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The alloy composition and sensitization behavior are crucial factors that cannot be overlooked in Al–Mg alloys. The effect of Zn addition on mechanical properties and sensitization behavior of 5083 alloy was investigated by tensile tests, weight loss experiments, and first-principles calculation in this paper. The results show that Zn can significantly improve the strength and sensitization behavior of the alloy. The tensile strength of the cold-rolled alloy increased from 459 to 498 MPa with the addition of 1.5%Zn. Furthermore, by incorporating 2.0% Zn, the mass loss of the cold-rolled alloy after sensitization at 170℃/1 h decreased from 49.3 mg·cm−2 to 19.5 mg·cm−2. First-principles calculations indicate that Mg clusters can only exist stably near grain boundaries, while Mg–Zn clusters can exist stably both within and at grain boundaries. This reduces the number of corrosive media at grain boundaries and weakens their continuity. Moreover, the energy barrier that T-phase precipitation needs to overcome during precipitation is lower than β-phase, which transforms the stabilization schedule of the alloy from 240℃/12 h without Zn to that 210℃/4 h with 2.0% Zn-containing. The synergistic effects of Zn content and sensitization on the properties of the alloy were revealed, which sheds some light for the design of the composition and processing method according to application scenarios. [ABSTRACT FROM AUTHOR]

The effect of the grain boundary (GB) misorientation on plastic deformation of Inconel 718 (IN718) alloy was investigated in this paper, using in-situ tensile experiment at 650 °C in combination with crystal plasticity finite element method (CPFEM). The results indicate that dislocations tend to accumulate at GBs to form stress concentration, but the degree of stress concentration does not necessarily increase with the increase of the GB misorientation. It is attributed to the slip transfer at the GBs, determined by the angle between the slip systems of the two adjacent grains. There is a significant uncertainty in the slip transfer for GB misorientation larger than 10°. However, the m α β ′ SF α + SF β criterion, which is a function of the Luster and Morris m α β ′ combining the Schmid factors of the two slip systems with the GB misorientation, has some statistical separation significance. Slip transfer tends to appear at GB misorientation less than 30° and m α β ′ SF α + SF β > 0.78 . This study clarifies the mechanism of the influence of GB misorientation on IN718 microplastic deformation and provides a new strategy to study the deformation behavior of superalloys. [ABSTRACT FROM AUTHOR]

The mechanical properties of the materials are determined by the size and morphology of fine microscopic features. Quantitative microstructural analysis is a key factor to establish the correlation between the mechanical properties and the thermomechanical treatment under which material condition has been achieved. As such, microstructural analysis is a very important and complex task within the manufacturing sector. Published standards are used for metallographic analysis but typically involve extensive manual interpretation of grain boundaries, resulting in measurements that are slow to produce, difficult to repeat and highly subjective. Computer vision and the evolution of artificial intelligence in the past decade can offer solutions to such problems. Deep learning and digital image processing techniques allow digital microstructural analysis to be automated using a fast and repeatable method. This paper proposes a novel boundary class semantic segmentation approach (BCSS) to identify each phase of the microstructure and additionally estimate the location of the grain boundaries. The BCSS is then combined with more traditional segmentation techniques based on the Watershed Transform to improve the identification and measurement of each feature within the microstructure using a new, hybrid automated digital microstructure analysis approach. The new method is validated on a published dataset of two-phase titanium alloy microstructure pictures captured using a scanning electron microscope. Measurements match the level of accuracy of accepted manual standards, and the method is demonstrated to be more reliable than other automated approaches. The influence of the subjective nature of manual labelling, required to train the proposed network, is also evaluated. [ABSTRACT FROM AUTHOR]

It is vital to reveal the effect of microstructure features on the corrosion behavior of pure aluminum anode in alkaline electrolyte for Al-air batteries. In this paper, we extensively studied the influence of mechanical deformation on the corrosion microstructure and then corrosion features of pure aluminum anode. The results demonstrate that the cold-rolling deformation could facilitate the grain size refinement of pure aluminum. Surprisingly, the aluminum anodes with smaller grain sizes and larger grain boundary areas could effectively improve the corrosion resistance. The further characterization of the microstructure indicates that the deformation is capable of causing the fracture of the cathodic secondary phase precipitate clusters thus weakening the galvanic corrosion effects. The results of this paper could gain insights into the design and processing of pure aluminum for Al-air battery. [ABSTRACT FROM AUTHOR]

