High Strain Rate Superplasticity Research Articles

Superplastic behaviour of Al-Mg-Zn-Zr-Sc-based alloys at high strain rates

Superplastic deformation behaviour of two aluminium-based AA7XXX type alloys with Sc and Zr additives distinguished by the presence and absence of coarse eutectic Al9FeNi particles are compared. An alloy with Al9FeNi particles exhibits high strain rate superplasticity at constant strain rate range of 5×10−3–8×10−2s−1 with elongation of 915% due to extensive dynamic recrystallisation. Effective activation energy of superplastic deformation, surface relief of deformed samples, and grain structure evolution are analysed. Particle-depleted zones are found in the alloy with Al9FeNi particles at strains higher than 1.25.

Mechanism of Low-Temperature Superplastic Deformation in Aluminum Alloys Containing a Dispersion of Nanoscale Al<sub>3</sub>(Sc,Zr) Particles

The ultrafine grained (UFG) structure with an average size of ∼0.8 μm was produced in an Al-Li-Mg-Sc alloy by equal-channel angular extrusion (ECAE) at 325oC with a total strain of ~16. Superplastic behavior was examined in the temperature range 150-250oC at strain rates ranging from 10-5 to 10-2 s-1. A maximum elongation-to-failure of 440% was recorded at 175oC (~0.5 Tm, where Tm is the melting point) and a strain rate of 2.8×10-5 s-1 with the corresponded strain rate sensitivity coefficient of 0.32. Mechanisms of low-temperature superplasticity (LTSP) and high-strain-rate superplasticity (HTSP) are essentially the same. The difference between superplastic behaviors at low and high temperatures is attributed to applied stress.

Effect of Sn Addition on Superplastic Properties of Zn-22mass%Al Eutectoid Alloy

Superplastic behavior of a Zn 22 mass % Al eutectoid alloy (SPZ) with small addition of Sn (SPZSn) was investigated. Granular grain size of about 0.3 μm was obtained by water quench after annealing SPZ and SPZ05Sn (addition of 0.05 mass % Sn into the SPZ) at 653 K for 2 h. The fundamental microstructure of the SPZ05Sn was similar to that of the SPZ, but, microstructure observation by STEM showed additive Sn was present at the α’ grain boundary in the SPZ05Sn. Excellent high strain rate superplasticity was achieved in the SPZ05Sn, with elongation of more than 1300 % at 523 K at strain rate of 10-1 S-1. Furthermore, large elongation of about 1100 % was recorded at 473 K at strain rate of 10-1 S-1. The large elongation and high strain rate sensitivity value of the SPZ05Sn tend to shift to higher strain rate region as compared to those of the SPZ. It was considered that the small addition of Sn into the SPZ effectively suppressed the grain growth of α and β phase during the superplastic deformation, because granular grains less than 2 μm is maintained after superplastic deformation at 523 K.

Effect of different processes on lamellar-free ultrafine grain formation, room temperature superplasticity and fracture mode of Zn–22Al alloy

The Zn–22 wt.%Al was processed using five different selected routes including thermal, thermo-mechanical and severe plastic deformation (SPD) techniques in order to produce ultrafine grained (UFG) microstructure for achieving room temperature (RT) and high strain rate (HSR) superplasticity, and to compare the same set of the results. After all routes, the microstructural evolutions and RT uniaxial tensile tests at different strain rates ranging from 1 × 10−3 to 1 × 100 s−1 were investigated. The smallest grain size was achieved to be 300 nm after the thermo-mechanical process including conventional hot and cold rolling steps. However, the lamellar microstructure of the alloy was not completely eliminated during that process. On the other hand, equal-channel angular pressing (ECAP) as one of the SPD techniques decreased the grain size down to 400 nm with a lamellar-free microstructure. The maximum elongation value was achieved to be 330% with the sample tested at 1 × 10−3 s−1 after a thermo-mechanical process including conventional hot and cold rolling, and an aging stage between these rolling steps. It was found that lamellar-free microstructure causes higher superplastic elongation even if it has higher grain size compared to the sample having partially lamellar structure.

High-strain rate superplasticity of Inconel 718 through grain refinement by high-pressure torsion

A Ni-based superalloy (Inconel 718) was successfully processed by high-pressure torsion (HPT) under a pressure up to 6GPa at room temperature. The grain size after the HPT processing was reduced to ~100nm. Tensile testing at 1073K led to the advent of superplasticity with a maximum elongation of 1190% at a strain rate of 2×10−2s−1 which is well in the range of high-strain-rate superplasticity. This superplastic flow was accompanied by a significant decrease in the flow stress.

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

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 2.0µm 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 50nm to ∼200nm. 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 10−4s−1, and 350% elongation was achieved at a high strain rate of 10−2s−1. Grain boundary sliding (GBS) was found to be the main deformation mechanism in region-II as the optimum superplastic region.

