Cooperative Grain Boundary Sliding Research Articles

Superplastic behavior of an in-situ TiB2 particle reinforced aluminum matrix composite processed by elliptical cross-section torsion extrusion

In this work, a severe plastic deformation method, namely elliptical cross-section torsion extrusion ( E -TE), was applied to achieve superplasticity of an in-situ TiB 2 /Al-Zn-Mg-Cu composite by obtaining the optimized microstructure of uniformly distributed particles and equiaxed-fine grains. The superplastic behavior and mechanisms of the E -TE processed composite were comprehensively investigated via tensile tests (at T = 350 °C–500 °C and ε ̇ = 10 −1 s −1 -10 −3 s −1 ) and microstructure characterizations. The tests demonstrated that the composite after a single pass of the E -TE gained a maximum elongation of 420% at 450 °C × 10 −3 s −1 . Even with the strain rate increasing to 10 −2 s −1 , the elongation still reached 360%, which implies a high strain rate superplasticity. Analysis based on the strain rate sensitivity ( m ≈ 0.20–0.35) and microstructure characterizations showed that the deformation and failure mechanisms of the composite vary with the temperature. When the temperature is within 350 °C–400 °C, the composite realizes deformation mainly through intragranular dislocation slip and fails by intragranular fracture. With the temperature of 425 °C–450 °C, the deformation is attributed to cooperative grain boundary sliding (CGBS) accommodated by dislocation activity and liquid phase, where a small fraction of liquid phases observed at the triple junctions of GBs facilitates the composite's superplasticity by reducing the stress concentration and deformation incompatibility. The failure is ascribed to the intergranular fracture by cavity formation/coalescence. For temperature within 475 °C–500 °C, numerous liquid phases formed at GBs and significantly weakened the GBs' strength, deteriorating the composite's superplasticity. The composite's failure is resulted from the intergranular fracture by strength weakening of GBs. • Particle reinforced aluminum matrix composites after elliptical cross-section torsion extrusion (E-TE) show superplasticity. • The E-TE processed composite exhibits an elongation of 360% at 450℃×10 -2 s -1 , implying a high strain rate superplasticity. • The superplastic deformation/failure mechanisms of the E-TE processed composite vary with temperatures. • The composite's superplasticity is facilitated by the appropriate liquid phase at the optimized temperature of 450℃.

Grain Boundary Sliding and Strain Rate Sensitivity of Coarse and Fine/Ultrafine Grained 5083 Aluminum Alloys

The viscoplastic behavior of coarse and bimodal fine/ultrafine grained (F/UFG) Al5083 alloy was investigated between 20 °C and 200 °C through tensile tests at various strain rates, and stress relaxation tests to deduce the strain rate sensitivity (SRS). The plastic strain fields were measured by correlation of SEM images. In the F/UFG material at high temperature, very high strains were measured in shear bands which sometimes crossed the whole gage width and exhibited intensive grain boundary sliding (GBS). Both the SRS and ductility rose with the temperature, and as the strain rate decreased, mainly due to a rising contribution of GBS, which accommodated a much larger fraction of the global strain in the F/UFG material. The boundary between the temperature–strain rate domains where grain refinement led either to strengthening or to softening was determined. Finite element simulations of tension and relaxation tests with viscoplastic grains and sliding grain boundaries captured the macro-scale behavior of the F/UFG material. It also provided some insight into the mechanisms of correlated and cooperative GBS and grain rotation along percolation paths (both inter and intragranular), probably, responsible for macro shear banding.

Cooperative grain boundary sliding in creep deformation of FeCrAl-ODS steels at high temperature and low strain rate

The high-temperature deformation mechanism of the FeCrAl-ODS ferritic steel was investigated at 1000 °C for the creep loading perpendicular to the elongated and aligned grains. The strain rate was varied in the range from the order of 10−2 to 10−7s−1. With decreasing strain rate from 10−2 to 10−5s−1, creep mechanism shifts from conventional dislocation creep pinned by oxide particles to grain boundary sliding (GBS) assisted concomitantly by diffusional creep. With further decreasing strain rate to 10−7s−1, deformation mechanism is drastically changed; group of three grains can move cooperatively, and cooperative GBS (CGBS) was originally recognized. The threshold stress for onset of CGBS was designated as σthI(CGBS). Rate limiting process of CGBS is dominated by dislocation movement over the oxide particles so as to relieve the stress accumulation due to CGBS. The σthI(CGBS) for CGBS corresponds to one third of the conventional threshold stress for dislocation creep.