In this paper, the deformation mechanisms of a Fe–2.25%Cr–1%Mo (P22) steel are studied after tensile tests at room temperature (RT) and 550 °C (HT), creep-fatigue tests at 550 °C and stress relaxation test at 550 °C (HTSR). The RT, HT and HTSR tests produced permanent strain of about 2.5% by different processes. Deformation features such as the orientation of the grains, local misorientation, grain boundary type and distribution, etc., were characterized using electron backscatter diffraction. Transmission electron microscopy was done subsequently to obtain greater insight into the dislocation substructures. The as-received sample had a ferritic–bainitic microstructure, and a large amount of local misorientation was found in the bainite, indicating plastic strain due to phase transformation. After tensile strain to 2.5% at room temperature, the dislocations formed very small substructures (~100–200 nm) in the ferrite grains, and most of the retained plastic strain seemed to be concentrated in the bainite after deformation. Both the ferrite and bainite showed much lower local misorientation and dislocation wall content after a tensile test to 2.5% strain at 550 °C. Compared to the RT tensile sample, after creep-fatigue at 550 °C, the substructures in ferrite opened up considerably and formed small-angle grain boundaries, with clean areas as large as 20–25 μm. After stress relaxation at 550 °C, the ferrite displayed even less substructure, while the bainite seemed to have greater amounts of local misorientation than in the creep-fatigue case. However, the degree of misorientation was less pronounced than in the room temperature tensile case. These results have been discussed with respect to the causes behind the phenomena observed herein and their possible effects on mechanical behaviour. [ABSTRACT FROM AUTHOR]

In this paper, we proposed a method of strengthening Sn-based solder to retard the microstructural evolution during thermal cycling and thus to improve the thermal fatigue life of Sn-based solder joint. We fabricated a multiple-elemental Sn-based solder by adding Ni, Bi and Sb into the Sn3.0%Ag0.5%Cu (SAC305) solder. Thermal cycling tests (− 55 °C ~ 150 °C) were performed on the prepared multiple-elemental Sn-based solder joints and SAC305 solder joints, and quasi-in situ observation was accompanied by the thermal cycling test to identify and compare the corresponding microstructural evolution process. It turned out that the intragranular dislocation movements were effectively hindered at the early stage of thermal cycling, which was due to the effects of solid solution strengthening. Furthermore, the motion of pre-existing grain boundaries and hysteresis of dislocation slip played a predominated role in the strengthened multiple-elemental solder joints. By contrast, the aforementioned two phenomena occurred concurrently in the non-strengthened solder joints. As a consequence, the multiple-elemental solder joints retained the integrity of solder joints after 1200 thermal cycles. This phenomenon indicates that the microstructural evolution was significantly delayed compared with the pristine SAC305 solder joints during thermal cycling. Therefore, SACNSB solder joints have a longer thermal fatigue life. The idea of retarding microstructure evolution during thermal cycling by strengthening Sn matrix can help open a new pathway to improve thermal fatigue life of Sn-based solder joints. [ABSTRACT FROM AUTHOR]

Dynamic recrystallization mechanism and microstructure evolution of a novel quasi-continuous network reinforced TiAl matrix composites, TiAl/B4C composites were studied in this paper. The isothermal compression experiments, the scanning electron microscopy, and the electron back-scattered diffraction were carried out. Besides the discontinuous dynamic recrystallization (DDRX), the continuous dynamic recrystallization occurred in γ-phase grains during the loading, characterized by crystal orientation accumulation. Additionally, a large number of 89 ± 3° grain boundaries associated with DDRX also appeared during the loading. With the temperature increasing or/and the strain rate decreasing, the volume fraction of recrystallized grains increased significantly, the < 010 > crystal direction of γ-phase grains within the matrix unit of present composites was gradually parallel to compression direction, and the texture changed from scatter to concentration. The variety of texture was mainly related to different dynamic recrystallization mechanisms in various conditions. [ABSTRACT FROM AUTHOR]

Grain boundary engineering (GBE) is one of the most successful processing strategies to improve the properties of polycrystalline solids. However, the extensive thermomechanical processes involved during GBE restrict its use to selected applications and materials. In this viewpoint paper, we discuss the opportunity provided by additive manufacturing (AM) technology to broaden the applicability of the GBE paradigm and, consequently, the design space for engineering materials. By integrating specially-designed thermomechanical processing within AM, it would be possible to produce bulk, near-net-shape parts with complex geometry and GBE microstructure. We discuss the major challenges in this endeavor and propose some possible strategies to achieve this goal, which we refer to as "additive-GBE". [ABSTRACT FROM AUTHOR]

Stainless-steel bipolar plates (BPPs) are of great significance in low-cost, easily processable, lightweight proton exchange membrane fuel cells (PEMFCs) despite the challenge presented by corrosion in protective coatings. Localized corrosion along the grain boundaries in a crystal film is common, but few preventive measures have been developed so far. Thus, we propose a novel strategy using a tantalum (Ta) and carbon (C) co-modification to improve the chemical stability of titanium carbide (TiC)-based coatings (Cr/Ta/TiC/C). During the film growth, the subjacent Ta atoms were thermally diffused throughout the columnar structure of TiC and reacted with the C layer. The reaction product, i.e., TaC, acted as a chemical passivator to the grain boundary. Combined with the C capping layer, these functional layers synergistically suppressed any localized corrosion. Therefore, corrosion current densities within the United States Department of Energy's technical recommendations were achieved in both potentiostatic and potentiodynamic polarization. Meanwhile, by controlling the Ta metal dispersion, the interfacial contact resistance between the multilayer structure and the carbon paper can be reduced to 7.1 mΩ·cm−2 at a compaction force of 140 N·cm−2. The substantial improvement in the corrosion resistance and conductivity of BPP places our work among the most efficient anticorrosion systems in PEMFC applications reported so far. [ABSTRACT FROM AUTHOR]

