Collector Plate Mechanism Research Articles

Grain boundary sensitization kinetics of cold-rolled Al–Mg alloys

As super-saturated solid solutions of Al-Mg, 5XXX series aluminum alloys are susceptible to sensitization via intergranular precipitation of the anodic β-phase, which promotes intergranular corrosion, exfoliation and stress corrosion cracking under environmental conditions. This study presents important updates to a Johnson–Mehl–Avarami–Kolmogorov (JMAK) type model for low-temperature sensitization, correlating the intergranular corrosion response to impingement of locally sensitized regions surrounding discrete β-phase grain boundary precipitates. It is demonstrated that the sensitization response of these alloys can be approached as a combination of two independent contributions: the geometric configuration of grain boundaries passing through the microstructure that are most prone to sensitization, and the rate that these boundaries sensitize due to the formation of the β-phase. This allows for the large sensitization response variations found between nominally identical materials produced by different suppliers, which originate due to a lack of constraints within current cold-rolled plate tempers, to be removed as a sample-dependent linear scaling factor that is separate of the rate kinetics. The JMAK model describes the kinetics of 5xxx series sensitization with excellent accuracy across all data available in the literature. The results of the model imply that sensitization at environmental temperatures proceeds via a site-saturated process, with the β-phase forming on a set density of preferential nucleation sites. It is shown that site-saturation allows for extension of the JMAK model to non-isothermal aging profiles and supports a diffusion pathway dominated by pipe diffusion to the interface followed by precipitate growth via the collector plate mechanism.

Growth kinetics of σ-phase precipitates and underlying diffusion processes in CrMnFeCoNi high-entropy alloys

Key mechanisms and elementary diffusion processes that control the growth kinetics of σ precipitates in high-entropy alloys were investigated in the present study. For this purpose, an off-equiatomic Cr26Mn20Fe20Co20Ni14 alloy with an initially single-phase FCC structure was subjected to isothermal heat treatments, which are known to promote the formation of σ phase, i.e., aging between 600 °C and 1000 °C for times ranging from 0.1 h to 1000 h. The growth kinetics of σ precipitates at grain boundaries of the FCC matrix and those located within the interior of the grains were analyzed separately. The latter precipitates are found to grow through direct substitutional diffusion of Cr-solutes towards and Mn, Fe, Co, and Ni away from them and the growth rate of the allotriomorphs can be rationalized by the collector plate mechanism of interfacial diffusion-aided growth. From the growth-kinetics data obtained in the present study, lattice interdiffusion coefficients as well as diffusivities along crystalline defects were obtained. Above 800 °C, the growth kinetics are dominated by lattice interdiffusion of Cr in the FCC matrix described by DL = 9.8 × 10-4 exp[(-300 kJ/mol)/(RT)] m2/s. At lower temperatures, the growth kinetics are enhanced by fast interdiffusion along dislocation pipes, which temperature dependence is given by DD = 5.0 × 10-3 exp[(-205 kJ/mol)/(RT)] m2/s. The Cr-diffusivity along σ/FCC interphase boundaries deduced from the thickening kinetics of grain boundary precipitates can be represented by the Arrhenius relationship DI = 0.5 × 10-4 exp[(-145 kJ/mol)/(RT)] m2/s, which is similar to that found for grain boundary interdiffusion in metals and alloys.

Growth Kinetics of Σ-Phase Precipitates and Underlying Diffusion Processes in Crmnfeconi High-Entropy Alloys

Key mechanisms and elementary diffusion processes that control the growth kinetics of σ precipitates in high-entropy alloys were investigated in the present study. For this purpose, an off-equiatomic Cr26Mn20Fe20Co20Ni14 alloy with an initially single-phase FCC structure was subjected to isothermal heat treatments, which are known to promote the formation of σ phase, i.e. aging between 600 °C and 1000 °C for times ranging from 0.1 h to 1000 h. The growth kinetics of σ precipitates at grain boundaries of the FCC matrix and those located within the interior of the grains were analyzed separately. The latter precipitates are found to grow through direct substitutional diffusion of Cr-solutes towards and Mn, Fe, Co, and Ni away from them and the growth rate of the allotriomorphs can be rationalized by the collector plate mechanism of interfacial diffusion-aided growth. From the growth-kinetics data obtained in the present study, lattice interdiffusion coefficients as well as diffusivities along crystalline defects were obtained. Above 800 °C, the growth kinetics are dominated by lattice interdiffusion of Cr in the FCC matrix described by DL = 9.8 × 10-4 exp[( 300 kJ/mol)/(RT)] m2/s. At lower temperatures, the growth kinetics are enhanced by fast interdiffusion along dislocation pipes, which temperature dependence is given by DD = 2.5 × 10-3 exp[( 210 kJ/mol)/(RT)] m2/s. The Cr-diffusivity along σ/FCC interphase boundaries deduced from the thickening kinetics of grain boundary precipitates can be represented by the Arrhenius relationship DI = 0.5 × 10-4 exp[( 145 kJ/mol)/(RT)] m2/s, which is similar to that found for grain boundary interdiffusion in metals and alloys.

