Diffusion Induced Grain Boundary Migration Research Articles

The structure dependence of oxidation behavior of high-angle grain boundaries of alloy 600 in simulated pressurized water reactor primary water

The intergranular degradation of seven different types of high-angle grain boundaries (HAGBs) were investigated on Alloy 600 after exposure to simulated pressurized water reactor primary water. All boundaries are susceptible to preferential intergranular oxidation (PIO) except for ideal coherent twin boundary. Diffusion induced grain boundary migration (DIGM) normally occurs and its depth is positively correlated with the PIO extent. Interestingly, the PIO susceptibility is independent on the grain boundary misorientation angle or Σ value, but related to the grain boundary atom packing density (GBAPD). Grain boundaries with higher GBAPD values show higher PIO resistance as the element diffusion is slower.

Simulating Diffusion Induced Grain Boundary Migration in Binary Fe–Zn

A recently developed phase-field model for simulating diffusion-induced grain boundary migration (DIGM) is applied to binary Fe–Zn. The driving force for the boundary migration is assumed to come from the coherency strain energy mechanism suggested by Sulonen. The effect of the angle of the grain boundary with the surface on the velocity of the boundary migration is studied in detail. The simulation results compare favorably with experimental observations, such as the oscillatory motion of the grain boundary, velocity of the moving grain boundary during DIGM, and the maximum value of mole fraction of Zn at the surface after 20 h of heat treatment.

Role of diffusion-induced grain boundary migration in the oxidation response of a Ni-30 Cr alloy

Surface deformation of Ni and Fe alloys often contributes to improved oxidation resistance at low and intermediate temperatures. While this beneficial effect has been attributed to fast diffusion mediated by high densities of dislocations and grain boundaries, their contributions ought to be limited due to the very rapid recovery and recrystallization at these temperatures. To elucidate the mechanisms responsible for the beneficial effect of deformation, a Ni-30 at.% Cr alloy in electropolished and ground conditions was oxidized in air at 600 °C for times up to 50 h. The electropolished surface produced a duplex scale with nodules consisting of external NiO and internal Cr2O3 oxides. Limited bulk Cr diffusion sustained Cr2O3 thickening, leading to a continuous thin Cr-depletion zone (<50 nm at 50 h) beneath the scale away from nodules. Alternatively, the ground surface generated a single oxide Cr2O3 layer. Since recovery and recrystallization took place within less than 10 min, the contribution of dislocations remained limited. Instead, the significantly faster Cr transport to the surface yielding exclusive selective Cr oxidation was enabled by diffusion-induced grain boundary migration (DIGM). Rather than diffusion along stationary grain boundaries as previously reported, DIGM and consequently grain growth of the sub-surface recrystallized region resulted in the originally deformed region to become uniformly Cr depleted over several micrometers. DIGM also plays a dominant role in the electropolished sample with the formation of a near Cr2O3 single layer and limited external NiO above grain boundaries.

Effect of temperature on the preferential intergranular oxidation susceptibility of thermally-treated Alloy 600

Thermally-treated Alloy 600 coupons were tested in pressurized water reactor simulated primary water at 320 °C and in high-pressure superheated steam at 400 °C to study the effect of a “semi-continuous” network of intergranular carbides on the stress corrosion cracking “precursors events”: preferential intergranular oxidation (PIO) and diffusion-induced grain boundary migration (DIGM). The intergranular carbides were partially consumed due to the environmental exposure, whereby the Cr released from the carbides contributed to the formation of the external oxide layer that limited both PIO and DIGM. The results were compared with previous studies in which similar coupons were exposed to H2-steam.