The paper studies the wetting of grain boundary triple junctions (GB TJs) by the second solid phase (intermetallic) δ in the Cu–In alloys. In this system, the portion of grain boundaries in a copper-based solid solution (Cu), which are "wetted" by the second solid phase δ, changes non-monotonically with increasing temperature. At first, the portion of such completely wetted GBs increases from zero to almost 100% when the sample is heated, and then quickly falls back to zero. The condition of complete wetting for the GB TJs (σGB > 1.73 σSS) is less stringent than for the GBs (σGB > 2 σSS). Therefore, if the transition from incomplete to complete wetting occurs with an increase in temperature, then all GB TJs should become completely wetted at a temperature TWTJ, lower than the temperature TWGB, at which all GBs become completely wetted. In this work on the Cu–In system, it was found experimentally for the first time that the wetting of the GB TJs also behaves non-monotonously. The percentage of wetted GB TJs also increases to 100% at first and then falls with increasing temperature. In this case, the portion of wetted GB TJs exceeds the portion of wetted GBs not only when it increases with increasing temperature, but also then, with the subsequent disappearance of fully wetted GBs. Grain boundary triple junctions in (Cu-In) solid solution are "wetted" by the δ intermetallic in broader temperature interval than grain boundaries. [ABSTRACT FROM AUTHOR]

The growth of graphene on Ni magnetic films is of great significance for graphene spintronics, whereas the existence of grain boundaries and twin crystal structures in Ni films is an obstacle for obtaining the large-scale and uniform graphene. In this paper, an epitaxial wafer-scale single-crystalline Ni(111) film with the flat and clean surface was successfully prepared on the commercial α-Al2O3(0001) substrate by a two-step method, which was demonstrated with several characterization methods. According to the abnormal grain growth mechanism, the clean and uniform sapphire surface plays a key role for the single crystallization of Ni films as it induces a weak interface energy difference between two atomic stacking structures (ABC and ACB), thus stimulating the evolution of Ni films from (111) out-of-plane textures to single crystals. Furthermore, an ultra-flat and wrinkle-free graphene monolayer was synthesized on the prepared Ni film, which further verified its high quality and effectiveness. [ABSTRACT FROM AUTHOR]

In the paper, we study the evolution of the structural phase state, deformation behavior, and fracture of ultrafine-grained near-β titanium alloy Ti–5Al–5Mo–5V–1Cr–1Fe after annealing in the temperature range 773–1073 K (0.4–0.55 Tm). The duration of the strain-hardening stage under tensile conditions is shown to be almost independent of the annealing temperature. The character of the strain-softening stage is largely determined by the alloy structure formed after annealing. It is found that annealing at the temperatures 773 and 873 K does not change the deformation behavior of the ultrafine-grained alloy under tension at room temperature. Deformation and fracture of the specimens localize in shear bands. Recrystallization occurring at the annealing temperature 973 and 1073 K leads to the transition of grain boundaries to a more equilibrium state and to a sharp decrease in strength and softening rate of the titanium alloy. It also affects the neck formation prior to fracture, giving a developed neck. Fracture surfaces are indicative of ductile dimple fracture of the alloy in all states. The dimple size depends on the size of structural elements after heat treatments. Based on the experimental data, the σ0 value and Hall–Petch coefficient k are determined. In the investigated grain size range (0.17–1.25 μm) the values are found to be, respectively, 680 MPa and 0.36 MPa m1/2. [ABSTRACT FROM AUTHOR]

Electrical conductivity/resistivity of elemental fcc metals, such as Al and Cu, has been investigated intensively for decades, both theoretically and experimentally. Since these metals are of great practical importance for electrical wiring, reducing their resistivity even by a few percent may have very strong impact on their application effectiveness. In this paper, we report on electrical resistivity measurements in Al single crystals grown by the Bridgman method. We found that their resistivity at room temperature decreases by 11.5% upon heat treatment in a boron environment at 600 °C, i.e., well below the melting temperature of Al (Tm = 660 °C). The residual resistivity indeed reaches its lower limit dictated by electron–phonon interaction at room temperature. We explain this effect by the boron-induced formation of distorted regions at the surface of the Al crystals. These regions are 30–50 μm in size and comprise finer grains with an average size of 5 μm, separated by low-angle grain boundaries. Resistivity reduction is mainly due to the getter effect, i.e., the removal of the impurity atoms from the crystal bulk by the outward diffusion to the distorted surface regions. [ABSTRACT FROM AUTHOR]