An investigation into the hot ductility behavior of AZ81 magnesium alloy

► Investigation of the hot ductility behavior in AZ81 alloy to characterize the ductility drops. ► Detrimental effect of grain boundary sliding on ductility values due to the resulting W-type cracks. ► DRX and CGBS as the main factor increasing the ductility at higher temperatures. ► Significant effect of γ precipitates on crack propagation in AZ81 alloy. ► The sharp ductility drop at very low strain rates due to the incipient melting of γ phase. The hot ductility behavior of AZ81 magnesium alloy, as a typical Mg–Al–Zn base cast alloy with high Al content, has been studied through applying a set of low strain rate hot tensile tests in the temperature range of 250–450 °C. The results indicated a ductility drop at 400 °C under the strain rates of 0.01 and 0.001 s −1 . This has been attributed to the occurrence of grain boundary sliding and the resulting W-type and R-type cracks at this deformation conditions. The dynamic recrystallization in large scales has been occurred and considered as the main factor increasing the ductility at higher temperatures. Under lower strain rate (0.0001 s −1 ), the ductility trough has been shifted to lower temperature (350 °C). This has been followed by an enhanced ductility regime up to 30% at 400 °C. The latter is attributed to the cooperative grain boundary sliding (CGBS) mechanism and the active grain boundary sliding of γ phase. By increasing the temperature to 450 °C the material exhibits a sharp drop in ductility variation which is explained considering the occurrence of incipient melting in this temperature range.

Information on deformation mechanisms in nanocrystalline Pd–10% Au inferred from texture analysis

There is still a lack of understanding of deformation mechanisms in nanocrystalline (nc) materials. Studies on microstructures formed in nc Pd–10% Au (grain size about 15 nm) after high pressure torsion revealed signatures of various deformation processes as cooperative grain boundary sliding (GBS), shear banding, dislocation slip and twinning. In order to estimate contributions of these processes to total strain, a comparison was made between torsion textures formed in nc and coarse grained (cg) samples after identical shear strain. The textures were measured with synchrotron radiation. Intensities of characteristic components of the shear texture are two times stronger in the cg sample than in the nc one, indicating that dislocation slip is less significant in the nc sample. It is proposed that numerous planes of cooperative GBS revealed by TEM contribute to plasticity of nc alloy.

Numerical Analysis for the Deformation Processes and Mechanisms of Grain Aggregates in Discrete Element Method

Large-scale deformation processes and mechanisms of the model aggregates comprising close-packed spherical particles are examined by the use of a discrete element method (DEM). It is demonstrated that the movement of grain groups, i.e., the cooperative grain-boundary sliding (CGBS), is essential in the large-scale elongation of polycrystalline materials. The strain energy of grain aggregates during CGBS is also estimated as a function of true strain, yielding a good agreement with the theoretical prediction of CGBS process. The discrete element analysis used in this study is an efficient technique in examining the large-scale deformation mechanism such as the superplastic deformation of polycrystalline materials.

Superplasticity and Superplastic Formability of Hot-Rolled AZ31 Magnesium Alloy

The superplasticity of a hot-rolled AZ31 Mg alloy was investigated by uniaxial tensile tests at temperature range 250-450oC and strain rate range 0.7×10-3-1.4×10-1s-1. Superplastic formability of the alloy was evaluated by gas bulging test at elevated temperatures. The threshold stress for grain boundary sliding (GBS) was calculated and the topography during superplastic deformation was observed by SEM. It is found that, at 400 oC and 0.7×10-3 s-1, the maximum elongation reaches 362.5%. GBS is the predominant deformation mechanism and characterized by a pronounced improvement in homogeneity with increasing temperatures, indicating a transformation of GBS mode from cooperative GBS (CGBS) to individual GBS (IGBS). The improved homogeneity of GBS can be interpreted in terms of decreased threshold stress with increasing temperatures. Gas bulging test demonstrates that the temperature for the best superplastic formability is 400 oC and a hemispherical part with a specific limiting dome height of 0.51 was obtained, suggesting good application prospect for this alloy.

Superplasticity and cooperative grain boundary sliding in nanocrystalline Ni 3Al

Cooperative grain boundary sliding (CGBS) has been shown to account for the majority of macroscopic strain seen in microcrystalline metallic systems undergoing superplastic deformation. While CGBS has been observed on the surface of microcrystalline samples deforming superplastically through the shifting of diamond scribe lines, there have been few transmission electron microscopy results showing such occurrences in the bulk of the material, or the details behind the micromechanism of CGBS. In this work, nanocrystalline Ni 3Al produced via high-pressure torsion is deformed superplastically in the electron microscope. High-temperature (∼700 °C) in situ tensile testing shows the nature of CGBS at the nanoscale through direct observation of this phenomenon.

