Grain Boundary Diffusion Of Cr Research Articles

Modification of grain boundary microstructure by controlling dissolution behavior of θ particles in Cr-containing hypereutectoid steel

To overcome the brittleness of high‑carbon steels, it is necessary to develop the strategy to remove the carbides on the grain boundaries and strengthen grain boundaries. In this paper, we achieved them by simply controlling the heat treatment conditions and the microstructure before heat treatment. It was found the dissolution behavior of the θ-cementite particles is changed greatly with and without Cr in the high‑carbon steels. In the Cr-free hypereutectoid steel, dissolution of θ particles at high temperatures is remarkably fast, but the dissolution behavior is greatly affected by the microstructure in the Cr-containing hypereutectoid steel. In the Cr-containing hypereutectoid steel, the dissolution of θ particles into the γ-austenite phase was rapid in steel with smaller θ particles in fine α grains, but slow in steel with larger θ particles in coarse α grains. The θ particles on the grain boundaries preferentially dissolved and rapidly disappeared in the initial stage of the dissolution treatment owing to the contribution of grain boundary diffusion, since the grain boundary diffusion of Cr is fast as comparable to C. However, the θ particles inside the grains slowly dissolved and then remained until the later stage, owing to slow dissolution via the partition local equilibrium (PLE) mode controlled by the lattice diffusion of Cr. Owing to the preferential dissolution of θ particles on the grain boundaries in the initial stage, large amounts of Cr atoms dissociated from the θ particles diffused along the grain boundaries with grain boundary diffusion, forming Cr-enriched grain boundaries. Such grain boundary microstructure control is expected to lead to the suppression of intergranular fracture, resulting in the improvement of fracture toughness.

Characterization of internal and intergranular oxidation in Alloy 690 exposed to simulated PWR primary water and its implications with regard to stress corrosion cracking

Oxidation testing of Ni-base Alloy 690 was conducted in simulated PWR primary water, and the internal and intergranular (IG) oxidation layers were systematically characterized by analytical transmission electron microscopy to obtain insight into its IG stress corrosion cracking (IGSCC) resistance and behavior. The internal oxidation layer consisted of upper NixFe1-xCr2O4 and innermost Cr2O3. Each internal oxidation layer had a crystallographic orientation relationship with the Alloy 690 matrix, indicating that the layers formed through solid-state reactions between inward diffusing oxygen and the alloying elements. Strong evidence that the Cr2O3 layer formed through the outward grain boundary diffusion of Cr was found, leaving severe Cr depletion along the grain boundary with a steep composition gradient toward the surface. The innermost Cr2O3 layer had a continuous and compact band-shaped morphology around the surface grain boundary. This Cr2O3 layer was revealed to be protective of the inward grain boundary diffusion of oxygen from the surface and therefore responsible for the prevention of grain boundaries from IG oxidation. IG Cr carbides suppressed the internal and IG oxidation further. The key findings obtained in this work may present the main reason for the high resistance of Alloy 690 to IGSCC under PWR normal operating conditions.

Microstructural evolution and high temperature oxidation characteristics of cold sprayed Ni-20Cr nanostructured alloy coating

Mechanically alloyed Ni-20Cr (wt%) nanocrystalline powder was cold spray deposited on SA 516 Grade 70 steel. The microstructure and high temperature oxidation characteristics of Ni-20Cr nanostructured alloy coated steel have been investigated. The cyclic oxidation was conducted in air at 900 °C for periods up to 50 h. The coated steel was examined prior to and after cyclic oxidation using SEM, EDX, FIB, TEM and SAED. The TEM analyses showed that the coating exhibits equiaxed nanoscale microstructure. Interestingly, the high temperature cyclic exposure significantly influences the grain growth despite the segregation and preferential formation of nano-islands of Cr2O3 within the coating matrix. Furthermore, the enhanced grain boundary diffusion of Cr in the nanostructured coating promotes the formation of denser Cr-rich protective surface oxide layer, which possesses a superior oxidation resistance, i.e. prevents the corrosion of substrate steel. The microstructural changes, crystallographic and chemical aspects of the Ni-20Cr nanostructured coating upon cyclic exposure are discussed in the context of its high temperature corrosion protection property.

Stability, phase separation and oxidation of a supersaturated nanocrystalline Cu-33 at.% Cr thin film alloy

A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150°C–500°C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤300°C, whereas heat treatment at temperatures of ≥400°C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400°C–500°C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate.

In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy

The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered.