Recently (Prokofjev in J Mater Sci 52:4265–4277, 2017. 10.1007/s10853-016-0681-2), the empirical expressions representing the surface tension of general-type high-angle grain boundaries in elemental solids as a function of their melting temperature and molar volume in the solid state at the melting temperature were proposed. In this paper, the application of the expressions to estimate the grain boundary surface tension in the allotropes of elemental solid, which do not melt but transform to the higher-temperature allotrope, is described. The parameters of the temperature dependences of grain boundary surface tension in allotropes of 22 elemental solids are presented. [ABSTRACT FROM AUTHOR]

In this paper, the shape instabilities of Nb in in situ Cu-Nb microcomposite wires after exposed to different annealing treatments have been analyzed using scanning electron microscopy and transmission electron microscopy technologies. The results suggest that the thermal stability is related to misorientation among the adjacent grains at the triple joint. Most of the triple joints are composed of low-angle grain boundaries in Nb ribbons and Cu-Nb interfaces of (111)Cu//(011)Nb. These triple joints provide dragging force to interface motion so that neither the grains nor the interface boundaries show substantial changes below 500 °C. Above 500 °C, the Nb ribbons start to dissociate at the triple joints within Nb phase due to the stored energy by misorientation/distortion in Nb. Grooves and pits formed at these regions tend to promote the spheroidization of the Nb phase. Such results may enrich the studies on the microstructure evolution of Cu-based microcomposite. [ABSTRACT FROM AUTHOR]

Study of microstructure evolution in the form of grain growth in polycrystalline materials has been an important goal for material scientists as it drastically affects physical and mechanical properties. Specifically, nanocrystalline materials, which are known for their superior mechanical properties, are highly susceptible to grain growth even at low temperatures and stresses. Various experiments and simulations carried out on nanocrystalline materials indicate that the microstructure evolution in these materials takes place due to grain boundary migration and grain rotation. Therefore, migration of grain boundaries and grain rotation-induced grain coalescence contribute in increasing the average grain size in the microstructure. In order to simulate microstructure evolution in polycrystalline materials, the multi-order parameter phase-field model is a popular approach and is widely used for studying evolution purely due to the grain boundary migration. In this work, we present a multi-order parameter phase-field model capable of capturing microstructure evolution due to grain boundary migration and grain rotation-induced grain coalescence. The model couples constitutive equation of viscous sliding-induced grain rotation and the classical phase-field model for curvature-driven grain boundary migration. This paper covers various topological and statistical aspects of the microstructure evolved in the presence of both these growth mechanisms in great detail. [ABSTRACT FROM AUTHOR]

This paper discusses the advantages of a room-temperature poling procedure during exposure to ultraviolet light for Pb(Zr0.52Ti0.48)O3 (PZT) films. The results of these experiments include the following: for 1.7-µm-thick chemical solution-deposited PZT films, the saturation photocurrent density after a 10 min white light exposure (190–1900 nm) (no DC bias field applied) increased up to 0.066 µA/cm2 with increasing Cr thickness of top electrode in Cr/Pt bilayer electrodes. Furthermore, the d33,f piezoelectric coefficients for UV-poled samples were 40 and 20% higher than those achieved from field-only poling at either room temperature or 150 °C. Additionally, the development of an internal bias field and pinching were investigated in major and minor polarization–electric field loops. It was found that ultraviolet illumination during the poling process produced photoinduced charge carriers that became trapped by local defects and/or grain boundaries in the films. [ABSTRACT FROM AUTHOR]

Oxide Dispersed Strengthened (ODS) ferritic stainless steels present well-known fine grains microstructures where dislocation movement is hindered by a dense precipitation of nano-oxides particles. Previous research, on the thermomechanical behavior at high temperature and strain rates, was focused on torsion tests (Karch in J Nucl Mater 459:53-61, 2014). Considering texture evolution and grain shape as indicators of the intragranular dislocation glide activity, it was shown that, for high temperature and strain rate, intragranular deformation was in competition with intergranular accommodation. The latter phenomenon was related to early damaging at grain boundaries. The occurrence of a transition phenomenon from an intragranular to an intergranular deformation mechanism, with increasing temperature, was recently confirmed by neutron diffraction spectroscopy (Stoica in Nature Commun 5:5178, 2014). In the present paper, hot extrusion (HE) tests are performed, avoiding damage due to the high stress triaxiality, and allowing further investigation of intragranular and intergranular plasticity at large strains. Three ferritic steels exhibiting various precipitation size and density were hot extruded. Microstructure evolution at different stages of deformation is investigated using the Electron Back-Scattered Diffraction (EBSD) technique. After extrusion at 1373 K (1100 °C), the microstructure of ODS steels consists of a mixture of small round shape grains and larger elongated grains containing low-angle grain boundaries. Texture measurements show the appearance of the α-fiber (<110>//extrusion direction) and an increase in its intensity during the extrusion process in the larger grains. The fragmentation of the large elongated grains by Continuous Dynamic Recrystallization (CDRX) partially occurs in ODS materials depending on precipitation reinforcement. For smaller grains, plastic deformation has no effect on crystallographic orientation and grain shape, indicating a grain boundary accommodation phenomenon as the major deformation mechanism. Precipitation density not only impacts the intragranular dislocation glide activity, but also reduces CDRX kinetics in coarse grains. [ABSTRACT FROM AUTHOR]