Cooperative grain boundary sliding in nanocrystalline materials

Superplastic behaviour of microcrystalline materials is now believed to be controlled by cooperative grain boundary sliding (CGBS). An increasing role of grain boundary mediated plasticity with decreasing grain size down to the nanoscale was predicted leading to the prospect of enhanced superplasticity in nanocrystalline materials. Nevertheless, materials with nanosized grains have revealed a significant decrease in plasticity contrary to theoretical prediction. Direct evidence of CGBS in nanocrystalline Ni3Al alloy from SEM surface analysis and in-situ TEM tensile testing was detected, confirming one similarity in the rheology of deformation processes between micro- and nanomaterials. Thus, differences in deformation behaviour of materials at these two length scales are related to the probability of sliding surface formation, sliding distance and related accommodation mechanisms.

Indentation Contact Deformation of 3Y-TZP at Elevated Temperatures

The contact deformation mechanisms of a superplastic ceramic, 3Y-TZP (3mol % yttria-partially stabilized tetragonal zirconia polycrystals) are examined in indentation creep tests at high temperatures. A large discrepancy between the uniaxial compression and the pyramidal indentation contact behaviors is observed. The indentation creep curves exhibit a hardening behavior, i.e., the penetration rate decreases as the penetration depth and/or the creep time increases. This fact implied that the cooperative grain-boundary sliding (CGBS) in a microscopically localized region, such as the sub-surface contact region beneath a pyramidal indenter, is very limited through the microscopic processes of localized grain interlocking.

Elevated temperature behavior of SePD materials: Superplasticity or enhanced ductility?

The generated results on superplastic flow (SPF) in materials produced by severe plastic deformation (SePD) have been analyzed. Formation of sliding surfaces with subsequent cooperative grain boundary sliding (CGBS) has been demonstrated to be a main deformation mechanism of superplasticity under conditions of microstructural stability. A substantial role of dislocation glide in accommodation processes is shown. When grain growth is observed during deformation, continuous grain boundary migration prevents formation of slide surfaces and CGBS is restrained. In this case, deformation of the material is more likely to occur by dislocation or diffusion creep than true SPF despite relatively large strains being reached.

Superplastic joining of 3Y-TZP

The microscopic processes and mechanisms for the superplastic joining of 3Y-TZP (3 mol% yttria partially stabilized tetragonal zirconia polycrystals) were examined. The cooperative grain-boundary sliding associated with superplastic deformation and flow plays an essential role in the joining at the interface between 3Y-TZP blocks. The mechanical strength of the joined interface was determined by three-point flexural loading, where three different types of joined interfaces were studied; the interfaces between (1) fine-grain-size 3Y-TZPs, (2) coarse-grain-size 3Y-TZPs, and (c) a fine-grain-size and a coarse-grain-size 3Y-TZPs. The strengths of these interfaces were examined by changing the compressive stresses applied to the interfaces during joining. It was concluded that the mutual migrations of grains across the interface through cooperative grain-boundary sliding (CGBS) dominates the strength evolution at the interface along with thermally activated diffusion processes.

Interfacial phenomena and microstructural evolution during superplastic deformation

AbstractCertain polycrystalline materials of fine equiaxed grains of size <10µm exhibit tensile ductility of several hundred to a few thousand per cent, called superplasticity, at intermediate strain rates and high temperatures. Experimental and theoretical results on mechanisms for superplastic deformation and concurrent microstructural evolution are reviewed to understand the role of grain boundary phenomena towards enhancement of grain growth and cavitation, etc. Grain boundary sliding (GBS) dominates superplastic flow, the extent of which varies between the grain and interphase boundaries in two‐phase alloys. Accommodation to GBS is provided by diffusional and dislocation mechanisms, which are manifested in the form of grain boundary migration (GBM), grain rotation and grain switching events. Although GBS helps in retaining the initial equiaxed grain shape of superplastic materials, the accommodation processes accelerate grain growth. In the lack of other accommodation processes the cavities are formed, being more pronounced at the boundaries of maximum sliding. The ppm‐level impurities present in superplastic materials do not influence the sliding and flow characteristics under typical superplastic conditions but they do promote cavitation and limit the ductility. The presence of precipitate or hard particles in quasi‐single‐phase superplastic materials inhibits GBM but provides sites for cavitation. Such materials exhibit more dislocation activities than anticipated for accommodation of GBS. The nature of microstructural changes noted during superplastic deformation supports the operation of cooperative GBS, in addition to a large variety of combinations between sliding and accommodation processes. Copyright © 2001 John Wiley & Sons, Ltd.

Superplasticity in Metals and Ceramics

Generality and distinctions in the mechanism of superplastic (SP) deformation between metals and ceramic materials are analyzed. It is established, that SP deformation depends on properties of grain boundaries and is determined by their structure and long-range area in a polycrystal Under optimal SP deformation conditions there operates a specific mechanism of deformation, namely, cooperative grain boundary sliding (CGBS). The possible mechanisms of formation of CGBS bands are considered Difference in SP behavior of metallic and ceramic materials is connected with occurrence of liquid and amorphous phases at grain boundaries during deformation.

