Showing papers on "Superplasticity published in 2013"

A transition from localized shear banding to homogeneous superplastic flow in nanoglass

Abstract: A promising remedy to the failure of metallic glasses (MGs) by shear banding is the use of a dense network of glass-glass interfaces, i.e., a nanoglass (NG). Here we investigate the effect of grain size (d) on the failure of NG by performing molecular dynamics simulations of tensile-loading on Cu50Zr50 NG with d = 5 to 15 nm. Our results reveal a drastic change in deformation mode from a single shear band (d ∼ 15 to 10 nm), to cooperative shear failure (d ∼ 10 to 5 nm), to homogeneous superplastic flow (d ≤ 5 nm). Our results suggest that grain size can be an effective design parameter to tune the mechanical properties of MGs.

Phase Composition and Superplastic Behavior of a Wrought AlCoCrCuFeNi High-Entropy Alloy

Abstract: A cast AlCoCrCuFeNi high-entropy alloy was multiaxially forged at 950°C to produce a fine homogeneous mixture of grains/particles of four different phases with the average size of ~2.1 μm. The forged alloy exhibited unusual superplastic behavior accompanied by a pronounced softening stage, followed by a steady-state flow stage, during tensile deformation at temperatures of 800°C–1000°C and at strain rates of 10−4–10−1 s−1. Despite the softening stage, no noticeable strain localization was observed and a total elongation of up to 1240% was obtained. A detailed analysis of the phase composition and microstructure of the alloy before and after superplastic deformation was conducted, the strain rate and temperature dependences of the flow stress were determined at different stages of the superplastic deformation, and the relationships between the microstructure and properties were identified and discussed.

High strain rate superplasticity in friction stir processed ultrafine grained Mg–Al–Zn alloys

Abstract: Friction stir processing of a rolled AZ31 alloy and a high pressure die cast AZ91 alloy produced ultrafine grained materials with average grain size of 0.8 μm and 0.5 μm, respectively. Tensile testing was done in the temperature range of 210–360 °C and strain rate range of 1×10 −4 –3×10 −2 s −1 . Higher amounts of β-precipitates in AZ91 thermally stabilized the microstructure up to 330 °C. Friction processed ultrafine grain AZ91 exhibited high strain rate superplasticity at all tested temperatures. The best elongation of 1251% was achieved at a strain rate of 1×10 −2 s −1 and temperature of 330 °C. The kinetic analysis of the superplastic data revealed a large variation in the kinetic parameter due to difficulty in using proper grain size values because of concurrent grain growth.

High strain rate superplasticity of a fine-grained AZ91 magnesium alloy prepared by submerged friction stir processing

Abstract: The as-cast AZ91 plate was subjected to normal friction stir processing (processed in air) and submerged friction stir processing (processed in water, SFSP), and microstructure and superplastic tensile behavior of the experimental alloys were investigated. SFSP results in remarkable grain refinement due to the enhanced cooling rate compared with normal FSP, with an average grain size of 1.2 μm and 7.8 μm. The SFSP AZ91 specimen exhibits considerably enhanced superplastic ductility with reduced flow stress and higher optimum strain rate, as compared to the normal FSP specimen. The optimum superplastic deformation temperature is found to be 623 K for both the normal FSP and SFSP AZ91 specimens. An elongation of 990% is obtained at 2×10−2 s−1 and 623 K for the SFSP specimen, indicating that excellent high strain rate superplasticity could be achieved. By comparison, maximum ductility of the normal FSP specimen strained at high strain rate is 158%. Grain boundary sliding is the main mechanism for the superplastic deformation of the normal FSP and SFSP specimens. The excellent high strain rate superplasticity of the SFSP alloy is attributed to its finer grain structure and higher fraction of grain boundary.

High-Temperature Mechanical Behavior of Extruded Mg-Y-Zn Alloy Containing LPSO Phases

Abstract: The high-temperature mechanical behavior of extruded Mg97−3xY2xZnx (at. pct) alloys is evaluated from 473 K to 673 K (200 °C to 400 °C). The microstructure of the extruded alloys is characterized by Long Period Stacking Ordered structure (LPSO) elongated particles within the magnesium matrix. At low temperature and high strain rates, their creep behavior shows a high stress exponent (n = 11) and high activation energy. Alloys behave as a metal matrix composite where the magnesium matrix transfers part of its load to the LPSO phase. At high-temperature and/or low stresses, creep is controlled by nonbasal dislocation slip. At intermediate and high strain rates at 673 K (400 °C) and at intermediate strain rates between 623 K and 673 K (350 °C and 400 °C), the extruded alloys show superplastic deformation with elongations to failure higher than 200 pct. Cracking of coarse LPSO second-phase particles and their subsequent distribution in the magnesium matrix take place during superplastic deformation, preventing magnesium grain growth.