The Role of Grain Boundaries during the Initial Oxidation Stages in Cu-Added Austenitic Stainless Steel at 700 °C Studied at the Atomic Scale

The early stages of the oxidation behavior (≤ 12 h, in air with 20% water vapor at 700°C) of Cu-added austenitic stainless steels with various grain sizes was investigated using atom probe tomography (APT), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), electron probe microanalyzer (EPMA) and X-ray diffraction (XRD). A thin Cr-rich oxide layer is formed over the entire surface of a small-grained (8 µm) sample, which is similar to that formed near grain boundaries of medium-grained (17 µm) and large-grained (27 µm) samples. Oxidation of all samples proceeds via the lattice and grain boundary diffusion of Cr, leading to the formation of a protective Cr-rich oxide first at the grain boundaries and then via lateral growth toward the grain interiors. APT and TEM studies clearly reveal that within 4 μm of the grain boundaries, the oxide layer exhibits a duplex-layer structure consisting of a thin Fe-rich (Fe,Cr)3O4 oxide (~55 nm) above and a protective Cr2O3 oxide (~40 nm) below as a diffusion barrier. In contrast, further away from the grain boundaries, a non-protective Fe-rich (Fe,Cr)3O4 oxide (~160 nm) and a Cr-rich (Fe,Cr)3O4 oxide (~40 nm) are formed as the outer and inner layers, respectively. The oxidation kinetics was studied in terms of mass gain measurements. The critical grain size (< 8 µm) to prevent the formation of fast-growing, non-protective Fe-rich oxides is discussed based on the experimental findings.

Surface oxidation of Alloy 617 in low oxygen partial pressure He–CO–CO2 environments at 750–850 °C

Surface Cr oxidation mechanism of Alloy 617 was investigated in He–CO–CO2 environments with PCO/PCO2 of either 9 or 1320 at temperatures 750–850°C. The activation energy for surface oxidation was 169.0±13.2kJ/mol in He–PCO/PCO2=9 and 234.4±8.9kJ/mol in He–PCO/PCO2=1320. The ambient oxygen partial pressure dependence of the measured oxidation rates was consistent with n-type behavior of Cr2O3, in which Cr interstitials are the primary mobile species. The oxidation rates measured were greater than that can be sustained only by lattice diffusion of Cr in Cr2O3, indicating that grain boundary diffusion of Cr plays a role in the Cr2O3 growth kinetics.

The effect of surface condition and cold work on the sulphidation resistance of 153MA at 700°C

The normal grain sized 153MA-sample experienced an almost linear weight gain curve in the sulphidizing environment while modified materials, with a finer grain size or cold worked structure, displayed more parabolic behaviour and lover weight gains. These effects may be related to more effective Cr-diffusion in the modified samples with a higher Cr-grain boundary diffusion in the fine grain-sample and a higher Cr-bulk diffusion rate in the cold worked sample. The sand blasted sample performed exceedingly well with a thin protective oxide layer compared to the etched and ground samples which both suffered significant weight loss and spalling. The deformed surface structure on the sand blasted sample enhances the Cr-bulk diffusion thus promoting a protective Cr-rich oxide formation.

Short-Circuit Diffusion of &lt;sup&gt;51&lt;/sup&gt;Cr and &lt;sup&gt;59&lt;/sup&gt;Fe in Nickel-Aluminium Intermetallics

Tracer grain-boundary diffusion (GBD) of 51 Cr and 59 Fe in Ni 3 Al modified by iron, chromium and zirconium was studied in the temperature interval from 773 K to 1273 K using the serial sectioning method. It was found that additions of Fe and Cr up to about 6 at pct. do not alter the activation enthalpy Q b of chromium GBD, whereas the alloying of Ni 3 Al with Zr (up to 1.6 at pct) leads to a significant increase in Q b for chromium GBD. The values of Q b for iron GBD in the same materials are smaller than activation enthalpies of Cr GBD and, they slightly increase with increasing concentration of either Fe, or Cr, A decrease in Q b for Fe GBD in Zr-doped alloys with increasing Zr concentration was observed. An interpretation of obtained results is proposed, based on the atomic size effect and on the different affinity between Zr and Cr and between Zr and Fe atoms.

