Kinetics Of Grain Boundary Diffusion Research Articles

Anisotropic grain boundary diffusion in binary alloys: Phase-field approach

In this study, we generalize the phase-field model of grain boundary diffusion in polycrystalline materials. Solute transport is described by the modified Cahn-Hilliard equation accounting for diffusion anisotropy, different interaction parameters in grain boundary region and grain bulk, as well as concentration-dependant mobility. The numerical results are provided for solute transport in plain parallel grains and pollycrystalline medium with various directions of fastest (slowest) solute diffusion. This model predicts subdiffusive power-law dependence for propagation of the concentration profile with the exponent varying in a wide range (α=0.25−0.37 for B-type, α=0.45 for C-type kinetics of grain boundary diffusion). Within the proposed phase-field approach, we confirm the validity of Levine-MacCallum conclusion on concentration profile dynamics and modify the ratios for estimation of the effective values of grain boundary mobility.

Rheology of partially molten plagioclase containing wetting silica-rich anhydrous melt

Abstract This work explores the effects of melt chemistry on diffusion controlled creep of partially molten labradorite plagioclase (An50) at anhydrous conditions. Using sol-gel and hot pressing techniques we produced: (1) nominally melt-free samples, with <1 vol. per cent residual glass confined solely to multiple-grain junctions; (2) two types of partially molten samples, containing respectively ∼1 and ∼5 vol. per cent silica-rich partial melts, wetting numerous grain boundaries by thin (<10 nm) amorphous films. Energy dispersive X-ray analysis showed that the amorphous phases of the latter materials contained ∼85 and 95 wt. per cent SiO2, thus representing different polymerization degrees. Infrared spectroscopy showed that the initial traces of water (∼0.05 wt. per cent) were dried out by annealing in air above 1100 °C. Uniaxial creep tests performed at temperatures and flow stresses ranging, respectively, between 1100–1250 °C and 3–60 MPa showed dominantly linear viscous flow, with a strong grain size dependence indicating grain boundary diffusion control. Counter-intuitively strength and activation energy increased with the content of melts. However, for the sample suite silica content covaries with melt proportion, and thus our results suggest that the kinetics of grain boundary diffusion controlled creep strongly depends on melt chemistry. Instead of acting as shortcut for diffusion, thin films of highly viscous amorphous phases may in turn considerably hinder grain boundary transport properties.

Radioactive isotopes reveal a non sluggish kinetics of grain boundary diffusion in high entropy alloys

High entropy alloys (HEAs) have emerged as a new class of multicomponent materials, which have potential for high temperature applications. Phase stability and creep deformation, two key selection criteria for high temperature materials, are predominantly influenced by the diffusion of constituent elements along the grain boundaries (GBs). For the first time, GB diffusion of Ni in chemically homogeneous CoCrFeNi and CoCrFeMnNi HEAs is measured by radiotracer analysis using the 63Ni isotope. Atom probe tomography confirmed the absence of elemental segregation at GBs that allowed reliable estimation of the GB width to be about 0.5 nm. Our GB diffusion measurements prove that a mere increase in number of constituent elements does not lower the diffusion rates in HEAs, but the nature of added constituents plays a more decisive role. The GB energies in both HEAs are estimated at about 0.8–0.9 J/m2, they are found to increase significantly with temperature and the effect is more pronounced for the CoCrFeMnNi alloy.

Oxygen partial pressure dependence of thermoelectric power factor in polycrystalline n-type SrTiO3: Consequences for long term stability in thermoelectric oxides

The Seebeck coefficient and electrical conductivity have been measured as functions of oxygen partial pressure over the range of 10−22 to 10−1 atm at 1173 K for a 10% niobium-doped SrTiO3 ceramic with a grain size comparable to the oxygen diffusion length. Temperature-dependent measurements performed from 320 to 1275 K for as-prepared samples reveal metallic-like conduction and good thermoelectric properties. However, upon exposure to progressively increasing oxygen partial pressure, the thermoelectric power factor decreased over time scales of 24 h, culminating in a three order of magnitude reduction over the entire operating range. Identical measurements on single crystal samples show negligible changes in the power factor so that the instability of ceramic samples is primarily tied to the kinetics of grain boundary diffusion. This work provides a framework for understanding the stability of thermoelectric properties in oxides under different atmospheric conditions. The control of the oxygen atmosphere remains a significant challenge in oxide thermoelectrics.

Diffusion Mechanisms in Nanocrystalline and Nanolaminated Au-Cu

Thermal anneal treatments are used to identify the temperature range of the two dominant diffusion mechanisms – bulk and grain boundary. To assess the transition between mechanisms, the low temperature range for bulk diffusion is established utilizing the decay of static concentration waves in composition-modulated nanolaminates. These multilayered structures are synthesized using vapor deposition methods as thermal evaporation and magnetron sputtering. However, at low temperature the kinetics of grain-boundary diffusion are much faster than bulk diffusion. The synthesis of Au-Cu alloys (0-20 wt.% Cu) with grain sizes as small as 5 nm is accomplished using pulsed electro-deposition. Since the nanocrystalline grain structure is thermally unstable, these structures are ideal for measuring the kinetics of grain boundary diffusion as measured by coarsening of grain size with low temperature anneal treatments. A transition in the dominant mechanism for grain growth from grain boundary to bulk diffusion is found with an increase in temperature. The activation energy for bulk diffusion is found to be 1.8 eV·atom-1 whereas that for grain growth at low temperatures is only 0.2 eV·atom-1. The temperature for transitioning from the dominant mechanism of grain boundary to bulk diffusion is found to be 57% of the alloy melt temperature and is dependent on composition.

