Diffusional Grain Research Articles

Diffusion and segregation of silver in copperΣ5(310) grain boundary

Grain boundary diffusion of ${}^{110m}\phantom{\rule{-0.16em}{0ex}}$Ag in Cu $\ensuremath{\Sigma}5(310)$ bicrystal is measured along and perpendicular to the $[001]$ tilt axis in both C and B kinetic regimes after the common Harrison's classification. The grain boundary diffusion coefficients, ${D}_{\mathrm{gb}}$, of the single grain boundary in a true dilute limit of solute concentration are determined in the C kinetic regime and the values of the triple products $P=s\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\delta}\ifmmode\cdot\else\textperiodcentered\fi{}{D}_{\mathrm{gb}}$ are measured in the B regime (here $s$ and $\ensuremath{\delta}$ are the segregation factor and the diffusional grain boundary width, respectively). A strong anisotropy of the grain boundary diffusion is established, which disappears above 826 K. It is the point at which the triple product $P$ demonstrates a downward deviation from the otherwise linear Arrhenius dependence at lower temperatures. These results substantiate a temperature-induced disordering transition of the grain boundary structure. The anisotropy of the product $s\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\delta}$ for the $\ensuremath{\Sigma}5(310)$ grain boundary is estimated.

Low-temperature spark plasma sintering of NiO nanoparticles

NiO nanoparticles of 20nm in diameter were spark plasma sintered between 400°C and 600°C for 5 and 10min durations. Application of 100MPa pressure from room temperature resulted in densities between 75% and 92%. The final grain size was between 26nm and 68nm. Lower densities were recorded when 100MPa was applied at the SPS temperature. Two shrinkage rate maxima of ∼3.4×10−3s−1 and ∼2×10−3s−1 were observed around 390±10°C and at the SPS temperature. The two shrinkage rate maxima were related to densification by particle sliding followed by diffusional grain boundary sliding during the heating. The strong effects of the surface and interfacial processes which are active during the SPS were highlighted.

Grain boundary self-diffusion in polycrystalline nickel of different purity levels

Grain boundary self-diffusion in Ni materials of two different purity levels (99.6 and 99.999 wt.%) was measured over wide temperature intervals using the radiotracer technique and applying the 63Ni radioisotope. The diffusion experiments were performed in both Harrison’s type B and type C kinetic regimes. The diffusional grain boundary width, δ, was found to be equal to 0.54 ± 0.1 nm, that is reasonably close to the accepted value of δ = 0.5 nm in face-centred cubic metals. The purer the material, the higher the grain boundary diffusivity and the lower the corresponding activation enthalpy of self-diffusion along general high-angle grain boundaries. Using the semi-empirical approach of Borisov et al., the average energy of high-angle grain boundaries was estimated to be about 0.79 and 0.98 J m −2 in the low- and high-purity Ni materials at room temperature, respectively.

Coble creep in a powder-metallurgical nickel aluminide of composition Ni–22.8Al–0.6Hf–0.1B (at. %)

Coble creep, which is controlled by mass transport along grain boundaries, has been identified in a powder-metallurgically prepared nickel aluminide with the nominal composition Ni–22.8Al–0.6Hf–0.1B (at. %). Diffusional creep rates∊as a function of temperatureT, stressσ, and grain sizeLare well described by∊ = 33δbDbΩσ(kTL3), whereδbDb= 3 × 10−6m3s−1× exp [– (313 kJ/mol)/(RT) (δbis the diffusional grain boundary width,Dbis the grain boundary diffusivity,Ris the gas constant, andTis the absolute temperature). The activation energy of 313 kJ/mol is unusually high as compared to that for volume diffusion.

A method of investigating the kinetics of high-temperature ductility variation of porous solids during sintering under pressure

A method is proposed making it possible to study the high-temperature kinetics of the variation of the coefficients of shear ductility of a porous solid and its material during sintering under pressure. An investigation was carried out into the variation of the ductility of copper powder compacts in the temperature range 700–950°C. Experimental data are compared with results obtained on the basis of the theory of diffusion-viscous flow and a general rheological theory of sintering. It has been established that in the initial stage of sintering at 950°C the coefficient of shear ductility of copper increases linearly with time. Under conditions of diffusion-viscous flow of a polycrystalline material, this is due to diffusional grain growth according to a parabolic law. It is shown that experimental values of shear ductility of copper are smaller than values obtained on the basis of the rheological theory of sintering. Values approximating most closely to experimental data have been obtained with a model of an ideal porous solid.

Liquid-phase sintering of very fine tungsten-copper powder mixtures

The sintering of tungsten-copper powder mixtures of low copper content can be intensified by employing tungsten powders of submicron particle size. The process of liquid-phase sintering of very fine tungsten-copper powder mixtures can be quantitatively described within the framework of the theory of local heterogeneous deformation of a polycrystalline solid with allowance for diffusional grain shape accommodation.