Papers by Keyword: Grain Boundary Sliding

Abstract: The Harmonic Structure (HS) design was implemented in a high-entropy CrMnFeCoNi alloy compact to study its deformation characteristics at elevated temperatures, with particular emphasis on comparison with the homogeneous (Homo) compacts. The HS compact was prepared by powder metallurgy, employing a mechanical grinding process with a planetary ball mill in an argon atmosphere. The rotational speed was set at 150 rpm, and the milling time was either 180 or 360 ks. The resulting powders were then exposed to spark plasma sintering at 1223 K for 1.8 ks under 50 MPa. Subsequently, the compacts were subjected to high-temperature compression tests at 1073 K or 1173 K, at varying initial strain rates over a range of temperatures. These tests were conducted after the sintering process was completed. Homo exhibited a work hardening at the initial stage of deformation, followed by a slight decrease in flow stress, which then remained nearly constant. In contrast, HS exhibited a distinctive softening in flow stress following initial work hardening. A thorough examination of the microstructure during the softening process revealed that adjacent Shell/Core units caused grain boundary sliding in the Shell region. Furthermore, each Core exhibited a rotation of approximately 2.3 degrees and a lateral displacement of 1.5 μm. Observation of the softening phenomenon during high-temperature deformation was confirmed through TEM analysis, revealing that this softening resulted from dynamic recrystallization within the Shell region. Consequently, dynamic recrystallization in the Shell was postulated, followed by rotation of the Shell-Core unit through grain boundary sliding of the UFG structure.

Abstract: The superplastic deformation behavior, microstructure evolution in the volume and on the FIB-milled surface of the samples of fine-grained AA5083-type alloy with an initial grain size of ~5 µm were investigated, and the role of deformation mechanisms was discussed for two superplastic deformation regimes (1) a strain rate of 1×10^-3 s^-1 and a temperature of 0.87T_i.m. and (2) a strain rate of 5×10^-3 s^-1 and a temperature of 0.97T_i.m.. The m values were ~0.45-0.55 and elongations to failure were ~300% and ~600% for the first and second regimes, respectively. According to the shifts of the marker grid lines after straining to e=0.41, GBS contributed ~33% and ~23% to the total strain in the low-temperature and high-temperature deformation, respectively. The dislocation-induced intragranular deformation provided ~30% for the low temperature regime and ~20 % for the high temperature regime, and remaining 30-50% of strain was localized in the striated zones formed at the across grain boundaries due to both GBS and diffusion creep deformation mechanisms. Considering the strain induced by grain elongation for the low and high temperature deformation regimes, it was concluded that diffusion creep contributed 23% and 34% of the total deformation, and the recalculated GBS contribution, including both FIB grid shifts and a portion of the strain localized in the striated regions, was 43% and 38%, respectively.

Abstract: Magnesium and its alloys display a non-usual relationship between flow stress and grain size at room temperature. Breaks in the Hall-Petch relationship have been reported in the literature. Inverse Hall-Petch behavior in which flow stress reduces with grain size decreasing has also been reported in pure magnesium and magnesium alloys with ultrafine and nanocrystalline structures. The present overview discusses these effects in terms of controlling deformation mechanisms. The distinct strength observed in pure magnesium and magnesium alloys with ultrafine grained structure is also discussed. It is shown that experimental data for fine and ultrafine grained magnesium alloys agree with a model suggested recently based on the mechanism of grain boundary sliding. It is also exhibited that the stability of the grain structure might control the strength of ultrafine grained samples.

Abstract: It is now well established that the grain size is the fundamental microstructural feature of all polycrystalline materials. In practice, a very wide range of grain sizes will be needed in order to fully evaluate the effect of grain size on the mechanical properties of metals. For many years this was a significant limitation because it was not possible to use conventional thermomechanical processing to produce materials with submicrometer or nanometer grain sizes. Recently, this problem has been addressed by developing alternative processing techniques based on the application of severe plastic deformation. This overview demonstrates that, although the flow stress increases with decreasing grain size at low temperatures and decreases with decreasing grain size at high temperatures, this clear dichotomy in behavior may be adequately explained by using a single theoretical flow mechanism based on the occurrence of grain boundary sliding.