Early stages of discontinuous precipitation reaction in an advanced Cr-Fe-Ni alloy isothermally aged at 800 °C

The initiation mechanism of the discontinuous precipitation (DP) reaction driven by migrating grain boundaries (GBs) in Alloy 33 (Cr-Fe-Ni-N) isothermally aged at 800 °C has been studied using analytical electron microscopy, which includes scanning/transmission electron microscopy (STEM/TEM), X-ray energy-dispersive spectroscopy (XEDS), and electron diffraction. Precipitation products including both the types of precipitated phases (FCC M23C6 and M6N with diamond-cubic structure) and the GB morphology generated in the early stages of the aging process have been investigated in detail to correlate the experimental findings of the present work with the DP initiation mechanisms reported in the literature. STEM-XEDS elemental maps and electron diffraction data confirmed the FCC M23C6 was the first precipitated phase at the GBs, generating a Cr-depleted zone along the boundary and around the intergranular precipitates. The most remarkable characteristic in the initiation of DP reaction observed in this alloy system refers to the consistent evidence that the GB migration occurred to a significant extent while the M23C6 precipitates remain at the original GB position. In all observations, GB migration occurred against the capillarity force of the concave-forward boundary curvature, thereby suggesting a strong chemical force acting on the GB. Micro and nano-scale analytical STEM-XEDS results corroborated the existence of a compositional gradient of Cr across all migrating GB, confirming that a chemical driving force is responsible for the displacement of the GB. It is concluded that in the overall precipitation phenomenon the necessary solute partitioning is operated by interface diffusion mechanism through the moving GB acting as reaction front. Extensive STEM-XEDS analyses of the precipitation products observed in this investigation have confirmed that diffusion-induced grain boundary migration (DIGM) plays a major role as precursor to the initiation of DP reaction in Alloy 33.

The intergranular oxidation behavior of low-angle grain boundary of alloy 600 in simulated pressurized water reactor primary water

The preferential intergranular oxidation (PIO) behavior of low-angle grain boundaries (LAGBs) with misorientation angle θ ranging from 5.7 to 14.0° on alloy 600 were investigated after exposure to simulated pressurized water reactor primary water. Interestingly, all LAGBs are susceptible to PIO. When θ <8.8°, diffusion-induced grain boundary migration (DIGM) does not occur as Cr is immobile and the PIO depth varies slightly among those LAGBs. As θ increases above 8.8°, Cr starts to diffuse outwards and DIGM occurs. The depths of PIO and DIGM are positively correlated in this case. Also, the PIO depth is not related to the change of θ, but instead poses an inverse relationship with the atom packing density of GB planes.

A study on the diffusion-induced grain boundary migration ahead of stress corrosion cracking crack tips through advanced characterization

Three austenitic alloys with different Ni content were stress corrosion cracking (SCC) tested in simulated pressurized water reactor (PWR) primary water at 320 and 360 °C. Diffusion-induced grain boundary migration (DIGM) associated with preferential intergranular oxidation (PIO) is observed ahead of all SCC crack tips. The occurrence of DIGM is revealed to be driven by PIO-induced diffusion of Cr, Fe, and Ni. The extent of DIGM is controlled by the lateral and in-depth elemental diffusion and PIO, with these processes interconnected. Temperature and alloy composition are revealed to affect the extent of DIGM by affecting these processes.

Эволюция структуры и зернограничного ансамбля никеля при диффузионном отжиге в условиях сегрегирующей примеси (серебра) по границам зерен

Investigations of the effect of an impurity (silver) segregating along the grain boundaries of nickel on the evolution of the structure and grain-boundary ensemble under conditions of diffusion annealing at a temperature of 823 K for 6 hours have been carried out. It is shown that, under these conditions, the development of a diffusion-induced recrystallization (DIR) process is observed in the Ni(Ag) system, in contrast to the Ni(Cu) system, in which the process of diffusion-induced grain boundary migration (DIGM) was observed. The estimates made have shown that a possible reason for the fact that diffusion-induced migration of grain boundaries practically does not occur in the Ni(Ag) system can be significantly lower than in the Ni(Cu) system the value of osmotic pressure as the driving force for the DIGM process.

Phase field modelling of diffusion induced grain boundary migration in binary alloys.

