Showing papers on "Superplasticity published in 2015"

Hardness−strength relationships in fine and ultra-fine grained metals processed through constrained groove pressing

Abstract: Fine grained (FG) and ultra-fine grained (UFG) materials processed by severe plastic deformation exhibit beneficial hardness and tensile properties. Constrained groove pressing (CGP) were employed for fabrication of FG and UFG sheet metals and accomplished into different types of metals and alloys, such as commercial pure aluminum, AA3003 aluminum alloy, commercial pure copper, nickel, titanium and low carbon steels. Tensile and hardness characteristics in the FG and UFG sheets have been assessed with the aim of evaluating the hardness−strength relationship frequently established for coarse-grained metals and alloys ( σ U T S / H V = 3.45 ). However, it was revealed that the FG and UFG materials do not obey widely used hardness−strength relationships in the conventional coarse grained structures. A new multiplicity factor less than 3, depending on the chemical composition of processed materials, is proposed in this study. This is attributed to different strain hardening response of the FG and UFG materials with slight work hardening before necking instability. In fine grained and ultra-fine grained structures failure does not occur in (or right after) the onset of necking point. That is, tensile deformation sustains significantly up to fracture point due to the role of superplasticity mechanisms.

Ultrafine-grained magnesium–lithium alloy processed by high-pressure torsion: Low-temperature superplasticity and potential for hydroforming

Abstract: A Mg–Li alloy with 8 wt% Li was processed by severe plastic deformation (SPD) through the process of high-pressure torsion (HPT) to achieve ultrafine grains with an average grain size of ~500 nm. Tensile testing with an initial strain rate of 10 −3 s −1 showed that the alloy exhibited superplasticity at a temperature of 323 K or higher. Tensile testing in boiling water confirmed that the specimens were elongated to 350–480% at 373 K under the initial strain rates of 10 −3 s −1 to 1 0 −2 s −1 with a strain rate sensitivity of ~0.3. The current study suggests that not only superplastic forming but also superplastic hydroforming should be feasible after the grain refinement using the HPT method.

Superplastic behaviour of AZ91 magnesium alloy processed by high-pressure torsion

Abstract: An investigation has been conducted on the tensile properties of a fine–grained AZ91 magnesium alloy processed at room temperature by high pressure torsion (HPT). Tensile testing was carried out at 423 K, 473 K and 573 K using strain rates from 1×10–1 s–1 to 1×10–4 s–1 for samples processed in HPT for N = 1, 3, 5 and 10 turns. After testing was completed, the microstructures were investigated by scanning electron microscopy and energy dispersive spectroscopy. The alloy processed at room temperature in HPT exhibited excellent superplastic behaviour with elongations higher than elongations reported previously for fine–grained AZ91 alloy produced by other severe plastic deformation processes, e.g. HPT, ECAP and EX–ECAP. A maximum elongation of 1308 % was achieved at a testing temperature of 573 K using a strain rate of 1×10–4 s–1, which is the highest value of elongation reported to date in this alloy. Excellent high–strain rate superplasticity (HSRSP) was achieved with maximum elongations of 590 % and 860 % at temperatures of 473 K and 573 K, respectively, using a strain rate of 1×10–2 s–1. The alloy exhibited low–temperature superplasticity (LTSP) with maximum elongations of 660 % and 760 % at a temperature of 423 K and using strain rates of 1×10–3 s–1 and 1×10–4 s–1, respectively. Grain–boundary sliding (GBS) was identified as the deformation mechanism during HSRSP, and the glide–dislocation creep accommodated by GBS dominated during LTSP. Grain–boundary sliding accommodated with diffusion creep was the deformation mechanism at high test temperature and slow strain rates. An enhanced thermal stability of the microstructure consisting of fine equiaxed grains during deformation at elevated temperature was attributed to the extremely fine grains produced in HPT at room temperature, a high volume fraction of nano ?–particles, and the formation of ?–phase filaments.

Mechanical properties of ultra-fine grained AZ91 magnesium alloy processed by friction stir processing

Abstract: The relationship of microstructure and mechanical properties in a friction stir processed (FSP) AZ91 magnesium alloy has been analyzed and compared with numerous investigations in the literature. Since the heat generation and sink during FSP drastically influences the final microstructure, several backing devices were used for controlling the stir zone temperature, producing a grain size refinement down to values close to 0.5 μm. This microstructure often determines excellent mechanical properties at room temperature and superplastic behavior at high temperatures. The yield stress at room temperature shows a sharp decrease in the Hall–Petch slope related to a favorable orientation for slip of the basal planes. Noticeable changes in ductility are explained in terms of grain size and texture effects on work hardening behavior which joins both contributions. Finally, the analysis of the tensile tests performed at high temperature, together with the data reported by other authors, have been used to obtain an unitary description of the Grain Boundary Sliding (GBS) mechanism in the AZ91 magnesium alloys.