High strain-rate superplasticity of fine- and ultrafine-grained AA5083 aluminum alloy at intermediate temperatures

Abstract Superplastic behavior of fine and ultra fine-grained AA5083 Al alloy was examined using the shear punch test. To achieve fine- and ultra fine-grained microstructures, a relatively new severe plastic deformation (SPD) process, namely Double Equal Channel Lateral Extrusion (DECLE) was employed. The strain rate sensitivity indices (m) of samples were evaluated after 1, 2, 4, and 6 passes for shear strain rates in the range of 3 × 10− 3 to 3 × 10− 1 s− 1 and temperatures in the range of 573 to 673 K. For microstructural observations, TEM images together with the corresponding SAED patterns were prepared and utilized. A considerable increase in the m-value was observed after the first pass of the operation for all testing temperatures. The best condition for achieving a good superplasticity for the alloy was found to be a single pass DECLE at 673 K in the strain rate range of 10− 2 to 10− 1 s− 1. This process condition resulted in an m-value of 0.43, indicative of a high strain rate superplastic deformation behavior. Further passes of the SPD process did not show any sign of superplasticity until the last pass of the operation, during which the m-value slightly increased, compared with the previous pass.

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

In this study, a fine-grained Mg–Y–Nd alloy with an average grain size of ~1.3μm and small second phases of ~280nm 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 683–758K and the strain rate ranges from 2×10−2 to 4×10−4s−1. Due to the fine-grained and stable microstructure, excellent high strain rate superplasticity (HSRS) of 900% was achieved at 758K and 2×10−2s−1, and the maximum elongation of 967% was obtained at 733K and 3×10−3s−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.

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

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 423K, 473K and 573K using strain rates from 1×10−1s−1 to 1×10−4s−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 573K using a strain rate of 1×10−4s−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 473K and 573K, respectively, using a strain rate of 1×10−2s−1. The alloy exhibited low-temperature superplasticity (LTSP) with maximum elongations of 660% and 760% at a temperature of 423K and using strain rates of 1×10−3s−1 and 1×10−4s−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 β-particles, and the formation of β-phase filaments.

High strain rate superplasticity in oxide ceramics

Oxide ceramic is a structural material with excellent mechanical properties and wide applications. Due to its inbeing brittleness, superplastic forming provides a feasible approach to process the structural oxide ceramics. However, low strain rate is a serious limitation on superplasticity applications of the oxide ceramics in the future. This paper reviewed experimental results and superplastic models in some recent reports, and focused on the major factors limiting the deformation strain rate, such as: Grain size, dynamic grain growth, grain boundary diffusivity, chemical bonding state, grain boundary state, cavity nucleation and growth. Some existing basic and new approaches to achieving high strain rate superplasticity in oxide ceramics were summarized. The superplastic behaviors and enhancement mechanisms were also discussed.

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

Homogenization treatment and equal channel angular pressing was applied to Mg–Y–Nd–Zr alloy cast ingot. Uniform grains with a grain size of ~1.5μm 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, 1×10−1s−1 and 475°C, 2×10−2s−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, 3×10−3s−1. They were attributed to the thermal stability of uniform fine-grain microstructure and the coordination function of secondary-phase.

Examining the mechanical properties and superplastic behaviour in an Al-Mg-Sc alloy after processing by HPT

Experiments were conducted using the solution treated Al-3 % Mg-0.2 % Sc alloy in order to examine the mechanical properties and superplastic behaviour after processing by high-pressure torsion. Hardness values of ~200 Hv were detected in the edge of the samples after 10 turns of HPT, although the microhardness distribution was inhomogeneous along the diameter of the discs. Excellent superplastic properties were achieved in this Al alloy after HPT processing and further tensile testing at temperatures over the range from 523 to 673 K, demonstrating good agreement with the theoretical model for grain boundary sliding. High strain rate superplasticity was consistently observed at temperatures from 573 to 623 K, with elongations to failure up to ~800 % at 573 K and at a strain rate of 1.4×10-2 s-1. Further analysis of the experimental data revealed that the stress exponent at the testing conditions displaying elongations >400 % was ~2 thereby demonstrating good agreement with the theoretical model for grain boundary sliding. The calculated activation energies lie in the range from ~98 to ~118 kJ mol-1 and are similar to the value obtained for the same alloy after ECAP which is within the range for grain boundary diffusion in pure Al and interdiffusion in Al-Mg alloys

Высокоскоростная сверхпластичность алюминиевого сплава 1570С с бимодальной структурой, полученной равноканальным угловым прессованием и прокаткой

Высокоскоростная сверхпластичность алюминиевого сплава 1570С с бимодальной структурой, полученной равноканальным угловым прессованием и прокаткой

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 room temperature superplasticity in Zn–5Al alloy at high strain rates by equal-channel angular extrusion