Mn Diffusion at Early Stage of Intercritical Annealing of 5Mn Steel

Mn distribution and austenite morphology at the early stage of intercritical annealing of 5Mn steel were investigated. It was experimentally demonstrated that a newly formed 20 nm-thick austenite was formed without the partitioning of Mn. The elemental analysis confirmed that the growth of austenite should be controlled by the diffusion of C prior to the diffusion of Mn at a low heating rate. The austenite growth started under negligible-partitioning local equilibrium mode and then switched to partitioning local equilibrium mode. Mn segregation at the γ/α interface suggested that the collector plate mechanism was the essential way of Mn partitioning at the early stage of austenite growth.

A Collector Plate Mechanism-Based Classical Intergranular Precipitation Model for Al Alloys Sensitized at Different Temperatures

The sensitization behavior of Al 5xxx alloys is mainly caused by the formation of Mg-rich precipitates at grain boundaries. In this study, a classical nucleation–growth–coarsening theory for the description of intergranular precipitation is formulated, which adopts a collector plate mechanism, an equivalent average Mg concentration at the grain boundary, and new coarsening mechanisms. Three coarsening mechanisms, the modified Lifshitz–Slyozov–Wagner, the Kirchner mechanism, and a combination of these two mechanisms, are compared. Modeling results reveal that the Kirchner mechanism will breakdown when continuity ( $$ \sqrt {N\pi R^{2} } $$ ) is close to 1. According to the new model, the coarsening still accounts for a small fraction (only 10 pct) in the final growth rate after aging at 343 K (70 °C) for 40 months, which is confirmed by the precipitate size distribution data. Thickness and continuity results predicted by the new model agree well with the experimental results obtained from scanning transmission electron microscopy images of Al 5083 H131 alloys aged at 343 K (70 °C) for different times. In addition, the new model is also applied to a high-temperature [453 K (180 °C)] situation, where coarsening of precipitates is observed.

The Use of Size Distributions in Determining Growth Mechanisms: The Growth of Grain Boundary Precipitates in Cobalt-20 Iron

Accurate prediction of microstructural stability in an alloy depends not only on a sound knowledge of the thermodynamics of the system but also of the kinetics of the phase changes involved. Conventionally, precipitate growth mechanisms have been inferred from the variation with aging time of various single parameters such as the mean, mode or maximum of the precipitate size distribution, which has then been compared to theoretical models of growth of an individual precipitate. In the present study, the development, with aging time at 1003 K (730 °C), of the size and shape distributions of grain boundary precipitates in Co-20Fe has been examined to determine the rate-controlling processes, and the conclusions compared to those from conventional analysis. The growth of the precipitates was well described by the grain boundary-dependent collector plate mechanism of Brailsford and Aaron. As the precipitates grew, low-energy facets were formed, which could move only by the propagation of ledges, and thickening was inhibited. The precipitates’ diffusion fields in the grain boundary overlapped and the size distributions of the longest aged specimens showed that local coarsening occurred under partial interface control.

Capillarity Effect Controlled Precipitate Growth at the Grain Boundary of Long-Term Aging Al 5083 Alloy

A model was developed to predict thickness and continuity of β phase (Al3Mg2) formed at grain boundaries of long-term aged Al 5083 alloy. In this model, a variable collector plate mechanism was adopted at the early stage of aging, then, at about 1 month (\( 2\sqrt {D_{\text{b}} t} \approx 100\;{\text{nm}} \)), the model transitions to a constant collector plate mechanism. Two concentration profiles of Mg, one for a semi-infinite bulk at short diffusion distances and one for a finite slab at long diffusion distances (\( 2\sqrt {D_{\text{m}} t} \approx 20\,{\text{pct}} \) of the grain size), were applied to this model for different aging times. Capillarity effects were used to determine the morphology of β phase at the grain boundary. Combining different collector plate mechanisms and Mg concentration profiles, the whole β phase growth process was divided into three stages (short-term Mg concentration profile-variable collector plate, short-term Mg concentration profile-constant collector plate, and long-term Mg concentration profile-constant collector plate). Finally, the model was solved numerically. Experimental results of β phase length and thickness were obtained using transmission electron microscopy (TEM) images of Al 5083 aged at 343 K (70 °C) for different thermal exposure times. Modeling results of β phase thickness and continuity agree well with experimental observations.

