusion.

Emission Nuclear Gamma-Resonance Spectroscopy of Grain Boundaries in Coarse-Grained and Ultrafine-Grained Polycrystalline Mo

Grain boundaries in coarse-grained Mo with grain boundaries of recrystallization origin and in ultrafine-grained (UFG) Mo obtained by high pressure torsion have been studied by the emission Mössbauer spectroscopy on the57Co (57Fe) nuclei. It is demonstrated that Co atoms diffuse along grain boundaries by interstitials. The temperature dependence of grain-boundary segregation factor of Co in coarse-grained Mo has been determined. It is shown that the state of Co atoms in grain boundaries and near-boundary areas in UFG Mo differs from that in coarse-grained Mo.

Emission Mössbauer spectroscopy of grain boundaries in polycrystalline molybdenum

Grain boundaries in polycrystalline molybdenum have been studied by emission Mossbauer spectroscopy on 57Co (57Fe) nuclei. It has been shown that Co atoms diffuse along the grain boundaries by the interstitial mechanism. The states of Co atoms in grain boundaries and in near-boundary regions of the crystallites have been analyzed. The values of the effective diffusion coefficient in the subboundary regions of the crystallites have been estimated. The temperature dependence of the grain-boundary segregation factor of Co in Mo has been determined.

Emission Mössbauer Spectroscopy of Grain Boundaries in Poly- and Nanocrystalline Metals

Capabilities of the emission nuclear gamma resonance (NGR) spectroscopy in studying grain boundaries in metals are considered. It is demonstrated that this method enables to evaluate grain-boundary segregation factors, to determine grain-boundary diffusion mechanisms, to estimate an effective diffusion coefficient in near-boundary areas, etc. Besides, this method permits to evaluate quantitatively differences in the properties of grain boundaries of polycrystalline and nanostructured materials.

Grain Boundary Diffusion and Segregation of Co and Cr in Zr-Based Alloys

51Cr and 60Co diffusion along grain boundary (GB) in polycrystalline Zr and -Zr-20%Nb were measured by means of the radiotracer technique in an overall temperature range [380-1000] K. The use of Harrison´s C and B kinetics resulted in direct data of the GB diffusivity (Dgb) and the apparent GB diffusivity (Pgb). The analyzed temperatures involved those of power reactors service. The GB segregation factors s were determined or evaluated in the limit of very dilute solute concentration.

Mössbauer Investigations of Grain-Boundary Diffusion and Segregation

Possibilities of grain-boundary diffusion and segregation studies using nuclear gammaresonance spectroscopy (NGR) are considered. It is shown that the results of the Mössbauer investigations testify the necessity to specify the classical Fisher’s model of grain-boundary diffusion, and a possible way of such specification is suggested. It is demonstrated that investigation of grain boundaries using emission Mössbauer spectroscopy appreciably supplement the information obtained from the diffusion profiles analysis. In particular, Mössbauer investigations make it possible to evaluate directly the grain-boundary segregation factor, to determine the grain-boundary diffusion mechanism, to estimate the rate of the diffusant pumping from a grain boundary core into the bulk, etc.

Grain boundary diffusion and segregation of Ge in Cu: Radiotracer measurements in different kinetic regimes

Abstract Grain boundary (GB) solute diffusion of Ge in Cu polycrystals was systematically investigated by the radiotracer serial sectioning technique using 68Ge and 71Ge isotopes. The diffusion measurements were carried out under conditions of Harrison's B- (923–626 K) and C- (576–451 K) type diffusion regimes. In the B-type diffusion regime, the triple product P = sδDgb (s is the segregation factor and δ the GB width) was found to follow the Arrhenius law P = 4.0 × 10−16 × exp(–58.6 kJ mol−1/RT) m3s−1, where R is the universal gas constant and T the temperature. Measurements in the C-regime directly yielded the GB diffusion coefficient Dgb, Dgb = 2.8 × 10−6 × exp(–84.8 kJ mol−1/RT) m2s−1. Combining the obtained P and Dgb values and assuming δ ≅ 5 × 10−10 m, the GB segregation factor for Ge in Cu was determined to follow an Arrhenius dependence s = 0.29 exp(26 kJmol−1/RT). The determined segregation coefficients s are discussed together with previous results on solute segregation in Cu and Ag, and it is c...

Nickel and selenium grain boundary solute diffusion and segregation in silver

For the first time, the grain boundary (GB) solute diffusion of Ni and Se in Ag polycrystals was investigated systematically by the radiotracer serial sectioning technique using 63Ni and 75Se isotopes. Measurements in the temperature ranges 589–989 K for Ni and 371–877 K for Se were carried out in Harrison's type B regime at high temperatures and in type C regime at low temperatures. In the B regime, the product sδD GB was determined ( s being the solute GB segregation factor, δ the GB width, D GB the GB diffusion coefficient), while in the C-regime D GB values were measured directly. The solid solubility of both solutes, Ni and Se, in Ag is very small. The equilibrium phase diagrams, however, are quite different: Se forms an intermetallic compound whereas Ni does not form any compound with Ag, which indicates dominating Se–Ag and Ni–Ni bonds, respectively. Comparing the solute D GB values with C regime measurements of GB self-diffusion in Ag, a drastic decrease of the Ni diffusivity and an increased activation enthalpy of Ni diffusion, H GB Ni, in Ag GBs is observed. This behaviour is discussed in terms of repulsive Ni-vacancy interactions and decreased jump frequencies of the Ni atoms which are mainly located at low energy positions in the GB. The measured D GB values of Se, on the other hand, are comparatively close to GB self-diffusion, whereas H GB Se is again larger than H GB Ag of GB self-diffusion. This behaviour is explained in terms of the formation of “embryos” of two-dimensional phases at high energy GB sites. Combining the obtained sδD GB and D GB values and assuming δ=0.5 nm, the GB segregation factors and segregation enthalpies of Ni ( H s Ni=−39.7 kJ/mol) and Se ( H s Se=−21.4 kJ/mol) in Ag were evaluated.

Effect of impurities on sintering and conductivity of yttria-stabilized zirconia

The effect of low concentrations of Fe2O3, Al2O3 and Bi2O3 on the sintering behaviour of (ZrO2)0.83 (YO1.5)0.17, made by alkoxide synthesis, has been investigated. The best results are achieved with Bi2O3 as a sinter agent and a relative density of 95% is obtained at 1200 K. The effects of these impurities on the electrical conductivity of the bulk and the grain boundaries has been investigated using frequency dispersion analysis (101-106 Hz). All investigated impurities have a negative influence on both the bulk and grain-boundary conductivity. For Fe2O3 and Al2O3 grain-boundary segregation factors of about two are calculated.