Showing papers on "Superplasticity published in 2003"

Multifunctional Alloys Obtained via a Dislocation-Free Plastic Deformation Mechanism.

Abstract: We describe a group of alloys that exhibit “super” properties, such as ultralow elastic modulus, ultrahigh strength, super elasticity, and super plasticity, at room temperature and that show Elinvar and Invar behavior. These “super” properties are attributable to a dislocation-free plastic deformation mechanism. In cold-worked alloys, this mechanism forms elastic strain fields of hierarchical structure that range in size from the nanometer scale to several tens of micrometers. The resultant elastic strain energy leads to a number of enhanced material properties.

X-ray micro-tomography an attractive characterisation technique in materials science

Abstract: X-ray tomography is a non-destructive technique which provides 3D information of materials. It is consequently very attractive in Materials Science since the relation between macroscopic properties and the micro-structure of a material is very frequently required. The aim of this paper is to present selected results obtained in various investigations of metallic materials such as superplastic deformation, materials in the semi-solid state and metallic foams. Depending on the studied features, several tomography analysis modes were used: conventional absorption mode, phase contrast and holotomography, a new technique, which provides the 3D distribution of the electron density in the bulk of the material. Furthermore micro-tomography enables one to perform in situ experiments either by using a mechanical test machine or a furnace.

Dynamic continuous recrystallization characteristics in two stage deformation of Mg-3Al-1Zn alloy sheet

Abstract: Dynamic recrystallization (DRX) characteristics of a Mg/3Al/1Zn (AZ31) alloy sheet were investigated at temperatures ranging from 200� /450 8C and constant strain rates of 1/10 4 � /2/10 4 s 1 . The average grain size of the as-received alloy was 12 mm and can be refined to 6 mm via deformation at 250 8C, 1/10 4 s 1 to a strain level of 60%. Grain refinement was less effectiv ea t higher temperatures due to rapid grain growth. The grain refinement was attributed to dynamic continuous recrystallization which involves progressive increase in grain boundary misorientation and conversion of low angle boundaries into high angle boundaries. During DRX, subgrains were developed through the conversion of dislocation cell walls into subgrain boundaries. The presence of precipitates was not essential for dynamic recrystallization in the magnesium alloy being investigated because of its limited slip systems, low stacking fault energy and high grain boundary diffusion rate. # 2003 Elsevier Science B.V. All rights reserved.

Texture evolution during large-strain hot rolling of the Mg AZ61 alloy

Abstract: Grain refinement in a Mg-based AZ61 alloy of initially coarse, recrystallized microstructure was successfully achieved by thermomechanical processing (TMP) consisting of two to three hot-rolling steps with large reductions per pass. Reductions as large as 85% (equivalent to a true strain of ≈1) were achieved without surface cracking. The underlying microscopic mechanisms operative during the TMP that allowed this hcp material to accommodate such large strains per pass were investigated by macro- and microtexture analysis. A significant decrease in the intensity of the initial basal texture was observed after the first pass. This was attributed to rotational dynamic recrystallization, a mechanism by which new recrystallized grains develop, with orientations favourable for basal slip. Upon subsequent passes, basal slip becomes the main deformation mechanism. Simultaneously, grain refinement takes place by continuous dynamic recrystallization. The fine-grained microstructure thus developed showed improved superplastic behaviour in comparison with that of similar alloys processed by more elaborate methods.

High strain rate superplasticity in a commercial 2024 Al alloy via friction stir processing

Abstract: A novel technique for producing high-strain-rate-superplastic (HSRS) microstructure via friction stir processing (FSP) in a commercial 2024 Al has been demonstrated. A maximum ductility of ∼525% has been achieved at a strain rate of 10 −2 s −1 and 430 °C. Current results suggest that friction stir processing can be developed as a simple yet effective technique for producing microstructure amenable for superplasticity at high strain rates and/or lower temperatures and at lower flow stresses.

Near-Perfect Elastoplasticity in Pure Nanocrystalline Copper

Abstract: Ductile metals and alloys undergo plastic yielding at room temperature, during which they exhibit work-hardening and the generation of surface instabilities that lead to necking and failure. We show that pure nanocrystalline copper behaves differently, displaying near-perfect elastoplastic behavior characterized by Newtonian flow and the absence of both work-hardening and neck formation. We observed this behavior in tensile tests on fully dense large-scale bulk nanocrystalline samples. The experimental results further our understanding of the unique mechanical properties of nanocrystalline materials and also provide a basis for commercial technologies for the plastic (and superplastic) formation of such materials.