Abstract: It is widely accepted that the dominant deformation mechanism of fine-grained superplasticity is through grain boundary sliding (GBS) that occurs in fine-grained materials. However, it has been reported that in “Class I” solid solution alloys, superplastic-like behavior controlled by trans-granular deformation occurs by solute drag creep. In this study, we have investigated superplastic behavior in a fine-grained aluminum solid solution alloy with a thermally unstable microstructure. To obtain fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum (Al−Mg) alloy. An equiaxial fine-grained microstructure with a grain size of 7.4 μm was obtained after FSP; however, this microstructure was unstable at high temperatures. Generally, for fine-grained superplasticity or GBS to occur or continue, the fine-grained microstructure must be smaller than 10 μm during high-temperature deformation. However, a large elongation of over 200% was observed at high temperatures despite the occurrence of grain growth. From microstructural observations, it was determined that a fine-grained microstructure is maintained in the early stage of deformation, but at strain levels greater than 100%, trans-granular deformation occurs. The microstructural feature of this trans-granular deformation is similar to the deformation microstructure of solute drag creep observed in “Class I” solid solution alloys. This indicates that a change in the deformation mechanism from GBS to solute drag creep takes place during high-temperature deformation. Here, based on our observations on our model system, which is a thermally unstable aluminum solid solution alloy, we discuss the possibility of a superplastic elongation occurring by means of a transition of the deformation mechanism.

Abstract: Recent Al7075 severe friction stir processing (FSP) data gave new insights regarding the relationship among processing, microstructure and high temperature behaviour. Grain boundary sliding, GBS, usually operates with fine, equiaxed and highly misoriented grains although, so far, the variable misorientation is missing from the constitutive equation. A collection of very fine microstructures comprising various grain size and misorientation values is employed to evidence the relative importance of grain size vs misorientation in the superplastic behaviour of the processed alloy. This relationship is included into a new GBS constitutive equation incorporating the average misorientation as a variable.

Abstract: Accommodation processes are crucial for grain boundary sliding in superplasticity though few have been reported on their positive experimental evidences. The present study achieved two-dimensional grain boundary sliding in ODS ferritic steel which had elongated and aligned grain structure and got direct observations of accommodation processes without the surface effect of floating grains: 1) In Region II, diffusional accommodation was confirmed through observing the change in marking-line spacing, which indicates volume inflow and outflow at grain boundaries. 2) Between Regions II and III, dislocation accommodation inside of the mantle region, as proposed by Gifkins, was confirmed through observing curves of marking lines near grain boundaries. 3) In Region III, dislocation accommodation inside of the core region, as proposed by Ball and Hutchison, was confirmed through observing slip bands and sub-boundaries passing through a grain. It is, then, derived that superplasticity relies not on a single mechanism but on several diffusional and dislocation accommodations contributing depending on strain rate condition.

Abstract: Ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) may show both enhanced ductility and strength and hence resolve the so-called strength-ductility paradox. To gain mechanistic insights into such resolution, the effect of high-pressure torsion (HPT) on the microstructure and mechanical behavior was studied using a cast Al-7 wt. % Si alloy. As expected, the grain size decreased while the fraction of high-angle grain boundaries and microhardness increased due to HPT processing. However, tensile testing at room temperature revealed a simultaneous increase in strength and ductility compared to the as-cast sample. The samples showing simultaneous increase in strength and ductility also showed an increased contribution from grain boundary sliding (GBS), even at room temperature, which is attributed to the existence of a high fraction of high-angle and high-energy grain boundaries. It is proposed that the occurrence of moderate GBS, providing ductility, in very small size grains provides Hall-Petch strengthening and this suggests a potential combination for simultaneously achieving high strength and high ductility in SPD-processed UFG materials.

Abstract: The processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk metal solids. These ultrafine grains in the bulk metals usually show superior mechanical and physical properties. Especially, the development of micro-mechanical behavior is observed after significant changes in microstructure through processing and it is of great importance for obtaining practical future applications of these ultrafine-grained metals. Accordingly, this presentation demonstrates the evolution of small-scale deformation behavior through nanoindentation experiments after HPT on various metallic alloys including a ZK60 magnesium alloy, a Zn-22% Al eutectoid alloy and a high entropy alloy. Special emphasis is placed on demonstrating the essential microstructural changes of these materials with increased straining by HPT and the evolution of the micro-mechanical responses in these materials by measuring the strain rate sensitivity.

Abstract: Magnesium alloys with refined grain structure are often superplastic at elevated temperatures with maximum elongations up to more than 1000%. The superplastic behavior of this material agrees with deformation by grain boundary sliding. Dislocation climb becomes the rate controlling mechanism at higher stresses but the rate controlling mechanism at lower stresses is not fully documented. This report examines the development of superplasticity in a magnesium ZK60 alloy and shows that an increase in stress exponent and decrease in elongation takes place at low stresses. Deformation mechanism maps are constructed considering Regions I, II and III and Coble creep.