Papers by Keyword: Grain Boundary Sliding (GBS)

Abstract: In this study, the superplastic behavior on a fine-grained aluminum solid solution alloy consisting of thermally unstable microstructures was investigated. In order to obtain the fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum alloy. An equiaxial fine-grained microstructure of 7.8 mm was obtained after FSP, but this microstructure was thermally unstable at high temperatures. Commonly, for fine-grained superplasticity to occur (or to continue grain boundary sliding (GBS)), it is necessary to keep the fine-grained microstructure to less than 10 mm during the high-temperature deformation. However, in this study, a large elongation of over 200% was observed at high temperatures in spite of the occurrence of grain growth. From the microstructural observations, it was determined that the fine-grained microstructure was maintained until the early stage of deformation, but the transgranular deformation was observed at a strain of over 100%. The microstructural feature of the abovementioned transgranular deformation is similar to the deformation microstructure of the solute drag creep occurring in "Class I"-type solid solution alloys. This indicates that the deformation mechanism transition from GBS to the solute drag creep occurred during high-temperature deformation. Here, the possibility of occurrence of the superplastic elongation through deformation mechanism transition is discussed as a model of the thermally unstable aluminum solid solution alloy.

Abstract: Abstract. This study examined the critical plastic formability limit of a fine-structure AZ31 magnesium alloy plate under warm and high temperature based on the strength of a magnesium alloy that has cavities at room temperature. The cyclic hot free-forging process as pre-form working following rolling at a light reduction ratio fabricated a fine-structure AZ31 magnesium alloy plate. The appearance of the cavities was examined in detail together with changes in the structure and preparation methods before further damage at high temperatures with increasing uni-and biaxial plastic deformation. The allowable deformation limit in the super plasticity process can be estimated from the strength of the deformed material and forming limit diagram (FLD) at room temperature. During high-temperature deformation, cavities are produced by stress concentrations at grain boundary triple points and striation bands due to grain boundary sliding. The cavitations growth behavior is dependent upon deformation conditions, and a high percentage of large cavities occupy the sample surface as a large amount of grain boundary sliding is present, i.e., as uniform elongation grows larger, the cavity size also increases. In a case where 200% uniaxial strain was applied to a fine-grained structure material at a temperature of 623K under a strain rate of 10-4s-1, the tensile strength at room temperature decreased about 13%, and elongation was 10% less, compared with that of a material to which no load was applied due to the influence of cavities. In a case of biaxial deformation, the values were 28% lower. It is possible to draw a FLD based on the cavity incidence fraction .

Abstract: Superplasticity is the ability exhibited by some fine-grained materials to be elongated a great deal with no failure. Such phenomenological definition accounts for the engineering view point of this remarkable property. From a fundamental basis, there is a full consensus to admit that it is essentially linked to the grain boundary motion under invariance of microstructure. Despite the great scientific effort carried out during the two last decades, or probably due to that, there is still a gap on the scientific comprehension of the equation describing superplasticity from a basic point of view, not to say about its potential extension to nanostructured materials. This paper presents the essential basis of a new model proposed to account for the main features of structural ceramics.

Abstract: Highly-textured, rolled AZ31 sheet material shows a significant drop in the plastic anisotropy (r-value; r=ew/et) in tension between 25°C and 200°C. This behavior was initially explained as a result of the increased activity of non-basal slip with increased temperature. Other authors suggested, however, that the mechanism responsible for this phenomenon was the activation of grain boundary sliding (GBS). Here, in-situ tensile tests have been carried out in an SEM at various temperatures in order to obtain further evidence of the role of GBS during moderate to high temperature deformation of Mg alloys, which remains highly controversial.

Abstract: In order to attain high-strain-rate superplasticity (HSRS) in ceramics, flow behavior was examined with ZrO₂ reference sample. The results suggest that the enhancement of the accommodation processes of grain boundary sliding (GBS) is important in addition to the careful controlling the microstructural factors, such as stable fine grain structure, reducing residual pores and so on. The spinel particles dispersion can simultaneously provide the following positive factors to ZrO₂: i) suppressed grain growth due to pinning effect of spinel particles, enhanced accommodation due to ii) accelerated relaxation of stress concentrations exerted by GBS through dislocation motion and iii) accelerated lattice diffusion caused by the dissolution of aluminum and magnesium into ZrO₂ from the spinel particles. The positive factors due to spinel dispersion make it possible to attain HSRS in ZrO₂ ceramics.