The Phase field method has been used to study Diffusion Induced Grain boundary Migration (DIGM) in binary alloys. Simulations are performed for a simple hypothetical binary substitutional solid solution. The model is developed by taking into account the elastic interactions arising due to changes in concentration across a grain boundary when solutes are diffusing along the grain boundary. This gives a coupling term in the Gibbs energy functional that acts as a driving force for grain boundary migration. Two different kinds of coupling terms are studied: one acting on both the abutting grains and the other acting on only one grain. The former gives a freedom of choice as to in which direction the grain boundary will move, and is considered more general, whereas the latter only moves in one direction, regardless of the shape of the grain boundary. The results from the model are in qualitative agreement with previous experimental studies.

Effect of solute segregation on diffusion induced grain boundary migration studied by molecular dynamics simulations

Diffusion-induced grain boundary migration (DIGM) is the phenomenon of normal grain boundary (GB) migration caused by the lateral diffusion of solutes along it. Despite its technological importance and the fact that DIGM has been first observed and studied since 1970, many aspects of it are still not fully understood. In this study, molecular dynamics simulations are used to investigate the physical origin of DIGM with particular focus on the effects of solute-GB interactions. For this purpose, a few binary alloy systems are deliberately selected, e.g., Al-Ti, Al-Ni, and Ni-Cu, in which strong solute-GB interactions including both solute segregation and anti-segregation occur. The simulation results showed that strong solute segregation and anti-segregation can both influence DIGM, although past experimental and theoretical studies on DIGM mostly focused on systems with segregation. Furthermore, it is shown that the direction of GB migration strongly depends on the solute-GB interaction type, e.g., segregation or anti-segregation, which causes an attraction or repulsion between the GB and solute atoms, respectively. It is thus proved that solute-GB interactions, in general, play an important role in driving DIGM. Furthermore, by combining two atomistic simulation techniques, i.e., the synthetic driving force method and interface random walk method, we are able to quantify the driving forces for DIGM. All observations made during the simulations are supported by atomic configurations and graphical analysis. It is hoped that this study sheds some light on this research area after more than a decade’s stagnation in this field.

A study on the surface and crack tip oxidation of alloy 600 through high-resolution characterization

Surface, stagnant grain boundary (GB), and crack tip oxidation of Alloy 600 were studied. The mirror-finished surface oxide has a duplex structure and the formation mechanisms of each layer are discussed. A more compact oxide film is observed on the cold-worked surface, while the nano-GBs are preferentially oxidized. The crack tip oxidation rate is around four orders of magnitude faster than that of the stagnant GB, and their oxidation mechanisms are believed to be the same. Diffusion-induced grain boundary migration (DIGM) is observed around the crack and oxidized GB tips. The role of Ni expulsion on the DIGM is discussed.

Correlation between Grain Boundary Migration and Stress Corrosion Cracking of Alloy 600 in Hydrogenated Steam

Solution-annealed Alloy 600 samples were tested under active load conditions in a superheated low pressure H2-steam environment over a range of oxidising potentials relevant to Primary Water Stress Corrosion Cracking (PWSCC). Increased SCC susceptibility was observed for potentials the more reducing than the Ni/NiO transition where a deeper Preferential Intergranular Oxidation (PIO) occurred. Advanced microstructural analysis clearly showed that SCC initiated along the grain boundaries that exhibited Diffusion-Induced Grain Boundary Migration (DIGM), as well as enrichments in Al and Ti. Rather than the result of a single dominant mechanism, the initiation of SCC in Ni-base alloys involves the synergistic interactions between DIGM and the formation of Al- and Ti-enriched oxides, promoting PIO that can fracture with applied stress.

Phase-field model for diffusion-induced grain boundary migration: An application to battery electrodes