Shear punch superplasticity in equal-channel angularly pressed Mg–12Li–1Zn alloy

Abstract: The superplasticity of a fine-grained Mg–12Li–1Zn alloy, processed by equal-channel angular pressing (ECAP), was studied by shear punch testing. The strain rate sensitivity index of 0.45 and activation energy of 71 kJ mol–1 is indicative of a superplastic shear behavior dominated by grain boundary sliding.

Plastic Flow and Microstructure Evolution during Thermomechanical Processing of a PM Nickel-Base Superalloy

Abstract: Plastic flow and microstructure evolution during sub- and supersolvus forging and subsequent supersolvus heat treatment of the powder-metallurgy superalloy LSHR (low-solvus, high-refractory) were investigated to develop an understanding of methods that can be used to obtain a moderately coarse gamma grain size under well-controlled conditions. To this end, isothermal, hot compression tests were conducted over broad ranges of temperature [(1144 K to 1450 K) 871 °C to 1177 °C] and constant true strain rate (0.0005 to 10 s−1). At low temperatures, deformation was generally characterized by flow softening and dynamic recrystallization that led to a decrease in grain size. At high subsolvus temperatures and low strain rates, steady-state flow or flow hardening was observed. These latter behaviors were ascribed to superplastic deformation and microstructure evolution characterized by a constant grain size or concomitant dynamic grain growth, respectively. During supersolvus heat treatment following subsolvus deformation, increases in grain size whose magnitude was a function of the prior deformation conditions were noted. A transition in flow behavior from superplastic to nonsuperplastic and the development during forging at a high subsolvus temperature of a wide (possibly bi- or multimodal) gamma-grain-size distribution having some large grains led to a substantially coarser grain size during supersolvus annealing in comparison to that produced under all other forging conditions.

Microstructure and texture evolution of a near-α titanium alloy during hot deformation

Abstract: The microstructure and texture evolution of a near-α titanium alloy during the tensile deformation at 900 °C was investigated using electron backscattered diffraction technology. During the tension at strain rate of 3×10−3 s−1, the flow stress reached a balance of hardening and softening, showing certain degree of superplasticity. The dynamic recrystallization was the principal softening mode. The mean size of the dynamically recrystallized grains increased with the flow stress decreasing. The initial textures were weakened or eliminated by the dynamic recrystallization process. Under deformation at strain rate of 3×10−3 s−1, a new texture component of ( 01 1 ¯ 0 ) [ 0001 ] generated and showed the largest values of Taylor factor. The origin of this dynamic recrystallization texture was clarified based on the Taylor's theory.

Deformation mechanisms during superplastic testing of Ti–6Al–4V–0.1B alloy

Abstract: Superplastic tensile tests on warm rolled and optimally annealed boron modified alloy Ti-6Al-4V-0.1B at a temperature of 850 degrees C and initial strain rate of 3 x 10(-4) s(-1) results in a higher elongation (similar to 500%) compared to the base alloy Ti-6Al-4V (similar to 400%). The improvement in superplasticity has been attributed to enhanced contribution from interfacial boundary sliding to the overall deformation for the boron modified alloy. The boundary sliding was facilitated by the starting microstructure which predominantly contains small equiaxed primary a grains with narrow size distribution. Dynamic processes such as coarsening and globularization of primary a phase occur under the test condition but do not significantly contribute to the observed difference in superplasticity between the two alloys. In spite of cavitation takes place around the TiB particles during deformation, they do not cause macroscopic cracking and early fracture by virtue of the cavities being extremely localized. Localized cavitation is found to correlate with increased material transfer due to faster diffusion.

Effect of microstructural inhomogeneity on superplastic behaviour of multipass friction stir processed aluminium alloy

Abstract: Microstructures required for superplasticity were fabricated by intermittent multipass friction stir processing (FSP) in a 6 mm 5086 aluminium alloy plate. Two processing parameters corresponding to two different heat inputs were used. Multipass FSP created a gradient microstructure with fine and coarse grain-depth features on the processed plates. Three sheets of 1.5 mm thickness with different microstructural features were extracted for deformation testing under superplastic conditions: a layer with only fine grains from the nugget layer (NL), a layer with a thermomechanical-/heat-affected layer containing coarse grains (TL), and a composite layer (CL) having both fine and coarse grains in equal proportions. High temperature tensile testing was conducted for different layers between 450–550 °C with strain rates ranging from 5×10−4 s−1 to 1×10−2 s−1 to determine the optimum superplastic conditions. The NL and CL were comparable in terms of ductility with a high m value of 0.44. The maximum ductility values were 325% for NL, 355% for CL and 230% for TL. The high ductility of the composite layers, despite their microstructural inhomogeneity, establishes multipass FSP as an effective bulk processing technique.

