Papers by Keyword: Grain Boundary Sliding

Abstract: 5083 Al alloy was friction stir processed (FSP) at room temperature under various experimental conditions. Two rotational speeds of 430 and 850 rpm with a single traverse feed of 90 mm/min (430-90, 850-90) were used, to investigate the effect of rotation speed. Also, another feed rate of 140 mm/min was used with a rotational speed of 430 rpm (430-140), to investigate the effect of feed rate. The processing conditions resulted in three different grain sizes of 0.95, 1.6 and 2.6 μm depending on the FSP parameters. The deformation behavior of the FSP samples was investigated at 250 C at three strain rates of 10^-4, 10^-3 and 10^-2 s^-1. The values of strain rate sensitivity, m was determined, as a function of grain size, and it decreased from 0.45 to 0.33 to 0.18 with increasing the grain size. True activation energy was calculated as 63, 95, 157 kJ/mole for the grain sizes of 0.95, 1.6 and 2.6 μm, respectively. These calculated values are comparable to grain boundary sliding of magnesium in aluminum (69 – 78 kJ mol^-1), magnesium in aluminum (~115 kJ mole^-1) and aluminum lattice diffusion (~143 kJ mol^-1 ). The change in the deformation mechanism with grain size was discussed in some details.

Abstract: The multilevel approach towards the study of a deformable solid considers a determining role in the deformation and failure of coupling factors and the necessity for combined deformation of its surface layers and volume. At high-cycle fatigue under the reverse bending conditions the plastic deformation is practically concentrated on surface layers. The paper presents the mechanisms of fatigue deformation for high-purity aluminium at various fatigue stages.

Abstract: High-temperature tensile deformation was performed using an oxide-dispersionstrengthened (ODS) ferritic steel,, which has grain structure largely elongated and aligned in one direction, in the perpendicular direction. In the superplastic region II, two-dimensional grain boundary sliding (GBS) was achieved, in which the material did not shrink in the grain-axis direction and grain-boundary steps appeared only in the surface perpendicular to the grain axis. In this condition, a classical grain switching event was observed. Using kernel average misorientation maps drawn with SEM/EBSD, dominant deformation mechanisms and accommodation processes for GBS were examined in the different regions. Cooperative grain boundary sliding, in which only some of grain boundaries slide, was also observed.

Abstract: Superplasticity of metallic alloys has been widely investigated for many years but despite numerous studies, some important questions regarding superplastic behavior of metals can be considered as still open. Among others, one can mention the role of grain boundary sliding as contributing mechanism of deformation, the microstructural requirements for getting superplasticity in metallic alloys or the link between mechanisms of deformation and mechanisms of damage. Interesting information were recently obtained by the use of new experimental techniques. One can mention for instance the development of fine grids allowing more detailed studies of the respective contributions of intra-and inter-granular strains in superplastic alloys or fast X-ray micro tomography allowing in situ 3D damage studies. The aim of this paper is to give information about current findings related to superplastic deformation of metallic alloys and to suggest some perspectives for future.

Abstract: Two-dimensional grain movements were microscopically observed in high-temperature shear deformation of an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure that was sheared in a direction perpendicular to the grain long axis. The microstructure was analyzed using electron back-scattered diffraction and electron channeling contrast imaging techniques before and after the shear deformation. Clear grain switching events, which are assumed to occur via grain-boundary sliding (GBS), were observed and the switching mechanism was characteristic of the core–mantle superplasticity model proposed by Gifkins; dislocation densities got much higher in narrow areas near the grain boundaries (mantles) than the grain interiors (cores). The mantle regions typically appeared in protruding portions of grains that was likely resistant to GBS, and low-angle boundaries were found to emerge at the core–mantle boundaries via slipping of dislocations within the mantle regions.

Abstract: TIMETAL^®54M (Ti-5Al-4V-0.75Mo-0.5Fe) or Ti-54M is an alpha/beta titanium alloy developed by TIMET. Earlier technical publications have confirmed that laboratory scale Ti-54M sheets exhibit superior SPF capabilities compared to Ti-6Al-4V (Ti-64), in terms of flow stress, temperature and strain rate. Various gauges of these sheets, ranging from 0.64mm to 4.57mm, were produced on a production scale using TIMET patented process. Coupon tests were performed in accordance to ASTM E2448 and the results confirmed that the SPF capability of these sheets at a temperature as low as 774°C, which is a significant cost benefit in the SPF fabrications. The present paper discussed basic properties of Ti-54M sheets along with SPF coupon tests results showing lower temperature superplasticity. The low temperature superplasticity of Ti-54M is due to the combined effect of alloying elements and finer primary alpha particle size.

Abstract: Superplastic deformation behavior was investigated for a dual-phase Mg-Ca alloy. The elongation-to-failure reached more than 120% with the strain rate sensitivity, m, over 0.4. The activation energy required for the deformation was estimated to be 98 kJ/mol which is close to the activation energy for grain boundary diffusion in magnesium. Therefore, the superplastic deformation mechanism was suggested to be the grain boundary sliding rate, which is controlled by boundary diffusion.

Abstract: The paper studies deformation mechanisms of nanocrystalline (NC) pure Al and its binary alloys with various distributions of an alloying element which can be Co or Mg via molecular dynamics simulations. It is revealed that a shear deformation of the pure Al is associated with the grain boundary sliding (GBS) and their simultaneous migration. Mg atoms in grain boundaries (GBs) of an Al-Mg alloy lead to GBS which does not accompany with a grain growth, while the deformation process of the corresponding alloy with a random distribution of Mg is close to that for the pure Al. Unlike Mg, GB segregations of Co atoms detain both GBS and GB migration and result in high strength of an Al-Co alloy. On the contrary, the strength of the alloy with the Co atoms distributed randomly is very low due to the structure amorphisation leading to the ease of plastic flow.

Abstract: Hydrogenation effect on the development of superplastic deformation in the submicrocrystalline Ti–6Al–4V alloy at temperatures (0.4–0.5)Т_melt is investigated. Hydrogenation of the submicrocrystalline Ti–6Al–4V alloy to 0.26 mass% during superplastic deformation is found to result in solid solution strengthening, plastic deformation localization, and as a consequence, decrease of the deformation to failure. Possible reasons for the decrease of the flow stress and increase of the deformation to failure in the submicrocrystalline Ti–6Al–4V–0.26H alloy during deformation under conditions of superplasticity and simultaneous hydrogen degassing from the alloy are discussed.

Abstract: The evolution of surface, grains and dislocation structures during superplastic deformation was studied in Al–6.8%Mg–0.6%Mn–0.25%Cr alloy by SEM, EBSD, TEM techniques. The effective activation energy of superplastic deformation was calculated. Contribution of grain boundary sliding was defined during superplastic deformation. Low value of grain boundary sliding, significant dynamic grain growth in stress direction, high dislocations activity and permanent continuous formation of sub-grain boundaries during superplastic deformation were found.