Grain Boundary Behavior in an Ultrafine-Grained Aluminum Alloy

The low-angle grain boundary (LAGB) stability in an ultrafine-grained (UFG) Al alloy during monotonic loading was investigated by orientation imaging microscopy (OIM). The experiments show that boundaries with misorientation below ~2.5 deg are stable, while boundaries with misorientation between 2.5 and 15 deg are unstable. Similar results were observed during cyclic loading of UFG Al alloy. This behavior was rationalized using the nonequilibrium thermodynamics concept. On the other hand, it is postulated that the grain boundary behavior in microcrystalline alloy is different because of the larger dislocation mean free path.     

Evaluation of Precipitation Hardening Characteristics of Rheology-Forged Al 7075 Aluminum Alloy Using Nano- or Microindentation and Atomic Force Microscopy

The mechanical and tribological properties of rheo-formed Al 7075 wrought alloys are investigated using nano- or microindentation and nanoscratch techniques, incorporating optical microscopy and atomic force microscopy (AFM). The results are compared to results from a Vickers hardness test. The peak hardness and surface roughness of specimens aged for 24 hours are obtained for Al 7075 alloy. The tribological characteristics of rheologically formed materials are investigated using the constant load scratch (CLS) method. Using this technique, the heat treatment condition for rheologically formed wrought Al 7075 alloys is optimized.     

Measuring Grain Boundary Character Distributions in Ni-Base Alloy 725 Using High-Energy Diffraction Microscopy

We use high-energy X-ray diffraction microscopy (HEDM) to characterize the microstructure of Ni-base alloy 725. HEDM is a non-destructive technique capable of providing three-dimensional reconstructions of grain shapes and orientations in polycrystals. The present analysis yields the grain size distribution in alloy 725 as well as the grain boundary character distribution (GBCD) as a function of lattice misorientation and boundary plane normal orientation. We find that the GBCD of Ni-base alloy 725 is similar to that previously determined in pure Ni and other fcc-base metals. We find an elevated density of Σ9 and Σ3 grain boundaries. We also observe a preponderance of grain boundaries along low-index planes, with those along (1 1 1) planes being the most common, even after Σ3 twins have been excluded from the analysis.

     

Influence of Process Parameters on the Mechanical Behavior of an Ultrafine-Grained Al Alloy

Aluminum alloys with nanocrystalline (NC) and ultrafine grain (UFG) size are of interest because of their strengths that are typically 30 pct greater than conventionally processed alloys of the same composition. In this study, UFG AA 5083 plate was prepared by quasi-isostatic (QI) forging of cryomilled powder, and the microstructure and mechanical behavior was investigated and compared with the behavior of coarse-grained AA 5083. Forging parameters were adjusted in an effort to strengthen the UFG material while retaining some tensile ductility. Different forging parameters were employed on three plates, with approximate dimensions of 254 mm diameter and 19 mm thickness. The overarching goal of the current effort was to increase strength through minimized grain growth during processing while maintaining ductility by breaking up prior particle boundaries (PPBs) with high forging pressures. Mechanical tests revealed that strength increased inversely with grain size, whereas ductility for some of the experimental materials was preserved at the level of the conventional alloy. The application of the Hall-Petch relationship to the materials was studied and is discussed in detail with consideration given to strengthening mechanisms other than grain size, including dispersion (Orowan), solid solution, and dislocation strengthening.     

Stress-Induced Grain Growth in an Ultra-Fine Grained Al Alloy

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 °C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 °C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative.     

Stress-Induced Grain Growth in an Ultra-Fine Grained Al Alloy

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 °C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 °C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative.     

Precipitates and Grain Boundary Strength of an Fe-Mn-Ni Alloy

The effect of grain boundary (GB) precipitates on the GB strength of an age-hardened Fe-7.8Mn-8.2Ni alloy was investigated. Premature intergranular fracture was observed after age hardening due to the precipitation of ??-MnNi precipitates at prior austenite grain boundaries. However, the conversion of GB ?? precipitates to austenite by a short second aging at 793?K (520?°C) after peak aging at 713?K (440?°C) resulted in a remarkable improvement of GB strength. The result strongly supports the proposition that the weak bonding of GB ?? precipitates to the matrix is the main reason for GB embrittlement in age-hardened Fe-Mn-Ni alloys.     