High strain rate superplasticity in friction stir processed Al–Mg–Zr alloy

Abstract: Al–4Mg–1Zr extruded bar was subjected to friction stir processing (FSP), resulting in generation of a fine microstructure of 1.5 μm grain size. Superplastic deformation behavior of FSP Al–4Mg–1Zr alloy was investigated in strain rate range of 1×10−3 to 1 s−1 and temperature range of 350–550 °C and compared with that of as-rolled one. It is indicated that the FSP alloy exhibited significantly enhanced superplasticity at a high strain rate of 1×10−1 s−1, and a maximum elongation of 1280% was obtained at 525 °C and 1×10−1 s−1. Further, the FSP Al–4Mg–1Zr alloy exhibited excellent thermal stability at high temperature, and a large elongation of 1210% was observed at 550 °C and 1×10−1 s−1. Moreover, FSP resulted in a significant decrease in the flow stress in Al–4Mg–1Zr alloy. At a strain rate of 1×10−2 s−1, the flow stress (∼7 MPa) of FSP Al–4Mg–1Zr at 450 °C was comparable to that of as-rolled alloy at 550 °C.

Superplasticity and grain boundary sliding in rolled AZ91 magnesium alloy at high strain rates

Abstract: The superplastic deformation characteristics and microstructure evolution of the rolled AZ91 magnesium alloys at temperatures ranging from 623 to 698 K (0.67–0.76 Tm) and at the high strain rates ranging from 10−3 to 1 s−1 were investigated with the methods of OM, SEM and TEM. An excellent superplasticity with the maximum elongation to failure of 455% was obtained at 623 K and the strain rate of 10−3 s−1 in the rolled AZ91 magnesium alloys and its strain rate sensitivity m is high, up to 0.64. The dominant deformation mechanism in high strain rate superplasticity is still grain boundary sliding (GBS), which was studied systematically in this study. The dislocation creep controlled by grain boundary diffusion was considered the main accommodation mechanism, which was observed in this study.

Superplastic behavior of an Al/Mg alloy at elevated temperatures

Abstract: The superplastic properties and microstructural evolution of a 0.2% Zr and 1.6% Mn modified 5083 aluminum alloy with an initial grain size of 6.2 μm were examined at strain rates ranging from 10−5 to 10−1 s−1 in the temperature interval 500–580 °C. The maximum elongation-to-failure of 1150% was found at 570 °C, which is near the solidus temperature of 572 °C, and an initial strain rate of 2.8×10−3 s−1. The corresponding strain rate sensitivity coefficient, m, was about 0.6. It was shown that increasing the temperature from 550 to 570 °C results in reduced cavitation and expanding the optimal interval of superplasticity toward lower strain rates due to the disappearance of the threshold stress. The influence of temperature on the mechanisms of superplastic deformation is discussed.

Superplasticity and grain boundary sliding characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet

Abstract: A ‘Two-Stage Deformation Method’ was proposed to enhance the superplasticity of Mg–3Al–1Zn (AZ31) alloy sheet. This method exploited the capability of the material to undergo dynamic recrystallization (DRX) at optimum DRX conditions of 250 °C and constant strain rate of 1×10 −4 s −1 . Stage I was aimed at refining the coarse microstructure of the as-received alloy to result in fine equiaxial grains measuring less than 10 μm, which deformed by grain boundary sliding accommodated by intragranular slip. Subsequently, Stage II was performed at a higher deformation temperature, whereby viscous glide mechanism accommodated by lattice diffusion was predominant. By altering the deformation mechanisms at different strain levels, elongation-to-failure of 320 and 360% was attained at 400 and 450 °C, respectively.

Paradoxes of Severe Plastic Deformation

Abstract: Severe plastic deformation (SPD) can lead to emergence of microstructural features and properties in materials which are fundamentally different from the ones well known for conventional cold deformation. In particular, the instances of unusual phase transformations resulting in development of highly metastable states associated with formation of supersaturated solid solutions, disordering or amorphization and their further decomposition during heating, high thermal stability of the SPD-produced nanostructures, and the paradox of strength and ductility in some SPD-processed metals and alloys are discussed.

Diffusion bonding in superplastic magnesium alloys

Abstract: The superplastic characteristic and diffusion bonding behavior were investigated in a commercial AZ31 magnesium alloy sheet having equiaxed grain size with an average size of 16.8 μm. The alloy behaved in a superplastic manner from 623 to 723 K, and, the present material was successfully diffusion bonded at the superplastic temperature by selecting appropriate pressure and time. The maximum of lap shear strength was 0.85 at a bonding pressure of 3 MPa and a bonding temperature of 673 K, with a bonding time of 3 h. In the specimen with high ratio of lap shear strength, the bond line was not identified by optical microscopy.