Multi-pass equal-channel angular extrusion/pressing (ECAE/P) was applied to the eutectic Zn–5Al alloy to achieve high strain-rate (HSR) superplasticity in that alloy at room temperature (RT) by producing ultrafine-grained (UFG) microstructure. ECAE processing transformed the coarse-grained lamellar/spherical microstructure into a unique bimodal structure having equiaxed Zn-rich η-phase with a mean grain size of 540nm and spherical Al-rich α-phase with an average grain size of 110nm. The α-phase particles accumulated mainly along the η-phase boundaries. This unique microstructure brought about an extraordinary improvement in HSR superplasticity of the alloy even at RT. While the strength values decreased after ECAE, the elongation to failure increased substantially. The maximum elongation was 520% at the strain rate of 10−3s−1, still high elongation of about 400% was achieved at a high strain rate of 10−2s−1. This extraordinary improvement in HSR superplasticity of Zn–5Al alloy was attributed to the morphologically unique bimodal microstructure in UFG regime formed after ECAE. The grain boundary sliding (GBS) was found to be the main deformation mechanism for this alloy in superplastic regime.

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.

Influence of processing speed on microstructures and mechanical properties of friction stir processed Mg–Y–Nd–Zr casting alloy

Mg–3·99Y–3·81Nd–0·53Zr (WE43) casting alloy was subjected to friction stir processing (FSP) at a constant rotation rate of 800 rev min−1 and various processing speeds of 30, 60, 90 and 120 mm min−1, and microstructures and mechanical properties of the experimental materials were investigated. With the processing speeds increasing, the average grain size first decreases and then increases, and the finest microstructure (∼2 μm) is prepared at the processing speed of 60 mm min−1. Owing to much finer microstructure, the mechanical properties of the WE43 alloy after FSP are significantly improved. The maximum tensile strength and elongation of the FSP WE43 alloy obtained at 60 mm min−1 are 290 MPa and 17·2% respectively. The fracture morphology shows that fine dimples and tearing edges can be observed on the FSP specimens. In addition, high temperature tensile tests show that the FSP WE43 alloys exhibit excellent high strain rate superplasticity, with a large elongation of 631% at 748 K at a strain rate of 2×10−2 s−1.

Microstructural evolution and mechanical properties in a Zn–Al eutectoid alloy processed by high-pressure torsion

Experiments were conducted to examine the evolution of microstructure and mechanical properties in a Zn–22% Al eutectoid alloy processed by high-pressure torsion (HPT). Measurements of the Vickers microhardness revealed significant weakening in the alloy after HPT, and microstructural analysis showed that the initial duplex structure, consisting of equiaxed grains and lamellae, was retained at the disk centers after HPT, whereas equiaxed fine grains were observed at the disk edges. Direct evidence is presented for a transformation of the lamellae into equiaxed very fine grains and subsequent dynamic recrystallization in the early stages of HPT. The reduction of the lamellar structure and the loss of Zn precipitates account for the weakening in the alloy in HPT processing. Excellent high strain rate superplasticity was recorded after HPT, with elongations up to ∼1800% at 473K at a strain rate of 10−1s−1. The experiments show that the maximum elongations are displaced to faster strain rates with increasing numbers of HPT turns.

A high-strength aluminium-based alloy with advanced superplasticity

The structure, mechanical properties and superplastic behaviour of a high-strength Al–(3.5–4.5)Zn–(3.5–4.5)Mg–(0.7–0.9)Cu–(1.0–3.0)Ni–(0.25–0.30)Zr (wt.%) alloy are investigated. A 570MPa YS and a 610MPa UTS are achieved after T6 heat treatment. The alloy exhibits superplasticity in the temperature ranged from 380 to 480°C at constant strain rates from 2×10−3 to 1×10−1s−1 with a strain rate sensitivity index greater than 0.5 and an elongation of 400–800% due to the stable ultrafine structure with 1.8μm grain size. An EBSD study of the deformation behaviour demonstrated substantial grain refinement resulting from dynamic recrystallisation. High strain rate superplasticity is available due to zirconium addition that provides Zener pinning of grain boundaries, and nickel addition that forms Al3Ni eutectic particles which improved superplasticity due to grain refinement by particle stimulated nucleation.

High Strain Rate Superplasticity and Microstructure Evolution in a Coarse-Grained Mg-Gd-Y-Zr Rolled Sheet

Superplasticity at high deformation rates is desirable in order to make superplastic forming more practical. High strain rate superplastic behavior and microstructure of the rolled Mg-Gd-Y-Zr alloy sheet were investigated. For the purposes, tensile tests at the strain rate of 0.01 s-1were conducted, which revealed that the sheet exhibited elongations of 180%~266%. Post-deforming microstructures were characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, while crystallographic orientation information was obtained from macro-texture analysis. The results show that the high strain rate superplasticity was attributed to class-I creep accommodated by dynamic recrystallization. It is suggested from microstructural analysis results that the interaction between second phases and dislocation facilitated dynamic recrystallization. The macro-texture at the strain of 0.8 still exhibited some characteristics of the crystal rotation arising from dislocation slip despite the occurrence of DRX.