Transmission Electron Microscopic Investigations of Grain Boundary Beta Phase Precipitation in Al 5083 Aged at 373 K (100 °C)

The fine-scale microstructure of an Al 5083 alloy sensitized at 373 K (100 °C) for 3, 7, 14, 30, 45, and 90 days has been investigated using transmission electron microscopy (TEM) to study the evolution of the β phase (Al3Mg2) at grain boundaries. In fully sensitized Al 5083, the β phase (Al3Mg2) mostly forms heterogeneously at grain boundaries. TEM observations showed that the grain boundary precipitation of β phase was discrete up to 14 days of aging, and grain boundaries were fully covered by the β phase after 30 to 45 days of aging. The early stages of β growth on the grain boundaries are consistent with the collector-plate mechanism. We find no evidence of either Mg depletion or segregation at the grain boundaries. The initial growth rates as well as the thicknesses after long aging times are greater than can be accounted for by bulk diffusion of Mg to the grain boundaries. We suggest that dislocation pipe diffusion is necessary to account for the β growth rates.

Transmission electron microscopy characterization of Al 2Cu precipitate growth kinetics by in situ heat treatment

Growth kinetics of the Al 2Cu (θ) precipitate in an Al–1.5Cu alloy thin film were characterized in situ using transmission electron microscopy (TEM). The sizes of θ precipitates during isothermal growth in TEM were measured during aging at 250, 275 and 300 °C. The results of precipitate growth were analyzed based on the model of the “collector plate mechanism” for bulk alloys. The lengthening of the θ precipitate in thin film varies as t 0.29 for short annealing times (up to ∼10–20 min) at temperatures between 250 and 300 °C. This is in good agreement with the analysis in bulk alloys. For longer annealing time, however, the t 0.29 time law does not fit as the distances of Cu volume diffusion exceed one half of the film thickness.

Grain-boundary compositional measurements in complex high-temperature Ni-base alloys using x-ray spectroscopy

Thermal treatment of advanced Ni-base alloys used in critical aircraft engine components can have a significant effect on both the microstructure and properties of these alloys. One important property that can be strongly affected by heat treatment is the time-dependent fatigue crack propagation rate along grain boundaries. It is possible for heat treatments to affect the morphology, chemistry, andphase distribution along the grain boundaries in these complex multiphase alloys. Structure-property correlations require the measurement of each of these parameters. By far, the least tractable of these is the measurement of grain boundary chemistry.X-ray spectroscopy in the AEM has been shown to be a very powerful method for measuring grain boundary chemistry in single-phase Fe and Ni-base alloys, among others. In the case of a singlephase Ni-base alloy such as IN600, heat treatments in the range of 500-750&C cause precipitation of Crrich carbides at grain boundaries and the depletion of Cr from the grain-boundary segments between precipitates via a collector-plate mechanism. Measurement of Cr-depletion at the grain boundaries and Cr-depletion profiles across boundaries is straightforward.

Growth kinetics of Al2Cu in an Al-1.5Cu thin film by in Situ TEM

Cu alloying in Al interconnection lines on semiconductor chips improves their resistance to electromigration and hillock growth. Excess Cu in Al can result in the formation of Cu-rich Al2Cu (θ) precipitates. These precipitates can significantly increase corrosion susceptibility due to the galvanic action between the θ-phase and the adjacent Cu-depleted matrix. The size and distribution of the θ-phase are also closely related to the film susceptibility to electromigration voiding. Thus, an important issue is the precipitation phenomena which occur during thermal device processing steps. In bulk alloys, it was found that the θ precipitates can grow via the grain boundary “collector plate mechanism” at rates far greater than allowed by volume diffusion. In a thin film, however, one might expect that the growth rate of a θ precipitate might be altered by interfacial diffusion. In this work, we report on the growth (lengthening) kinetics of the θ-phase in Al-Cu thin films as examined by in-situ isothermal aging in transmission electron microscopy (TEM).

Growth of Grain Boundary Precipitates as a Function of Misorientation in an Al-5 WT% Cu Alloy

AbstractThe size evolution of θ phase (CuAl2) precipitates as a function of time was used to study the growth of grain boundary precipitates in an Al-5 wt % Cu alloy. The kinetics were: modeled using the Brailsford and Aaron treatment of the collector plate mechanism. It was found, for a given time, that the size varied in a reproducible manner as a function of misorientation between the grains. Precipitate size was found to vary from 5 x 10-6 cm to 5 x 10-5 cm, while misorientation varied from 20 to 50 degrees for a given heat treatment period. Grain boundary misorientation was determined to be the most important factor influencing precipitate size for a given grain boundary. The grain boundary plane orientation plays a secondary role in the growth of precipitates. From these data, the relationship between grain boundary misorientation and grain boundary diffusion has been determined.