High strain rate superplasticity of submicrometer grained 5083 Al alloy containing scandium fabricated by severe plastic deformation

Abstract: High strain rate superplasticity (HSRS) was obtained in a commercial 5083 Al alloy by introducing a ultrafine grained structure of 0.3 mm through severe plastic deformation and by adding a dilute amount of scandium (Sc) as a microstructure stabilizer. Tensile tests were carried out on the as-processed sample at temperatures of 623/823 K and initial strain rates of 1/10 3 /1/10 0 s 1 . The maximum elongation to failure of 740% was obtained at 773 K and 1/10 2 s 1 . HSRS of the alloy was attributed to the combined effects of dynamic recrystallization and preservation of fine recrystallized grains by the presence of Sc. The mechanical behavior of the alloy at 773 K was characterized by a sigmoidal behavior in a plot of stress vs strain rate in the double logarithmic scale. The origin of the sigmoidal behavior was discussed in terms of microstructural evolution during superplastic deformation. An examination of the fractured samples revealed that failure occurred in a brittle manner related to cavitation rather than necking. Cavity stringers were formed parallel to the tensile axis by interlinkage of jagged-shaped isolated cavities along grain boundaries aligned to the tensile axis. # 2002 Elsevier Science B.V. All rights reserved.

Formidable Increase in the Superplasticity of Ceramics in the Presence of an Electric Field

Abstract: The ductility of silicon nitride-based ceramics is dramatically enhanced in the presence of a pulsed electric field/current that induces movement of the charged species present in the grain boundar ...

Achieving superplasticity in ultrafine-grained copper: influence of Zn and Zr additions

Abstract: Equal-channel angular pressing (ECAP) was applied to samples of pure Cu, a Cu–30%Zn alloy and alloys of Cu–0.18%Zr and Cu–30%Zn–0.13%Zr. All materials exhibited ultrafine grain sizes in the range of ∼0.1–0.4 μm after ECAP but the microstructures were inhomogeneous and the grains were elongated in the as-pressed condition. Tensile testing showed that superplasticity was not achieved in any of these materials at a temperature of 573 K but at 673 K the Cu–Zn–Zr alloy exhibited superplastic elongations at strain rates at and below ∼10 −3 s −1 . The results demonstrate that the presence of Zn and Zr are both beneficial in promoting the occurrence of superplastic ductilities. Zirconium is needed because it increases the recrystallization temperature, inhibits grain growth and, therefore, serves to retain a small grain size at elevated temperatures, and zinc is beneficial because it introduces solute atoms into the matrix so that dislocation creep is inhibited and superplastic flow can occur more easily.

Comparison of superplastic behavior in two 5083 aluminum alloys

Abstract: Superplastic elongation is generally controlled by both strain-rate sensitivity and particles causing cavitation. The superplastic tensile behavior of 5083 Al (nominally Al–4.6% Mg) alloys from two different sources has been examined. It is found that for nearly the same grain size, one of the alloys (alloy A) has a slightly lower strain-rate sensitivity m. This alloy also contains a significantly larger number of hard particles and inclusions. Because this alloy exhibits somewhat lower flow stress during constant strain-rate tests, a reduced tendency for cavity initiation was expected. However, the larger density of particles tends to increase cavitation by providing more nucleation sites. Then the lower m value leads to more rapid cavity growth and interlinkage. Thus, while the presence of fine grain size and high-angle grain boundaries are important for superplastic flow, this research shows that in similarly processed alloys small changes in m values and variations in alloy chemistry that generate cavity-causing particles can have a medium to large effect on superplastic elongation.

Low Temperature Superplasticity and Its Deformation Mechanism in Grain Refinement of Al-Mg Alloy by Multi-Axial Alternative Forging

Abstract: In practical application, an appearance of low temperature superplasticity (LSTP) is one of necessaries conditions. In this paper, to estimate an appearance and deformation mechanisms of this superplasticity, the role of grain boundary sliding (GBS), intragranular deformation and the change of microstructure during superplastic deformation have been investigated for ultrafine-grained Al-Mg alloy with a grain size of less than 1 μm using Multi-Axial Alternative Forging (MAF) technique. In these materials, it shows that the elongation and strain rate sensitivity (m-value) were 340% and 0.39, respectively, at 473 K under a strain rate of 2.8 x 10 -3 s -1 . These results show that superplastic appearance is possible at 473 K. The void formed at 473 K elongated in parallel to the tensile direction, with a length of 15 μm and a width of 5 μm. The intragranular deformation contribution was estimated from the aspect ratio of the grains after deformation and its contribution ratio was about 33.5 %. Therefore, for the appearance of lower temperature superplasticity with large elongation and m-value, the role of intragranular deformation was the most important factor together with GBS under these conditions. As described above, the MAF technique is one of the most effective methods to produce ultrafine-grained material and appearance of lower temperature superplasticity.