Abstract: Thermomechanical processing to enable superplasticity in AA5083 materials includes cold working followed by heating prior to hot blow forming. Upon heating for forming at 450°C, a B-type ({110}) rolling texture is replaced by a near-random texture with a weak superimposed cube orientation parallel to the sheet normal. The presence of refined grains 7 – 8μm in size reflects the predominance of particle-stimulated nucleation of recrystallization prior to forming. The subsequent evolution of microstructure, texture and cavitation behaviour during biaxial deformation in the solute drag creep (SDC) and grain boundary sliding (GBS) regimes will be presented.

Abstract: The Earth deforms dominantly by solid-state creep. Diffusion creep is known to be important. It is less clear whether mechanisms in which grain boundary sliding is accompanied by other processes (dislocation activity), and/or are associated with stress exponents closer to 2 than to 1 are important. Since the mechanisms of superplasticity are themselves not fully resolved, we cannot say for sure whether the Earth deforms superplastically. Models for diffusion creep are relevant for the Earth and possibly for superplastic materials. Modelling shows that large strains may not necessarily obliterate initial textures because grain rotations, although they occur, slow down as microstructures evolve. Modelling also predicts major strength anisotropy induced by grain shape alignment. Models for two-phase diffusion creep can be constructed for when the second phase is inert (insoluble). If both phases are soluble and can participate in diffusion, the basic theory for single phase diffusion creep cannot be applied and new insight is required.

Abstract: Nanocrystalline (n-) Au shows a large internal friction accompanied with the modulus defects above ~200 K. After the creep test, the strong <111> preferred texture changed to rather random one but the mean grain size was unchanged. In situ STM observation indicated that the crystallites can independently move during the creep deformation. Quasi-two phase state composed of solid crystallites and anelastic/viscoelastic grain boundaries (GBs) is proposed to explain these characteristic mechanical properties of n-Au. Further, GBs show the glass-transition-like change at around 200 K, anelastic/viscoelastic transition at ~30 MPa and dynamical state change above ~200 MPa. The high vacancy-type-defect concentration plays an important role on stabilization of the quasi-two phase state in n-Au.

Abstract: Composites containing 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) reinforced with multiwalled carbon nanotubes (CNTs) with various amounts of CNTs (3Y-TZP / X wt% CNT, X= 0, 0.5, 1.5, 3 and 5) were processed by spark plasma sintering. Microscopic analysis proves that CNTs were well dispersed and embedded in grain boundaries of the sintered body. High temperature mechanical properties have been investigated using mechanical spectroscopy and low stress (6 MPa) creep. The isothermal spectrum (measured at 1600 K) consists of a mechanical loss peak at a frequency of about 0.1 Hz, which is superimposed on an exponential increase at low frequency. The absence of a well-marked peak in monolithic 3Y-TZP is justified considering that restoring force decreases at low frequencies or high temperatures due to the elasticity of neighboring grains. Therefore, strain is no more restricted and the mechanical loss increases exponentially, which is correlated to macroscopic creep. However, with CNT additions the mechanical loss decreases and a better resolved peak was observed. In parallel, the results have shown that the creep rate drastically decreases with CNT additions. These results can be interpreted by the pinning effect of CNTs which can hinder grain boundary sliding at high temperatures, resulting in a creep resistance improvement.

Abstract: A way to overcome the low deformability of magnesium alloys at room temperature is toincrease the temperature of forming operations. The stress exponent n, which is known to be a keyparameter in the control of plastic stability, generally decreases when temperature increases.Nevertheless, low n-values are not enough to ensure large capacity of deformation since fracturecan also result from strain induced cavitation. In the present investigation, both the mechanisms ofhigh temperature deformation and damage were studied in selected Mg alloys. Since damage datacan also give information on the deformation mechanisms, the strain induce cavitation behaviourwas mainly studied thanks to X-ray micro tomography which provides 3D information like thecavity shapes or the variation with strain of the number of cavities. Moreover, additionally toconventional post mortem analyses, it was attempted to perform the 3D damage characterisation inin situ conditions, namely directly during high temperature deformation tests.