Development of hardness homogeneity and superplastic behavior in an aluminum–copper eutectic alloy processed by high-pressure torsion

Abstract: An Al–33% Cu eutectic alloy was processed by high-pressure torsion (HPT) at a pressure of 6.0 GPa for 1/4 to 10 turns. Examination after processing showed a gradual evolution to a reasonable level of hardness and microstructural homogeneity after 5 or more turns. Tensile tests were conducted at a temperature of 723 K on specimens processed through 5 and 10 turns of HPT. These specimens exhibited excellent superplastic properties with a maximum elongation of ∼1250% at strain rates lower than 10−3 s−1. The results also showed that the maximum elongation is displaced to a faster strain rate when the HPT processing is conducted to a higher number of turns. An analysis demonstrates that superplastic flow in the Al–Cu alloy processed by HPT is well described by a theoretical relationship that was developed for conventional superplastic materials.

High temperature deformation and microstructural instability in AZ31 magnesium alloy

Abstract: The high temperature deformation behaviour of AZ31 magnesium alloy is analysed by comparing numerous investigations, including work by these authors and by other researchers. Three main deformation mechanisms are observed, i.e grain boundary sliding, solute drag creep and climb-controlled dislocation creep. A combined set of constitutive equations, which takes into account the concurring effect of these different deformation mechanisms, is proposed. Grain boundary sliding is observed to cause a superplastic behaviour in fine-grained materials, but grain growth due to excessively prolonged high temperature exposure invariably results in a transition to either viscous glide or dislocation climb as a rate-controlling mechanism. On the basis of these considerations, the differences observed by testing the same material under constant strain rate or by strain rate change experiments are rationalised by quantifying the effect of static and dynamic grain growth and dynamic recrystallisation. This procedure provides a unitary description of the high temperature deformation of AZ31 in a wide range of strain rates and temperatures.

Superplastic forming (SPF)/diffusion bonding (DB) forming method of titanium alloy cylindrical three-layer structure

Abstract: The invention belongs to a sheet-metal forming technology, and relates to a superplastic forming (SPF)/diffusion bonding (DB) forming method of a titanium alloy cylindrical three-layer structure. A novel blank structural form and a blank production method are adopted, the requirements on coating welding stopping agent and positioning a cylinder on a three-layer structure can be met, and a hot isostatic pressure DB and cylinder SPF step-by-step formation method is adopted for the cylinder blank, so that the technical difficulty and complexity can be reduced, and the structural flexibility can be improved. A partition ring is creatively additionally arranged among the cylinders, the dropping of the welding stopping agent, caused by the collision of the cylinders, can be avoided, an air inlet passage and an exhaust passage are directly machined and formed on the blank, no air passage needs to be machined on a mold, and the structural complexity of a tool can be reduced. The SPF/DB formation of the titanium alloy cylindrical three-layer structure can be stably and reliably realized, and the formation quality and the technological stability can be improved. The SPF/DB technological method is enlarged from the formation of a flat sandwich structure to the formation of the cylindrical rotation sandwich structure, so that the application field of the process can be enlarged, and good economic and technical benefit can be realized.

Superplastic behavior and deformation mechanism of Ti600 alloy

Abstract: The superplastic deformation behavior and mechanism of Ti600 alloy at elevated temperature were investigated. Results show that Ti600 alloy exhibits excellent superplastic behavior in the temperature range of 840–960 °C at 5×10 −4 s −1 and all of the tensile elongations exceed 220%. Optical microstructure shows that the grains still remain equiaxed and refined after deformation. However, primary α phase increases with the increasing of temperature. TEM observation indicates that the intragranular dislocation movement is very active and is accompanied by the occurrence of dynamic recrystallization, which is beneficial to promote the grainboundary sliding and to relieve the stress concentration. The superplastic deformation mechanism of Ti600 alloy is grainsgroup sliding accommodated by dislocation movement and dynamic recrystallization. The model of this mechanism is a corrected Ball–Hutchinson Model.

Accommodation mechanisms for grain boundary sliding as inferred from texture evolution during superplastic deformation

Abstract: To gain insight into accommodation mechanisms for local stress concentrations produced by grain boundary sliding (GBS), we systematically examined texture evolution within a superplastic magnesium alloy undergoing deformation at a relatively low deformation temperature (at which basal slip is known to be the preferred slip system in magnesium). Although we did observe an overall weakening of the initial basal texture during superplastic deformation, we also observed within the interior of the specimen a convergent evolution that depends on loading direction. We attribute this texture evolution within the bulk to the competing effects of (a) orientation divergence due to grain rotation accompanied by GBS and (b) convergent evolution due to slip, which acts primarily as an accommodation mechanism for GBS. In contrast, at the near-surface, we found the initial orientation to be preserved, indicating that slip accommodation is less important near the surface than within the bulk.