Grain-boundary diffusion of chromium in Ni3Al intermetallic modified by Fe, Cr and Zr

Tracer grain-boundary diffusion (GBD) of chromium Cr-51 in slightly hypostoichiometric (xAl < 25 at%) Ni3Al alloys containing several at. pct of Fe, Cr and Zr was studied using both serial sectioning and residual activity methods. Measurements of the product of the diffusion coefficient in grain boundary and the width of grain boundary, P, were carried out in the temperature interval from 773 to 1273 K. It was found that the addition of Fe and Cr up to about 6 at% does not alter the diffusion characteristics of Cr grain-boundary diffusion. The addition of a small amount of Zr, on the other hand, tends to increase the activation enthalpy and pre-exponential factor of Cr grain-boundary diffusion.

High angle grain boundary diffusion of chromium in niobium bicrystals

Grain boundary diffusion of Cr in [100][001] symmetrical Nb bicrystals with varying tilt angles, θ, was investigated in the temperature range of 958–1197°C. For low angle boundaries, the grain boundary diffusion is dominated by the dislocation pipe mechanism, in which the activation energy Q′ is independent of θ and the pre-exponential factor (sD′δ)0 varies linearly with sin(θ/2). For high angle boundaries, the grain boundary diffusion coefficients, sD′δ, fall into minima, whereas the activation energies Q′ attain maxima at the CSL boundaries, ∑13 (22.6, 67.4°), ∑17 (28.1, 61.9°) and ∑5 (36.9, 53.1°). The pre-exponential factor (sD′δ)0 also varies with θ, but the variation is random. The results of the study agree with those of the f.c.c. system, confirming that grain boundary diffusion is strongly affected by the boundary structure and associated energies.

Low angle grain boundary diffusion of Cr in Nb bicrystals

Abstract Grain boundary diffusion of Cr in [10]/[112] symmetrical bcc bicrystals of Nb has been investigated in the temperature range from 950 to 1197°C with tilt angles, θ, from 2 to 21°. For 0·17°, the activation energy is independent of θ, with an average value of 181-2kJ mol−1, which is about half of the activation energy of lattice diffusion. The grain boundary diffusion coefficients are greater than the lattice diffusion coefficients by five or six orders of magnitude and vary linearly with sin (θ/2), in agreement with the dislocation-pipe model. These conclusions are generally consistent with the findings in fcc bicrystals.

A note on grain-boundary diffusion of chromium in niobium bicrystals

Abstract The grain-boundary diffusion of Cr in Nb bicrystals with 8°[110]/[112] symmetrical tilt boundaries was measured in the temperature range 947–1150°C, using electron probe microanalysis. The dependence of the grain-boundary diffusion coefficient D' on temperature T is given by with an activation energy of 42961 cal mol−1

Corrosion behavior of stainless steel by oxygen and tellurium at low oxygen potentials

Oxidation behavior of SUS-316 stainless steel was investigated under oxygen pressures ranging from 10 −23 Pa (as controlled by a Mo MoO 2 buffer) to 10 −13 Pa (as controlled by a Ni NiO buffer). Te-induced corrosion was studied under the oxygen pressure controlled by the Mo MoO 2 buffer. Based on the kinetic and metallographical data and thermodynamic consideration, the reaction mechanism was discussed. Under the oxygen pressures controlled by the Mo MoO 2 buffer, a single oxide film was formed on the specimen surface and its growth rate was governed by lattice and grain-boundary diffusion of Cr in the stainless steel substrate. Under the oxygen pressures controlled by the Ni NiO buffer, a duplex oxide film was formed and its growth rate depended on the outward diffusion of Fe ions in the outer oxide layer. In the case of simultaneous reaction by O and Te, it was found that the uptakes of these elements increases beyond about 10 Pa Te pressure and the reaction rate was controlled by the outward diffusion of Fe and Ni in the inner reaction layer.

Effect of grain size of stainless steel on oxidation rate under oxygen pressure controlled by Mo/MoO2 oxygen buffer.

Oxidation behavior of the stainless steel fuel cladding is one of the important information to clarify the oxygen balance in the fast breeder reactor fuel pin during irradiation. In our previous work(1), oxidation behavior of SUS-316 stainless steel was investigated under oxygen potentials controlled by Mo/MoO2 and NbO2/Nb2O5 oxygen buffers, and the following information was obtained : The reaction product due to oxidationwas a mixture of Cr2O3 and MnCr2O4, and the oxidation rate was controlled by the outward diffusion of Cr in stainless steel ; this diffusion process was ascribed to the grain boundary diffusion because the activation energy, calculated from the temperature dependence of the parabolic rate constant, was close to that of the grain boundary diffusion of Cr in stainless steel rather than that of the lattice diffusion(2).