Polycrystalline graphene nanoribbons usually experience a brittle rupture before failure, and the irreversible breaking occurs in grain boundaries (GBs). In this paper, molecular dynamic simulations are conducted to study the fatigue properties of bi-crystal graphene nanoribbons under a periodic in-plane compression. It is found that ribbon edges rather than GBs dominate the dynamic stability when the ribbon width is smaller than 5 nm. The fatigue failure is closely related to the misorientation angle between two grains because of the distinct edge energy. It always starts from the edges with higher energy and results in a localized damage at the edges. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the role that the atomic structure of grain boundaries (GBs) in Ni has in the interaction with point defects produced during the collision cascades. Using molecular dynamics and statics, we study the influence of different GB structures on the production and evolution of defects, through calculation of defect formation energy, vacancy diffusion barrier, interstitial-vacancy annihilation barrier, and the corresponding energetic and kinetic influence ranges near pristine and damaged GBs. We observe that, as previously reported, GBs preferentially absorb interstitials over vacancies, which result in accumulation of vacancies in the bulk grain and interstitials to highly localize at the GBs. Also, we find that, in several layers near the GBs, the formation energy of defects is substantially reduced, indicating that the GBs serve as a sink for the radiation-produced defects. Further investigation also reveals that the sink strength of the GBs is significantly stronger for interstitials than vacancies. Also in terms of kinetic behavior, we find that the energy barrier of vacancy diffusion reduces as the defect migrates to the close vicinity of the GBs, implying that in the presence of thermally assisted events, vacancies diffuse toward the GBs via low-barrier processes, which result in healing of the damaged boundaries at high temperatures. Moreover, we observe that in the neighboring region of the defect-loaded GBs, the annihilation of a close vacancy-interstitial pair is spontaneous. Finally, via continuous ion bombardment simulation, we study the stability of the GBs in severe irradiation condition. We find that the GBs lose their stability as the number of trapped interstitials increases at the GB plane. Besides, we conclude that the GBs have high efficiency in defect removal on picosecond time scale and act as efficient defect sinks, despite the instability and saturation observed in repeated ion bombardments. [ABSTRACT FROM AUTHOR]

This paper describes fabrication of Si nanograins through allotropic phase transformation by concurrent application of high pressure and intense straining using high-pressure torsion (HPT). Single-crystalline Si(100) wafers were processed by HPT under a pressure of 24 GPa at room temperature. X-ray diffraction and Raman analysis revealed that the HPT-processed samples were composed of metastable Si-III and Si-XII phases and amorphous phases in addition to the original diamond-cubic Si-I phase. It was found that nanograins formed because the Si-I diamond phase had transformed to high-pressure phases (Si-II, Si-XI, and Si-V) having metallic nature, and it then became easier to generate a high density of dislocations to form grain boundaries. The high-pressure phases were further transformed to the Si-XII and Si-III phases via the Si-II phase upon unloading and they existed as metastable phases at ambient pressure. Subsequent annealing at 873 K gave rise to reverse transformation to Si-I but with nanograin sizes. Although no appreciable photoluminescence (PL) peak was observed from the HPT-processed sample, a broad PL peak centered around 600 nm was detected from the annealed sample due to quantum confinement in the Si-I nanograins. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to describe the five-parameter grain boundary character distribution (GBCD) of polycrystalline silicon and compare it to distributions measured in metals and ceramics. The GBCD was determined from the stereological analysis of electron backscatter diffraction maps. The distribution of grain boundary disorientations is non-random and has peaks at 36°, 39°, 45°, 51°, and 60°. The axis-angle distribution reveals that most of the grain boundaries have misorientations around the [111], [110], and [100] axes. The most common grain boundary type (30 % number fraction) has a 60° misorientation around [111] and of these boundaries, the majority are twist boundaries. For other common boundaries, symmetric tilt configurations are preferred. The grain boundary character distribution of Si is distinct from those previously observed for metals and ceramics. The measured grain boundary populations are inversely correlated to calculated grain boundary energies available in the literature. [ABSTRACT FROM AUTHOR]