Evolution of texture and grain misorientation in an Al-Mg alloy exhibiting low-temperature superplasticity

Low-temperature superplasticity (LTSP) at 250 °C and 1 × 10−3 s−1 was observed in a 5083 Al-Mg base alloy after thermomechanical treatments (TMTs). With a higher TMT rolling strain, the fraction of high-angle grain boundaries increased, which was favorable for the further operation of grain-boundary sliding (GBS) and LTSP. The near-brass {110}〈112〉, S {123}〈634〉, and Cu {112}〈111〉 texture components in the as-thermomechanically treated specimens gradually evolved into a random orientation distribution during LTSP straining from 30 to 100 pct. Static annealing at 250 °C itself could not alter the existing texture. The grain-misorientation distribution curves also showed that, after 100 pct LTSP elongation, the misorientation angles approached the random distribution. In the latter case, the low-, medium-, and high-angle boundaries each would partition around 10, 20, and 70 pct, respectively. When the LTSP elongation was greater than 150 pct, the macrodeformation anisotropy (R) ratio would reach a plateau value of ∼0.8. During the initial stage, a group of over 60 grains proceeded cooperative grain-boundary sliding (CGBS); most individual grain boundaries started to slide at the later stage. It seems that it is the high-angle boundaries, not the special coincidence-site lattice (CSL) boundaries, which could govern the LTSP performance.

Superplasticity of materials as effect of self-organizing of grain-boundary processes

For an explanation of superplastic deformation of metals many models were proposed allowing an explanation of a number of macroscopic characteristics of structural superplasticity (SP), such as dependence of stress on average size of grain, factors of high-speed sensitivity, high-speed interval of display of effect and some others. At the same time some experimental phenomenona of SP were studied, for example: cooperative grain-boundary sliding (CGBS), scale effect, effect of running neck. In the given work based on the physical model offered in a cited work, the synergetic model of effect of superplasticity is advanced and specific characteristic laws of SP are discussed.

Texture and Microstructure Evolution During Superplasticity of the Metal Matrix Composite PM 2014 Al–20% Al2O3

The effects of superplastic deformation (SPD) on grain structure and texture evolution of the Al 2014–20%Al2O3 composite produced via powder metallurgical method were investigated in conjunction with surface relief observations. Samples were deformed in tension at different strains in the range of true strain rates 10-4–100 S-1 at temperature 500C. It was found that SPD strains have a large effect on both texture and grain size. At the initial stage of superplastic flow, deformation bands are formed in the aluminum matrix along the tension direction. Simultaneously, the initial 〈111〉 fiber texture is broken and formation of 〈110〉 fiber texture takes place. Further strain up to 50% leads to grain structure change from banded structure to nearly equiaxed. The 〈111〉 fiber texture is restored and texture intensity decreases with increasing strain. Strain rate dependence of texture developments showed evidence for dislocation activity in examined strain range.The relation between microstructure evolution and texture development during superplastic deformation of the PM 2014–20% Al2O3 composite is explained from the viewpoint of cooperative grain boundary sliding (CGBS).

Superplasticity resulting from cooperative grain boundary sliding

A physical model of superplastic deformation based on the experimental evidence for the existence of cooperative grain boundary sliding (CGBS) is proposed. This approach shifts the focus of attention to the analysis of CGBS bands formation conditions and to the evaluation of their amount necessary for superplastic flow. The model accurately reproduces the observed dependencies of the flow stress on the strain rate, gives a new physical meaning to the superplasticity strain rate interval, and provides a basis for explanation of such phenomena as the high strain rate superplasticity and influence of a sample thickness on superplastic properties of materials.

Grain boundary sliding during superplastic stretching of AA7475 cylindrical cups

Topographic and microstructural examinations were performed at the deformed surface and also in the bulk superplastically stretched 7475 aluminum alloy cups. Sliding of grain groups at shear surfaces spaced at approximately four grain diameters was observed at strain level, e=0.6. The spacing between shear surfaces of such a cooperative grain boundary sliding (CGBS) decreases when the strain increases; it is approximately three grain diameters at e=0.85. The pattern of the CGBS surfaces is consistent with that predicted using slip line field theory. Long range correlated migration of sliding grain boundaries, the formation of dispersoid free zones and fibres have also been observed. These processes can be rationalized in terms of CGBS.

Geometrical aspects of superplastic flow

A number of models have been proposed in order to describe the geometrical aspects of the progress of superplastic deformation. Geometrical models of superplastic flow considering sliding of individual grains and models of cooperative grain boundary sliding (CGBS) have been critically analyzed. Experimental evidence supporting models that treat CBGS as a sequential sliding of grains has been presented. This approach is further developed in terms of movement of cellular dislocations in two-phase materials.