Evolution of Grain Boundary Precipitates in an Al-Cu-Li Alloy During Aging

The grain boundary microstructure of Al-Cu-Li alloy AA2050 was investigated for different isothermal aging times to rationalize intergranular corrosion (IGC) characteristics. In the underaged condition, the dominant grain boundary precipitates are fine T₁ (Al₂CuLi). Extended aging revealed that grain boundaries were decorated by large T₁ precipitates and S′ phase (Al₂CuMg), with S′ growth not dimensionally constrained. Such a transition in the precipitate type at grain boundaries is a unique feature of the Al-Cu-Li system.     

Study of Silver and Copper Nanoparticles by Electron and Atomic Force Microscopy

Powder Metallurgy and Metal Ceramics - Electron and atomic force microscopy was used to study nanosuspensions of silver (50 mg/L) and silver (85 mg/L) and copper (25 mg/L) mixtures in edible...     

Quantitative Atomic Force Microscopy Characterization and Crystal Plasticity Finite Element Modeling of Heterogeneous Deformation in Commercial Purity Titanium

Using a four-point bend sample of commercial purity titanium deformed to a surface strain around 1.5 pct, the active dislocation slip and twin systems in a microstructural patch of about 15 grains were quantitatively analyzed by a technique combining atomic force microscopy (AFM), backscattered electron (BSE) imaging, and electron backscattered diffraction (EBSD). Local shear distribution maps derived from z-displacement data measured by AFM were directly compared to results of a crystal plasticity finite element (CPFE) simulation that incorporates a phenomenological model of the deformation processes to evaluate the ability of the CPFE model to match the experimental observations. The CPFE model successfully predicted most types of active dislocation slip systems within the grains at correct magnitudes, but the spatial distribution of strains within grains differed between the measurements and the simulation.     

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.     

Grain Cluster Microstructure and Grain Boundary Character Distribution in Alloy 690

The effects of thermal-mechanical processing (TMP) on microstructure evolution during recrystallization and grain boundary character distribution (GBCD) in aged Alloy 690 were investigated by the electron backscatter diffraction (EBSD) technique and optical microscopy. The original grain boundaries of the deformed microstructure did not play an important role in the manipulation of the proportion of the Σ3 ⁿ (n = 1, 2, 3…) type boundaries. Instead, the grain cluster formed by multiple twinning starting from a single nucleus during recrystallization was the key microstructural feature affecting the GBCD. All of the grains in this kind of cluster had Σ3 ⁿ mutual misorientations regardless of whether they were adjacent. A large grain cluster containing 91 grains was found in the sample after a small-strain (5 pct) and a high-temperature (1100 °C) recrystallization anneal, and twin relationships up to the ninth generation (Σ3⁹) were found in this cluster. The ratio of cluster size over grain size (including all types of boundaries as defining individual grains) dictated the proportion of Σ3 ⁿ boundaries.     

Erratum to: Stress-Induced Grain Growth in an Ultra-Fine Grained Al Alloy

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 °C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 °C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative.     

The Use of Atomic Force Microscopy to Study Crack Tips in Glass

This article presents a review of the application of atomic force microscopy (AFM) to crack-tip corrosion during subcritical crack growth in glass. The two principal experimental techniques used in this type of study are (1) the direct observation of crack motion by scanning the tip of a crack during crack growth and (2) the examination of fracture surfaces once the specimen has been fractured in two. The first technique has been used to demonstrate and quantify water condensation at crack tips during subcritical crack growth and is particularly useful at low crack velocities. The second technique has been used to quantify the crack-tip corrosion process and the shape of the crack tip during crack growth. In this article, we discuss experimental results showing that the environment that develops at the tips of freshly fractured glass surfaces in soda lime glass can corrode the glass surfaces near the crack tip. Soda lime silicate glass contains mobile alkali ions that will exchange with hydronium ions in solution at the crack tip, forming a highly basic solution that is corrosive to glass. Experimental evidence for such corrosion has been obtained by the atomic force microscope, which demonstrates a displacement of the two fracture surfaces near the crack tip that can be as much as 20 nm, depending on how long the crack is held open at the fatigue limit. Despite the corrosion and displacement of the crack surfaces, the crack tip itself appears to remain sharp, suggesting that the fatigue limit in soda lime silicate glass is not due to crack-tip blunting. Most likely, the fatigue limit is a consequence of ion exchange at the crack tip, in which hydronium ions in the crack-tip solution exchange with sodium ions in the glass. As hydronium ions are larger than sodium ions, this exchange process leaves a compressive stress within the fresh fracture surface of the glass that resists crack motion and results in a stress-corrosion fatigue limit, as first proposed by Bunker and Michalske. In agreement with this mechanism, no fatigue limit is observed for silica glass, which also exhibits no ion exchange. As the crack-tip solution in silica glass is only mildly acidic, pH ≈ 5, corrosion does not occur at crack tips of this glass as supported by the observation that no crack-tip displacements are observed in silica glass by AFM. As the proposed ion exchange mechanism used to explain the stress corrosion limit in glass is at variance with the belief that the fatigue limit in glass is the result of crack-tip blunting, we discuss the possibility of plastic deformation at crack tips in glass and conclude that the available experimental data does not support such a model. At the present time, chemical reaction based crack growth theories are most consistent with the body of crack growth data that is available on glass and are probably the best explanation for the phenomenon.     