Different mechanisms have been suggested by many authors as controlling the rate of superplastic flow in different materials. From the viewpoint of computational effort and aesthetics, it is highly desirable to explain the phenomenon, independent of the material/system considered, on a common basis. With this aim, a mesoscopic grain boundary sliding-controlled deformation model was proposed sometime ago as being responsible for superplastic flow in materials of different kinds. In this paper, a rigorous numerical computational procedure for the experimental validation of the model, which takes into account all the physical requirements of the model, is presented. The soundness of the new procedure is established by analysing the experimental data pertaining to many systems belonging to different classes of materials, and matching the results of the analysis with the experimental findings. [ABSTRACT FROM AUTHOR]

In this paper, rapid solidified MgZnY (at.%) alloy powders produced by an inert gas atomizer were consolidated using a severe plastic deformation technique of high pressure torsion (HPT) at room temperature and 373 K. The behavior of powder consolidation, matrix microstructural evolution, and mechanical properties of the powders and compacts were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, microhardness, and tensile testing. As the HPT processing temperature increases, the powders are more plastically deformed due to decreased deformation resistance, grain boundaries are more in equilibrium, powder bonding is enhanced due to increased interparticle diffusion, hence, tensile ductility and strength increases. On the other hand, hardness decreases with the increased processing temperature, due to less dislocation density. [ABSTRACT FROM AUTHOR]

This paper reviews findings on the anisotropy of the grain boundary energies. After introducing the basic concepts, there is a discussion of fundamental models used to understand and predict grain boundary energy anisotropy. Experimental methods for measuring the grain boundary energy anisotropy, all of which involve application of the Herring equation, are then briefly described. The next section reviews and compares the results of measurements and model calculations with the goal of identifying generally applicable characteristics. This is followed by a brief discussion of the role of grain boundary energies in nucleating discontinuous transitions in grain boundary structure and chemistry, known as complexion transitions. The review ends with some questions to be addressed by future research and a summary of what is known about grain boundary energy anisotropy. [ABSTRACT FROM AUTHOR]

A model that was proposed originally to account for optimal superplasticity in metals and alloys with grain size in the micrometer range and later extended in a few subsequent papers to cover optimal superplastic deformation in ceramics, sub-micrometer-grained and nanostructured materials and intermetallics is described, with an emphasis on the current ideas used in this model and the mathematical procedure used at present (yet to be published in detail) for validating the proposals. The central assumption is that the rate controlling deformation process is confined to high-angle grain/interphase boundary regions that are essential for grain boundary sliding developing to a mesoscopic scale (defined to be of the order of a grain diameter or more) and for superplastic flow setting in. The strain rate equation was validated against experimental observations concerning metals, alloys and ceramics of micrometer- and sub-micrometer grain sizes, nanostructured materials and intermetallics. [ABSTRACT FROM AUTHOR]

Equal-channel angular pressing (ECAP) is an effective fabrication process for obtaining ultrafine-grained materials. This paper examines the development of homogeneity in materials processed by ECAP with emphasis on samples of pure aluminum and an Al-6061 alloy processed by ECAP for up to 8 passes at room temperature. The Vickers microhardness was recorded on the polished cross-sectional planes of each as-pressed billet and the results are plotted in the form of contour maps to provide a pictorial depiction of the hardness distributions throughout the cross-sections. The factors influencing the homogeneity are examined, including the die corner angle within the ECAP die and the number of imposed passes. It is shown that good homogeneity may be achieved through ECAP processing when the number of passes in ECAP is reasonably high. [ABSTRACT FROM AUTHOR]

During the past decade, fabrication of bulk nanostructured metals and alloys using severe plastic deformation (SPD) has been evolving as a rapidly advancing direction of nanomaterials science and technology aimed at developing materials with new mechanical and functional properties for advanced applications. The principle of these developments is based on grain refinement down to the nanoscale level via various SPD techniques. This paper is focused on investigation and development of new SPD processing routes enabling fabrication of fully dense bulk nanostructured metals and alloys with a grain size of 40–50 nm and smaller, namely, SPD-consolidation of powders, including nanostructured ones, as well as SPD-induced nanocrystallization of amorphous alloys. We also consider microstructural features of SPD-processed materials that are responsible for enhancement of their properties. [ABSTRACT FROM AUTHOR]

This paper presents a brief overview of the field of structural nanocrystalline materials. These are materials in either bulk, coating, or thin film form whose function is for structural applications. The major processing methods for production of bulk nanocrystalline materials are reviewed. These methods include inert gas condensation, chemical reaction methods, electrodeposition, mechanical attrition, and severe plastic deformation. The stability of the nanocrystalline microstructure is discussed in terms of strategies for retardation of grain growth. Selected mechanical properties of nanocrystalline materials are described; specifically strength and ductility. Corrosion resistance is briefly addressed. Examples of present or potential applications for structural nanocrystalline materials are given. [ABSTRACT FROM AUTHOR]