Hot deformation and processing maps of DC cast Al-15%Si alloy

Abstract: The hot deformation behavior of Al-15%Si alloy has been studied by employing both processing maps and microstructural observations. The material of investigation was deformed by compression in the temperature range of 300–500 °C and strain rate range of 0.001–5 s −1 . Processing maps were calculated to evaluate the efficiency of the hot working and recognize the instability regions of the flow behavior. The investigated alloy possesses the optimum hot-working conditions at temperature of 500 °C and strain rate of 5 s −1 corresponding to the peak efficiency of 33%, since the material undergoes dynamic recrystallization to produce a equaxied grain structure with fraction of ~53.5% of high angle boundaries. At higher temperature and lower strain rate, the material exhibited superplasticity with its maximum at temperature of 500 °C and strain rate of 0.001 s −1 . Flow instability occurs at deformation temperatures ranging from 350 to 450 °C and strain rates ranging from 0.5 to 5 s −1 at strain of 0.2, and the instability domain reduces with the increasing of strain and is gradually limited to deformation temperatures ranging from 350 to 400 °C and strain rates higher than about 1 s −1 .

High temperature plasticity in yttria stabilised tetragonal zirconia polycrystals (Y-TZP)

Abstract: The literature data on the superplastic deformation of high purity yttria stabilised tetragonal zirconia polycrystals is reviewed in detail. It is shown that, based on the existence of a threshold stress, the single mechanism of grain boundary sliding (GBS) accommodated by diffusional processes can explain the superplasticity of these materials over all the ranges of temperature, stress, grain size, and surrounding atmosphere that have been studied. The origin of the threshold stress and its quantitative dependence on temperature and grain size is explained in terms of the segregation of yttrium atoms at the grain boundaries. A new model for GBS accommodated by lattice or grain-boundary diffusion is presented which can explain the transition of the stress exponent from 2 to 1.

Superplastic behavior and microstructural stability of friction stir processed AZ91C alloy

Abstract: Superplastic behavior of a solution treated and friction stir processed (FSP) AZ91C alloy is studied. These studies are conducted in the temperature range of 300–375 °C and strain rates (SRs) in the range of 1 × 10−4–3 × 10−3 s−1. Microstructural stability of the FSP alloy is also studied in comparison to the AZ31, AZ61, and AZ91 alloys processed by various routes. High SR sensitivity in the range of 0.33–0.39 and grain size stability till 350 °C is observed for the FSP alloy. The FSP AZ91C alloy showed better thermal stability in comparison to AZ31 and AZ61 alloys. Kinetics of superplastic deformation of the FSP alloy is found to be slower as compared to AZ31 and AZ61 alloys processed by various routes, which is due to the presence of significant amount of second phase precipitates, such as, β-Mg17(Al,Zn)12, Mg2Si, and Al8Mn5 in the FSP alloy. However, these precipitates contributed for better thermal stability of the microstructure of FSP AZ91C alloy.

Superplasticity, flow and fracture mechanism in an Al–12.7Si–0.7Mg alloy

Abstract: The superplastic behavior of an Al–12.7mass%Si–0.7mass%Mg alloy was investigated under different conditions. Reasonable superplastic elongations were achieved in the fine-grained (9.1 μm) Al–Si–Mg alloy at temperatures ranging from 733 to 793 K at initial strain rates ranging from 1.67×10 –4 to 1.67×10 –3 s −1 . A maximum elongation to failure of 379% was demonstrated with a strain rate sensitivity, m , of 0.52 and an activation energy for flow, Q , of 156.7 KJ/mol at 793 K at an initial strain rate of 1.67×10 –4 s −1 , which is close to the lattice diffusion activation energy of aluminum. The dislocation activity within Al grains indicated that intragranular slip is the accommodation mechanism of grain boundary sliding. EBSD (Electron Backscatter Diffraction) results revealed that most grain boundaries were high angle boundaries and therefore indicated that boundary sliding and grain rotation occurred during deformation. A deformation mechanism map was plotted for the Al–Si–Mg alloy at 793 K and it is shown that the experimental datum points are in excellent agreement with the predictions of the map. Most cavities were formed around silicon particles and the cavity formation mechanism was proposed. The observation on the fracture surface revealed the presence of filaments. The filament quantity or density increased with increasing testing temperature, which can be interpreted by the transition of dislocation viscous glide creep to grain boundary sliding mechanism at elevated temperatures. The formation of filaments was related to the deformation mechanisms and the lattice diffusion at elevated temperatures. The superplastic fracture in the Al–Si–Mg alloy exhibited a diffuse necking and was a pseudo-brittle fracture. The fracture mechanism was intergranular fracture.