IC-218合金再结晶时的晶界迁移

采用TEM和ODF（取向分布函数）等手段对IC－218合金再结晶过程中的晶界迁移行为进行了研究，发现小角晶界的活动相当活跃，而一般大角晶界失去了明显可运性。TEM观察表明，大角晶界的迁移是一个界面前γ' γ两相“溶解”于界面区，然后以不连续沉淀方法重新析出γ' γ两相的过程；另一方面，穿过γ' γ两相的小角晶界保持与γ相中相同的位错墙结构。     

Development of Grain Boundary Precipitate-Free Zones in a Ni-Mo-Cr-W Alloy

In this study, the morphology and development of precipitate-free zones (PFZs) near grain boundaries (GBs) in low coefficient of thermal expansion (CTE) Ni-Mo-Cr-W alloys (based on Haynes 244) have been investigated as a function of thermal history and composition using electron microscopy techniques. It is shown that the formation of wide, continuous PFZs adjacent to GBs can be largely attributed to a vacancy depletion mechanism. It is proposed that variations in the vacancy distributions that develop after solution heat treatment (SHT) and subsequent quenching and aging greatly influence the development of the γ′-Ni₂(Mo,Cr) precipitates during the aging process and result in the development of PFZs of varying sizes. The relatively large (5 to 10 μm) PFZs are distinct from the smaller, more common PFZs that result from solute depletion due to GB precipitation that are typically observed after prolonged aging. During the course of this investigation, heat treatment parameters, such as aging time, SHT temperature, cooling rate after SHT, and heating rate to the aging temperature—all of which change vacancy concentration and distribution adjacent to GBs—were investigated and observed to have significant influence on both the size and morphology of the observed PFZs. In contrast to results from other Ni-based alloys studied previously, PFZ development in the current alloys was observed across a broad range of aging temperatures. This appears to be due to the high misfit strain energy of the γ′ precipitates, resulting in a nucleation process that is sensitive to vacancy concentration. It is also shown that a slightly modified alloy with higher Mo concentrations develops smaller, more typical PFZs; this is presumably due to an increased driving force for γ′ precipitation which overshadows the influence of misfit strain energy, thereby decreasing the sensitivity of precipitation on vacancy concentration.

     

Floating Grain Formation and Macrosegregation in a 2024 Al Alloy Prepared by Hot-Top DC Casting with a 2024 Al Alloy Insert

For the first time, the method of inserting a 2024 Al alloy rod into a melt of the same alloy during hot-top direct chill casting was used to verify the origin of floating grains and changes in the shape of the sump. Based on the as-cast structure and measured temperature distribution observed, we found that the 2024 Al alloy insert and the grains formed from melt surfaces and the interior walls of the launder or hot-top melted in the high-temperature region of the upper part of the hot-top; additionally, most of the dendrite fragments were captured by feathery grains. We confirmed that the nuclei in the center part of the liquidus isotherm region were able to form coarse-cell dendritic structures in the center part of the slurry zone due to the broad vertical width of this zone. Then, the coarse dendritic structures settled in the mushy zone, and fine dendrites formed in the periphery. In addition, with the insertion of the 2024 Al rod, a hillock-shaped liquidus isotherm was formed in the center part of the slurry zone, and the size of the floating grains in the billet processed with the insert was larger than that in the billet processed without the insert. However, surprisingly, the application of the 2024 Al rod reduced negative centerline segregation because it reduced the horizontal component of shrinkage flow.