Grain boundary migration is driven by the boundary's curvature and external loads such as temperature and stress. In intercalation electrodes an additional driving force results from Li-diffusion. That is, Li-intercalation induces volume expansion of the host-electrode, which is stored as elastic energy in the system. This stored energy is hypothesized as an additional driving force for grain boundaries and edge dislocations. Here, we apply the 2D Cahn-Hilliard$-$phase-field-crystal (CH-PFC) model to investigate the coupled interactions between highly mobile Li-ions and host-electrode lattice structure, during an electrochemical cycle. We use a polycrystalline FePO$_{4}$/ LiFePO$_{4}$ electrode particle as a model system. We compute grain growth in the FePO$_{4}$ electrode in two parallel studies: In the first study, we electrochemically cycle the electrode and calculate Li-diffusion assisted grain growth. In the second study, we do not cycle the electrode and calculate the curvature-driven grain growth. External loads, such as temperature and stress, did not differ across studies. We find the mean grain-size increases by $\sim11\%$ in the electrochemically cycled electrode particle. By contrast, in the absence of electrochemical cycling, we find the mean grain-size increases by $\sim2\%$ in the electrode particle. These CH-PFC computations suggest that Li-intercalation accelerates grain-boundary migration in the host-electrode particle. The CH-PFC simulations provide atomistic insights on diffusion-induced grain-boundary migration, edge dislocation movement and triple-junction drag-effect in the host-electrode microstructure.

Observation and quantification of the diffusion-induced grain boundary migration ahead of SCC crack tips

Crack tips prepared from Ni-based alloys after stress corrosion cracking testing in simulated pressurized water reactor primary water have been studied by high-resolution characterization. Diffusion-induced grain boundary migration (DIGM) was observed in all the cracks and its potential role on the crack propagation is discussed in detail. The extent of DIGM observed changed with grain boundary type and alloy composition, which could lead to different extent of retardation effects on the crack propagation. A method is proposed to quantify the role of DIGM on crack propagation based on the results obtained in this work.

Characterization of initial intergranular oxidation processes in alloy 600 at a sub-nanometer scale

Intergranular oxidation in Alloy 600 exposed to 480 °C hydrogenated steam was examined at the nano-scale level with analytical electron microscopy and atom probe tomography. The fundamental processes of minor element oxidation and diffusion-induced grain boundary migration (DIGM) were explored. Intergranular oxidation was observed, with Ti and Al oxidation preceding Cr oxidation. Extensive DIGM is observed, with concentrations of minor elements far exceeding bulk values. Calculations are performed which support that the mechanism of DIGM causes large-scale segregation of minor elements. Also, discrete oxide particles ahead of the oxide front provide evidence for classical intergranular internal oxidation at temperatures below 500 °C.

Effects of boundary migration and pinning particles on intergranular oxidation revealed by 2D and 3D analytical electron microscopy

To evaluate the beneficial effects of intergranular carbides on inhibiting stress corrosion cracking (SCC), Alloy 600 samples in the thermally-treated (TT) and solution-annealed (SA) conditions were analyzed after exposure to 480 °C hydrogenated steam. Intergranular oxidation was observed in both samples, along with diffusion-induced grain boundary migration (DIGM). Compositional mapping revealed DIGM to be more severe in 600SA, while conventional intergranular solute diffusion involving static boundaries appeared dominant in 600TT. 3D serial sectioning of the boundaries revealed strong variations in the intergranular oxide for 600TT, particularly in the depth of oxide penetration. This was attributed to Cr carbide precipitates, present due to the thermal treatment, pinning against DIGM. Immobilizing the boundary via carbide pinning reduces oxide growth by effectively starving it of the ready solute supply otherwise available to a migrating boundary. Because of this interaction between boundary migration, carbide pinning, and oxide growth, the intergranular oxidation in 600TT is highly variable and discontinuous compared to 600SA, where DIGM is unchecked and easier oxide growth produces near-uniform coverage of the boundary. This marked decrease in boundary oxide coverage likely contributes to the improved SCC resistance of 600TT. These results demonstrate the necessity of investigating such phenomena using 3D analysis methods.