Grain refinement and high strain rate superplasticity in alumunium 2024 alloy processed by high-pressure torsion

Abstract: An Al-2024 alloy was processed by high-pressure torsion (HPT) to produce an ultrafine-grained structure with a grain size of ~240 nm. A maximum elongation of ~750% was attained with an initial strain rate of 1×10−2 s−1 at 673 K, demonstrating the advent of high strain rate superplasticity through grain refinement by the HPT processing. Evaluation of the strain-rate sensitivity and the activation energy for the deformation confirmed that grain boundary sliding through grain boundary diffusion is the rate-controlling process for the superplastic deformation of the HPT-processed Al-2024 alloy.

Achieving high strain rate superplasticity in Mg–Y–Nd–Zr alloy processed by homogenization treatment and equal channel angular pressing

Abstract: Homogenization treatment and equal channel angular pressing was applied to Mg–Y–Nd–Zr alloy cast ingot. Uniform grains with a grain size of ~1.5 μm and spherical secondary-phase preferentially located at the grain boundaries were obtained. Excellent high-strain rate superplasticity of 860% and 960% was achieved at 500 °C, 1×10 −1 s −1 and 475 °C, 2×10 −2 s −1 , respectively, for the first time such a high-strain rate superplasticity was achieved in magnesium alloys processed by ECAP. The maximum elongation of 1120% was achieved at 475 °C, 3×10 −3 s −1 . They were attributed to the thermal stability of uniform fine-grain microstructure and the coordination function of secondary-phase.

Superplastic deformation mechanisms in fine-grained Al–Mg based alloys

Abstract: The superplastic deformation behaviour at elevated temperatures and constant strain rates of two fine-grained AA5083 type aluminium alloys was investigated. The first alloy with chromium contains Al 6 (Mn,Cr) particles and the second alloy without chromium has Al 6 Mn particles. The effective activation energy of superplastic deformation and the activation parameters for the grain boundary relaxation was calculated. The microstructure evolution and contributions of grain boundary sliding, intragranular deformation and diffusion creep to total superplastic deformation were studied by SEM, EBSD, FIB, TEM techniques. Low values of grain boundary sliding and permanent continuous formation of sub-grain boundaries were found in both alloys. Significant dynamic grain growth during superplastic deformation and large value of intragranular deformation were found in the alloy without chromium. Intragranular deformation is not significant and the superplasticity is primarily a result of diffusion creep in the chromium containing alloy with Al 6 (Mn,Cr) particles.

Effects of strain rate and temperature on hot tensile deformation of severe plastic deformed 6061 aluminum alloy

Abstract: The hot ductility of severe plastic deformed AA6061 was studied at different temperatures and strain rates. The large processed by ECAP specimens with the dimensions of about 100 mm×100 mm×14 mm were then subjected to cold rolling (CR) in order to fabricate the long Ultra-fine grained sheets. According to microscopic observations and hot tensile tests, the combination of two severe plastic deformations (i.e. ECAP+CR) can affect significantly the refinement of grain/subgrain and the ductility of the studied alloy. The results were then compared with those of as-received and CRed cases. In case of ECAP followed by the cold rolling process, the effect of temperature on the ductility, strain rate sensitivity, activation energy and Zener–Hollomon parameter was higher than strain rate. It can be suggested that the possible mechanism dominated the hot tensile deformation during tensile testing is dynamic recovery and dislocation creep. Although the maximum hot ductility was obtained at 500 °C, an increase in the volume fraction of cavities and their distribution led to a decrease in hot ductility and superplasticity of AA6061 alloy processed by SPD .

Low Temperature Sintering Cu6Sn5 Nanoparticles for Superplastic and Super‐uniform High Temperature Circuit Interconnections

Abstract: Brittle intermetallics such as Cu6 Sn5 can be transformed into low cost, nonbrittle, superplastic and high temperature-resistant interconnection materials by sintering at temperatures more than 200 °C lower than its bulk melting point. Confirmed via in situ TEM heating, the sintered structure is pore-free with nanograins, and the interface is super-uniform.