Ultrafine-grained materials are attractive for achieving superplastic elongations provided the grains are reasonably stable at elevated temperatures. Since the strain rate in superplasticity varies inversely with the grain size raised to a power of two, a reduction in grain size to the submicrometer level leads to the occurrence of superplastic flow within the region of high strain rate superplasticity at strain rates >10−2 s−1. This paper tabulates and examines the various reports of superplasticity in ultrafine-grained materials. It is shown that these materials exhibit many characteristics similar to conventional superplastic alloys including strain rates that are consistent with the standard model for superplastic flow and the development of internal cavitation during the flow process. [ABSTRACT FROM AUTHOR]

This paper gives an overview of “Grain boundary engineering (GBE) for advanced materials by magnetic field application” based on recent experimental work performed on different kinds of structural and functional materials. It is shown that magnetic field application has a great potential and unique advantage as “non-contact processing” for microstructure control, irreplaceable by any other existing processing methods. The control of grain growth and texture by magnetic fields has been found to be generally applicable to many metallic materials, irrespective of whether they are ferromagnetic or not. Grain growth which is controlled by grain boundary migration was found to be strongly affected by magnetic field application. Recent attempts at the grain boundary engineering by magnetic field application through phase transformation have revealed that magnetic phase transformation can provide us a new approach to grain boundary engineering for iron alloys and steels, as well as a new nanocrystalline material produced by magnetic crystallization from the amorphous state. The possibility of engineering applications of enhanced densification using magnetic sintering and magnetic rejuvenation has been discussed for iron powder compacts and deformation-damaged iron alloys, respectively. [ABSTRACT FROM AUTHOR]

In this paper we discuss various contact damage modes in thin TiN films on steel substrate with increasing load. To understand the displacement at different points along the depth of the film, we have used a TiN–AlTiN multilayered film in which each layer acts as a strain marker and we have also calculated the stresses theoretically using an elastic model of spherical indentation of a bi-layer. The study has helped to understand the physics behind different fracture phenomena, such as confinement of columnar sliding to the middle of the film, the genesis of lateral cracks during unloading, etc. We also emphasize the co-existence and competition of different modes of fracture in the film, rather than a single mode, at a particular combination of film thickness, substrate hardness and load and describe the way different modes interact in the spatial domains when they do coexist. [ABSTRACT FROM AUTHOR]

As a tribute to the scientific work of Professor David Brandon, this paper delineates the possibilities of utilizing in situ transmission electron microscopy to unravel dislocation-grain boundary interactions. In particular, we have focused on the deformation characteristics of Al–Mg films. To this end, in situ nanoindentation experiments have been conducted in TEM on ultrafine-grained Al and Al–Mg films with varying Mg contents. The observed propagation of dislocations is markedly different between Al and Al–Mg films, i.e. the presence of solute Mg results in solute drag, evidenced by a jerky-type dislocation motion with a mean jump distance that compares well to earlier theoretical and experimental results. It is proposed that this solute drag accounts for the difference between the load-controlled indentation responses of Al and Al–Mg alloys. In contrast to Al–Mg alloys, several yield excursions are observed during initial indentation of pure Al, which are commonly attributed to the collective motion of dislocations nucleated under the indenter. Displacement-controlled indentation does not result in a qualitative difference between Al and Al–Mg, which can be explained by the specific feedback characteristics providing a more sensitive detection of plastic instabilities and allowing the natural process of load relaxation to occur. The in situ indentation measurements confirm grain boundary motion as an important deformation mechanism in ultrafine-grained Al when it is subjected to a highly inhomogeneous stress field as produced by a Berkovich indenter. It is found that solute Mg effectively pins high-angle grain boundaries during such deformation. The mobility of low-angle boundaries is not affected by the presence of Mg. [ABSTRACT FROM AUTHOR]

In this paper the results of secondary phases determination in a SAF 2205 duplex steel, heat treated at 780, 850 and 900 °C for 10–40 min, are presented. χ-phase is the first phase to precipitate at all the temperatures, followed by σ-phase. The main drop of toughness is related to very low contents of χ-phase and σ-phase. Hardness is not a sensitive parameter to low amounts of secondary phases. [ABSTRACT FROM AUTHOR]

This paper introduces an analyse of the fractal dimension by Richardson’s method. Two different ways to calculate the fractal dimension are presented with their related calculation errors and applied the Von Koch curves. A Monte-Carlo simulation of the evolution of the grains’ boundaries when heating shows that the interfaces lose their fractal characteristics as reported in experimental work. This result is interpreted by dissipation of the energy during the evolution of the grain boundary. [ABSTRACT FROM AUTHOR]