Grain Boundary Chemistry in Al-Cu Metallizations as Determined by Analytical Electron Microscopy

AbstractAl with additions of Cu is commonly used as the conductor metallizations for integrated circuits (ICs). As the packing density of ICs increases, interconnect lines are required to carry ever higher current densities. Consequently, reliability due to electromigration failure becomes an increasing concern. Cu has been found to increase the lifetimes of these conductors, but the mechanism by which electromigration is improved is not yet fully understood. In order to evaluate certain theories of electromigration it is necessary to have a detailed description of the Cu distribution in the Al microstructure, with emphasis on the distribution of Cu at the grain boundaries. In this study analytical electron microscopy (AEM) has been used to characterize grain boundary regions in an Al-2 wt.% Cu thin film metallization on Si after a variety of thermal treatments. The results of this study indicate that the Cu distribution is dependent on the thermal annealing conditions. At temperatures near the θ phase (CuAl2) solvus, the Cu distribution may be modelled by the collector plate mechanism, in which the grain boundary is depleted in Cu relative to the matrix. At lower temperatures, Cu enrichment of the boundaries occurs, perhaps as a precursor to second phase formation. Natural cooling from the single phase field produces only grain boundary depletion of Cu consistent with the collector-plate mechanism. The kinetic details of the elemental segregation behavior derived from this study can be used to describe microstructural evolution in actual interconnect alloys.

The evolution of microstructure in Al-2 Pct Cu thin films: Precipitation, dissolution, and reprecipitation

The precipitation, dissolution, and reprecipitation processes of Al2Cu (θ phase) in Al-2 wt pct Cu thin films were studied. The films were characterized in the as-deposited condition, after annealing at 425 °C for 35 minutes, and after rapid thermal annealing (RTA) at 345 °C, 405 °C, and 472 °C. In the as-deposited samples, the precipitates had a fine even distribution throughout the thin film both at aluminum grain boundaries and within the aluminum grains. Annealing below the solvus temperature caused the grain boundary precipitates to grow and precipitates within the center of aluminum grains to diminish. Annealing above 425 °C caused the θ-phase precipitates to dissolve. Upon cooldown, the θ phase nucleated at aluminum grain boundaries and triple points in the form of plates.In situ heating and cooling experiments documented this process in real time. Analytical microscopy revealed that there is a depletion of copper at the aluminum grain boundaries in regions free of precipitates. The θ-phase precipitates nucleated and grew at the grain boundariesvia a collector plate mechanism and drew copper from the areas adjacent to the aluminum grain boundaries.

Measurement of solute segregation to grain boundaries in the AEM: A review

The segregation of solute or imparity elements to grain boundaries can occur by three well-defined processes. The first is Gibbsian segregation in which an element of minimal matrix solubility confines itself to a monolayer at the grain boundary. Classical examples include Bi in Cu and S or P in Fe. The second process involves the depletion of excess matrix solute by volume diffusion to the boundary. In the boundary, the solute atoms diffuse rapidly to precipitates, causing them to grow by the ‘collector-plate mechanism.’ Such grain boundary diffusion is thought to initiate “Diffusion-Induced Grain Boundary Migration,” (DIGM). This process has been proposed as the origin of eutectoid transformations or discontinuous grain boundary reactions. The third segregation process is non-equilibrium segregation which result in a solute build-up around the boundary because of solute-vacancy interactions.All of these segregation phenomena usually occur on a sub-micron scale and are often affected by the nature of the grain boundary (misorientation, defect structure, boundary plane).

Growth mechanisms of grain boundary allotriomorphs in Al-4Pct Cu at high homologous temperatures: Interfacial vs direct volume diffusion

The lengthening and thickening kinetics of θ allotriomorphs in Al-4 pet Cu were measured at 350°, 400°, and 450°C. Three different analyses all indicated that interfacial diffusion (i.e., a right angle collector plate mechanism) was the predominant growth mechanism at 350° and 400°C, but that volume diffusion directly to the growing allotriomorphs made a significant contribution to growth at 450°C. Using the data of this and previous investigations one may conclude that volume diffusion of solute directly to the allotriomorph does not contribute noticeably to growth belowT/T (solidus) = 0.78, plays a substantial but still a minority role atT/T (solidus) = 0.84 and 0.90 and becomes the major contributor atT/T (solidus) ≥ 0.91 when the matrix is fee.