Grain refinement in technically pure aluminium plates using incremental ECAP processing

Abstract: Ultrafine grained materials are capable of superplastic elongation at strain rates approximately two orders of magnitude faster than those currently employed for commercial superplastic forming operations. However, such operations require the material in the form of thin sheets. Therefore, in this work, a new approach to produce ultrafine grained plate samples using a modified equal channel angular pressing (ECAP) method, namely incremental ECAP, was proposed. Unlike conventional ECAP, incremental ECAP works in small steps in which deformation and feeding are associated with two different tools acting asynchronously. Eight passes were applied to technically pure aluminium, with the sample rotation by 90° around the Z axis, which resulted in two full rotations and accumulated strain equal to 9.2. It was demonstrated that grain refinement under these conditions occurs very efficiently. Eight passes resulted in grain size reduction to below 500 nm and very high fraction of high angle grain boundaries of about 80%. This was attributed to the activation of different slip systems in consecutive passes (thanks to sample rotation) and the lack of redundant strain, which results in early establishment of equiaxial grain structure. These two features confirm incremental ECAP to be one of the most effective severe plastic deformation methods in terms of grain size refinement and high angle grain boundaries formationreduction to below 500nm and very high fraction of high angle grain boundaries of about 80%. This was attributed to the activation of different slip systems in consecutive passes (thanks to sample rotation) and the lack of redundant strain, which results in early establishment of equiaxial grain structure. These two features confirm incremental ECAP to be one of the most effective severe plastic deformation methods in terms of grain size refinement and high angle grain boundaries formation.

Strong and superplastic nanoglass.

Abstract: The strength–ductility tradeoff has been a common long-standing dilemma in materials science. For example, superplasticity with a tradeoff in strength has been reported for Cu50Zr50 nanoglass (NG) with grain sizes below 5 nm. Here we report an improvement in strength without sacrificing superplasticity in Cu50Zr50 NG by using a bimodal grain size distribution. Our results reveal that large grains impart high strength, which is in striking contrast to the physical origin of the improvement in strength reported in the traditional nanostructured metals/alloys. Furthermore, the mechanical properties of NG with a bimodal nanostructure depend critically upon the fraction of large grains. By increasing the fraction of the large grains, a transition from superplastic flow to failure by shear banding is clearly observed. We expect that these results will be useful in the development of a novel strong and superplastic NG.

Effect of equal-channel angular pressing on room temperature superplasticity of quasi-single phase Zn–0.3Al alloy

Abstract: Quasi-single phase (dilute) Zn–0.3Al alloy was subjected to severe plastic deformation via equal-channel angular extrusion/pressing (ECAE/P), and the effects of ECAP on its room temperature (RT) and high strain rate (HSR) superplasticity and deformation mechanism were investigated. Multi-pass ECAP may refine the coarse-grained microstructure into the fine grained (FG) one. The grain size of Zn-matrix phase decreased down to 2.0 µm after ECAP. Many spherical Al-rich precipitates decomposed and homogeneously distributed inside the matrix phase. They are ultrafine grained (UFG) α-particles with the grain sizes ranging from 50 nm to ∼200 nm. This special microstructure having FG and UFG micro-constituents brought about an improvement in RT superplasticity even at HSRs. While multi-pass ECAP decreased flow stress of the alloy, its elongation to failure increased substantially depending on the initial strain rates. The maximum elongation was 1000% at a low strain rate of 10−4 s−1, and 350% elongation was achieved at a high strain rate of 10−2 s−1. Grain boundary sliding (GBS) was found to be the main deformation mechanism in region-II as the optimum superplastic region.

Improvement of high strain rate and room temperature superplasticity in Zn–22Al alloy by two-step equal-channel angular pressing

Abstract: The Zn–22Al alloy was subjected to equal-channel angular pressing (ECAP) to improve its high strain rate (HSR) superplasticity at room temperature (RT). A well-designed two-step ECAP process formed an ultrafine-grained (UFG) microstructure with an average grain size of 200 nm as the lowest one obtained so far after ECAP processing of this alloy. Also, agglomerate- and texture-free microstructure with UFG Al-rich α- and Zn-rich η-grains separated mostly by high-angle grain boundaries (HAGBs) was produced by this process. The maximum RT elongation was achieved to be 400% with a strain rate sensitivity of 0.30 at a very high strain rate of 5×10 −2 s −1 after the two-step ECAP process. This elongation value is the highest one obtained at RT and at all strain rates for this alloy up to now. The current results demonstrate that such an improvement in superplasticity of Zn–22Al alloy after the two-step ECAP process can enhance its applications where RT and HSR superplasticity are strongly needed.