It is now recognized that grain boundary sliding (GBS) is often an important mode of deformation in polycrystalline materials. This paper reviews the developments in GBS over the last four decades including the procedures available for estimating the strain contributed by sliding to the total strain, ξ, and the division into Rachinger GBS in conventional creep and Lifshitz GBS in diffusion creep. It is shown that Rachinger GBS occurs under two distinct conditions in conventional creep depending upon whether the grain size, d, is larger or smaller than the equilibrium subgrain size, λ. A unified model is presented leading to separate rate equations for Rachinger GBS in power-law creep and superplasticity. It is demonstrated that these two equations are in excellent agreement with experimental observations. There are additional recent predictions, not fully resolved at the present time, concerning the role of GBS in nanostructured materials. [ABSTRACT FROM AUTHOR]

This paper presents an example of grain boundary engineering (GBE) for improving intergranular-corrosion and weld-decay resistance of austenitic stainless steel. Transmission and scanning electron microscope (TEM and SEM) observations demonstrated that coincidence site lattice (CSL) boundaries possess strong resistance to intergranular precipitation and corrosion in weld decay region of a type 304 austenitic stainless steel weldment. A thermomechanical treatment for GBE was tried for improvement of intergranular corrosion resistance of the 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of CSL boundaries indicated a maximum at the small roll-reduction. The corrosion rate was much smaller in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface. The optimized 304 stainless steel showed an excellent resistance to weld decay during arc welding. [ABSTRACT FROM AUTHOR]

Recycling of iron and steel becomes an universally important issue from the viewpoint of energy and resource saving. Impurity elements like Sn and Cu tend to accumulate in steels by repeated recycling and remarkably degrade mechanical properties of recycled iron alloys due to segregation-induced intergranular embrittlement. The goal of this work is to study the potential of magnetic annealing for the control of grain boundary segregation and intergranular embrittlement in iron alloy. This paper reports several important findings regarding the effect of magnetic annealing on segregation-induced brittleness in iron-tin alloy. Of particular importance is the observations that the concentration of tin at grain boundaries in iron is decreased by magnetic annealing and fracture toughness of iron-tin alloy is drastically improved to the level as high as pure iron. [ABSTRACT FROM AUTHOR]

This paper reports an experimental investigation on the effect of multiple twinning on the interface population in two low stacking-fault alloys. This is an important topic for grain boundary engineering because annealing twinning is the indirect cause of improved intergranular corrosion resistance in this class of materials. Proportions of S 3n (n= 1-5) boundaries were analysed in both a brass specimen and a superalloy specimen where the boundaries had been processed so as to be very mobile and less mobile respectively. When S 3 twin boundaries (as distinct from S 3 grain boundaries) are discounted, the S 3n distribution for both specimens had a peak at S 9, because S 3 + S 9 ? S 3 occurs more frequently than S 3+S 9 ? S 27. The distributions and reactions between various S 3n values are described and discussed in detail. A novel trace analysis procedure is used to extract information from S 3 boundaries to decide whether or not they are annealing twins, and so provide a convenient means to assess proportions of twin and non-twin S 3s. The data show unambiguously that a significant proportion of S 3s are not on 111, and these boundaries have on average higher angular deviations from the exact S 3 reference misorientation than do other S 3s. A population of S 3s which were vicinal to annealing twins were also recorded. These data support the contention that profuse annealing twinning produces concurrently many not-twin S 3s, which are pivotal in grain boundary engineering. [ABSTRACT FROM AUTHOR]

The paper is dedicated to a major topic of Grain Boundary Engineering: evolution and stability of granular microstructures. The various mechanisms of grain microstructure stabilisation are considered. The role of grain boundaries, vacancies and especially triple junctions as dragging factors in grain growth of polycrystals is comprehensively discussed. A hierarchy of efficiency of different mechanisms for grain growth inhibitionis presented. It can be utilized as a basis for an assessment of the stability of fine grained and nanocrystalline materials. [ABSTRACT FROM AUTHOR]

In this paper, recent progress in magnesium matrix composite technologies is reviewed. The conventional and new processes for the fabrication of magnesium matrix composites are summarized. The composite microstructure is subsequently discussed with respect to grain refinement, reinforcement distribution, and interfacial characteristics. The mechanical properties of the magnesium matrix composites are also reported. [ABSTRACT FROM AUTHOR]

The effect of environment on creep crack growth behaviors of many nickel-base superalloys is a well-documented and serious problem. Stress accelerated grain boundary oxidation (SAGBO) is accepted as the prior mechanism of the environment effect. In this paper, the crack growth behavior of Inconel 718 was investigated by high temperature moiré interferometry (HTMI), coupled with SEM/EDAX. Based on the results obtained from this research, the mechanism is proposed to be caused by the segregated Nb, which couples with the oxygen diffusing into the grain boundaries in front of the crack tip and forms an NbO layer on the grain boundaries, thereby causing the brittle elastic cracking behavior. [ABSTRACT FROM AUTHOR]