Thermal ageing and short-range ordering of Alloy 690 between 350 and 550 °C

Thermal ageing of Alloy 690 triggers an intergranular (IG) carbide precipitation and is known to promote an ordering reaction causing lattice contraction. It may affect the long-term primary water stress corrosion cracking (PWSCC) resistance of pressurized water reactor (PWR) components. Four conditions of Alloy 690 (solution annealed, cold-rolled and/or heat-treated) were aged between 350 and 550 °C for 10 000 h and characterized. Although no direct observation of ordering was made, variations in hardness and lattice parameter were attributed to the formation of short-range ordering (SRO) in all conditions with a peak level at 420 °C, consistent with the literature. Prior heat treatment induced ordering before thermal ageing. At higher temperatures, stress relaxation, recrystallization and α-Cr precipitation were observed in the cold-worked samples, while a disordering reaction was inferred in all samples based on a decrease in hardness. IG precipitation of M23C6 carbides increased with increasing ageing temperature in all conditions, as well as diffusion-induced grain boundary migration (DIGM).

The Study of Local Effect of Manganese on Microstructure Development of Admixed Fe-Mn-C Sintered Steels

AbstractThe present paper is focused on the effect of manganese on microstructure development of admixed Fe-Mn-C sintered steels along with diffusion characteristics of manganese in the iron matrix. Admixed systems were prepared on the base of sponge iron powder, with addition of 0.3% C and 3% Mn added as ferromanganese. Sintering at 1,023, 1,173, 1,323, 1,423 and 1,473 K for 180 s was carried out in laboratory tube furnace in an atmosphere of pure gases mixture 25% N2+75% H2with the dew point of 243 K. The results show that admixed sintered manganese steels exhibit heterogeneity of microstructure due to the local chemical heterogeneities of these materials, in particular for those areas with a high manganese concentration. On the basis of calculation of manganese apparent diffusion coefficient and penetration depth, results reveal that diffusion-induced grain boundary migration (DIGM) is the dominant alloying mechanism in sintered manganese steels.

Diffusion-induced grain boundary migration as mechanism for grain growth and defect annihilation in chalcopyrite thin films

Polycrystalline thin-film solar cell absorbers exhibit complex structure-property relationships. Their microstructure is affected by sophisticated synthesis processes and influences the photovoltaic performance. In Cu(In,Ga)Se2 – currently presenting the highest efficiencies of all thin film absorber materials – the evolution of the [Cu]/([In]+[Ga]) ratio prior to the final Cu-poor composition is crucial for the efficiency of layers deposited by co-evaporation. A precise understanding of the effect of Cu on the microstructure is necessary to simplify the deposition process and bridge the gap between champion cell and module performance. In the present investigation, domain size growth, stacking fault annihilation and strain relaxation during low-temperature CuInSe2 co-evaporation are shown to correlate with Cu deposition, implying a driving force for grain growth induced by diffusion.Synchrotron based x-ray diffraction and fluorescence permit to study the microstructural evolution of the thin film in situ during Cu-Se deposition in a specially adapted process chamber. Repeated interruptions of the Cu evaporation reveal the dependency of the microstructure evolution on Cu deposition. Diffusion-induced grain boundary migration (DIGM) - hitherto not considered in chalcopyrite thin film deposition - is proposed as a mechanism by which Cu deposition drives grain growth at temperatures considerably below the threshold for thermal activation.

Evaluation of the characteristics of 305SAC lead-free solder joints between a chip electrode and a Cu-pad in automotive electronics

The joint characteristics of 96.5Sn-3.0Ag-0.5Cu lead-free solder were confirmed by thermal life test, shear test, and the finite elements method (FEM). The thermal life test was performed as a reliability test considering the vehicle environment, and the stress concentration in the shear test was calculated using FEM. After the thermal test, shear strength decreased 8.2% in the R 1608 chips and 24.1% in the C 1608 chips. To grasp the cause of the shear strength reduction, we examined the fracture mode and analyzed the fracture surface and detected elements. Metal compounds, including Cu3Sn, Cu6Sn5, Ag3Sn, and Ni3Sn4 as intermetallic compounds, were formed by diffusion between materials as could be expected with aging at high temperature. The change in the thickness of metal compounds at the early stage followed by thermal diffusion-induced grain boundary migration (DIGM) of the Ni plating layer and the chip electrode (Ag, Cu) caused the reduction in shear strength. Thus, controlling the kinds and thicknesses of metal compounds under the high temperatures common in the operating environments of vehicles is necessary in the future.