The thermal stability of nanocrystalline Au–Cu alloys

Grain refinement to the nanocrystalline scale is known to enhance physical properties as strength and surface hardness. For the case of Au–Cu alloys, development of the pulsed electroplating has led to the functional control of nanocrystalline grain size in the as-deposited condition. The thermal aging of Au–Cu electrodeposits is now investigated to assess the stability of the nanocrystalline grain structure and the difference between two diffusion mechanisms. The mobility of grain boundaries, dominant at low temperatures, leads to coarsening of grain size, whereas at high temperature the process of bulk diffusion dominates. Although the kinetics of bulk diffusion are slow below 500 K at 10 − 20 cm 2 s − 1 , the kinetics of grain boundary diffusion are faster at 10 − 16 cm 2 s − 1 . The diffusivity values indicate that the grain boundaries of the as-deposited nanocrystalline Au–Cu are mobile and sensitive to low-temperature anneal treatments affecting the grain size, hence the strength of the material.

Compensation effect for grain boundary diffusion in a bicrystalline experiment

The paper is devoted to the problem of the compensation effect for grain boundary (GB) diffusion, i.e. the linear dependence of the pre-exponential factor of the GB diffusion coefficient on the activation energy. Specific features of GB diffusion as a thermally activated process namely, the influence of segregation factor, K, and variation of the GB width, δ, on the diffusion rate are discussed. A special diffusion experiment was designed to estimate the contribution of the separate component parts of the triple product, KδDGB (DGB is the GB diffusion coefficient). The experiment was performed with Al bicrystals. The variation of the GB width δ, and a value of the segregation factor K, due to GB structure change are estimated. It is concluded that DGB is the main GB structure-sensitive parameter in the triple product. This circumstance allows us to consider the GBs in Al bicrystals as a series of uniform objects and to describe the kinetics of GB diffusion in terms of the compensation temperature Tc and a “barrier” phase. The value of Tc for GB diffusion of Zn and Ge in Al bicrystals is practically the same and equals 709 and 706 K, respectively. The character of the “barrier” phase is discussed.

A study of diffusion behavior of elements lanthanum and oxygen in Mo–La 2O 3 cathode

The diffusion behavior of elements lanthanum and oxygen in Mo–La 2O 3 cathode has been carried out by using Auger Electron Spectroscopy. Lanthanum and oxygen ions (La 3+ and O 2+) diffuse from the grain boundaries to the surface. The experimental results were analyzed by kinetics of grain boundary diffusion. In the temperature range 1123 K–1423 K, the diffusion coefficients of La 3+ and O 2+ ions were found to fit with the following expression: D La =3.6703×10 −16 exp(−1.01639×10 5/ RT) m 2 s −1 D O =1.5122×10 −16 exp(−8.13066×10 4/ RT) m 2 s −1

Crenulation cleavage differentiation: implications of solution-deposition processes

Metamorphic differentiation associated with cleavage development in crenulated anisotropic rock fabrics is commonly due to a redistribution of mineral phases within a certain volume of the fabric. Such a volume redistribution can be explained by solution transfer of soluble minerals from sites of high chemical potential, fold limbs, to sites of low chemical potential, fold hinges. The processes involved are dissolution, diffusional transfer via grain-boundaries and redeposition. The driving force for the diffusion, differences in chemical potential, is relatable to stress and fabric variations around the microfolds. The rate of transfer is influenced by the initial solubility of the mineral grains, the kinetics of grain-boundary diffusion, the nature of grain contacts and the microfold wavelength. The sense of migration of dissolved species is dependent on spatial variations in the magnitudes of normal stress and mean stress combined with grain shape and grain orientation changes around the microfolds.

Abstract: Application of MeV backscattering to thin film diffusion problems

The temperature at which mixing processes and compound formation between two metals occur in a thin film structure is strongly decreased with respect to bulk materials. The MeV He+ backscattering technique is well suited with its mass and depth perception to investigate the thermal kinetics of grain boundary diffusion in both miscible and nonmiscible couples of thin films. In the Cu−Pb thin film couple, a nonmiscible system as predicted by the phase diagram, a grain boundary migration of Pb occurs in the temperature range 200−300°C and Pb atoms accumulate at the Cu surface following a t1/2 dependence. In Al−In couples, another nonmiscible system, grain boundary diffusion takes place during deposition with filling of boundaries so that no accumulation is detected after annealing. In the completely miscible In−Pb couples, the slight increase of the substrate temperature during evaporation allows a strong intermixing so that backscattering spectra indicate the presence of both elements on the sample surface.