Enhancement of superplasticity in a fine-grained Mg–3Gd–1Zn alloy processed by equal-channel angular pressing

Abstract: Superplastic behavior of an Mg–3Gd–1Zn (GZ31) alloy, processed by extrusion at 653 K followed by equal-channel angular pressing (ECAP) at 553 K, was investigated by shear punch testing (SPT). A fine grained microstructure with a grain size of 1.7 μm and large fraction of high-angle grain boundaries was achieved after processing by ECAP. Shear punch tests were conducted at temperatures in the range of 573–773 K and shear strain rates in the range of 0.003–0.130 s −1 . While the strain rate sensitivity (SRS) index of the as extruded condition was less than 0.22, a high SRS index of 0.51 at 673 K was achieved after ECAP. The SRS index of 0.51 and the activation energy of 73 kJ mol −1 imply that the responsible mechanism for superplasticity after ECAP is grain boundary sliding (GBS) accommodated by grain boundary diffusion.

Effect of thermal treatment on the bio-corrosion and mechanical properties of ultrafine-grained ZK60 magnesium alloy.

Abstract: The combination of solid solution heat treatments and severe plastic deformation by high-ratio differential speed rolling (HRDSR) resulted in the formation of an ultrafine-grained microstructure with high thermal stability in a Mg–5Zn–0.5Zr (ZK60) alloy. When the precipitate particle distribution was uniform in the matrix, the internal stresses and dislocation density could be effectively removed without significant grain growth during the annealing treatment (after HRDSR), leading to enhancement of corrosion resistance. When the particle distribution was non-uniform, rapid grain growth occurred in local areas where the particle density was low during annealing, leading to development of a bimodal grain size distribution. The bimodal grain size distribution accelerated corrosion by forming a galvanic corrosion couple between the fine-grained and coarse-grained regions. The HRDSR-processed ZK60 alloy with high thermal stability exhibited high corrosion resistance, high strength and high ductility, and excellent superplasticity, which allow the fabrication of biodegradable magnesium devices with complicated designs that have a high mechanical integrity throughout the service life in the human body.

Superplastic behavior and microstructure evolution of a fine-grained Mg–Y–Nd alloy processed by submerged friction stir processing

Abstract: In this study, a fine-grained Mg–Y–Nd alloy with an average grain size of ~1.3 μm and small second phases of ~280 nm was prepared by submerged friction stir processing (SFSP). Microstructure evolution and superplastic behavior of Mg–Y–Nd alloy processed by SFSP was investigated in the temperature ranges of 683–758 K and the strain rate ranges from 2×10−2 to 4×10−4 s−1. Due to the fine-grained and stable microstructure, excellent high strain rate superplasticity (HSRS) of 900% was achieved at 758 K and 2×10−2 s−1, and the maximum elongation of 967% was obtained at 733 K and 3×10−3 s−1. Because of the good deformation compatibility, cavities were easily formed at the grain boundaries instead of the interface between particles and matrix. Grain boundary sliding accommodated by lattice diffusion was the dominated deformation mechanism during superplastic deformation.

Effect of Sc and Zr additions on grain stability and superplasticity of the simple thermal–mechanical processed Al–Zn–Mg alloy sheet

Abstract: Effect of scandium and zirconium on grain stability and superplastic ductility in the simple thermal–mechanical processed Al–Zn–Mg alloys was investigated Tensile testing revealed that the Al–Zn–Mg alloy without Sc and Zr additions showed no superplasticity because of the larger grain size (>10 μm) and the poor stability of the microcrystalline structure during superplastic deformation However, the Al–Zn–Mg–025Sc–010Zr (wt%) alloy exhibited excellent superplastic (elongations of ≥500%) at a wide temperature range of 450–550 °C and high strain rate range of 5×10−3–5×10−2 s−1, and the maximum elongation of ~1523% was achieved at 500 °C and 1×10−2 s−1 Electron back scatter diffraction analysis and transmission electron microscopy results showed that superior superplastic ductility of the Al–Zn–Mg–025Sc–010Zr alloy can be ascribed to the complete transformation of low angle grain boundaries to high angle grain boundaries due to the occurrence of dynamic recrystallization and the presence of coherent Al3ScxZr1−x particles that effectively impede the growth of the grains during superplastic deformation Besides, strong β-fiber rolling textures gradually weakened, and random textures were predominant in the superplastic deformed alloy Analyses on the superplastic data revealed that the average strain rate sensitivity and the average activation energy of the Al–Zn–Mg–025Sc–010Zr alloy were ~037 and ~845 kJ/mol, respectively All results indicated that the main superplastic deformation mechanism was grain boundary sliding