Showing papers on "Superplasticity published in 2019"

Microstructure, mechanical properties and superplasticity of the Al–Cu–Y–Zr alloy

Abstract: The phase composition, mechanical properties, and superplastic deformation behavior of a novel Al-4.7Cu-1.6Y-0.3Zr alloy were analyzed. The precipitation of Al3(Zr,Y) dispersoids was observed during a homogenization treatment. The precipitates have an L12 structure and a mean size of 17 and 19 nm at 540 and 590 °C, respectively. The sheets exhibit a yield strength of 292 MPa, an ultimate tensile strength of 320 MPa, and an elongation of 5.3% after simple thermomechanical treatment and annealing at 100 °C. The Al-4.7Cu-1.6Y-0.3Zr alloy exhibits superplasticity with m > 0.4 and elongation of 300–400% within a temperature range of 550–580 °C and a strain rate range of 1 × 10−4 to 1 × 10−3 s−1.

The mechanism of L12 phase precipitation, microstructure and tensile properties of Al-Mg-Er-Zr alloy

Abstract: Erbium (Er) is a promising element to replace expensive Scandium (Sc) to improve the mechanical strength of aluminum-based alloys due to the formation of nanoscale dispersoids. However, the effect of Er on precipitation behavior and properties of such alloys is not fully investigated. The paper studied the Er effect on precipitation kinetics of the L12 structured phase by hardness measurements and electron microscopy techniques. Compact continuously-formed dispersoids, which contained similar Zirconium (Zr) and Er concentrations, nucleated homogeneously in a grain body and heterogeneously on dislocations and grain boundaries. Fan-shaped aggregations of the L12 structured Zr-rich Al3(Er,Zr) phase discontinuously precipitated near the grain boundaries. It was found that a two-stage treatment mode provided a maximum hardening effect due to high density of continuously-formed precipitates. The effect of Er and the annealing regimes on the precipitation behavior of the Al-3Mg-0.25Er-0.25Zr alloy were discussed. Mechanical properties, corrosion behavior, and superplasticity of the sheets exposed to simple thermo-mechanical treatments were studied.

Microstructural evolution and superplastic behavior of a fine-grained Mg–Gd alloy processed by constrained groove pressing

Abstract: In the current study, microstructural evolution and superplasticity of an extruded Mg–2wt% Gd sheet were studied after the constrained groove pressing (CGP) process. Microstructural observations by scanning electron microscopy and electron backscattered diffraction revealed that after 4 cycles of CGP, a rather homogeneous fine-grained microstructure with an average grain size of 4.3 μm, and a large fraction of high angle grain boundaries was obtained. By performing shear punch tests (SPT) at different temperatures and various shear strain rates, a peak strain rate sensitivity index (m-value) of 0.49 was obtained after 4 cycles of CGP process at 673 K, while peak m-values of 0.31 and 0.36 were obtained for the as-extruded and 2 cycle CGP process conditions, respectively. An m-value of 0.49 and an activation energy of 113 kJ/mol, obtained for the fine-grained material after 4 cycles of CGP, suggest that the dominant deformation mechanism in the superplastic regime is grain boundary sliding (GBS) controlled by grain boundary diffusion.

Superplasticity of fine-grained Mg–xGd alloys processed by multi-directional forging

Abstract: Microstructural development and superplastic behavior of Mg–xGd (x = 1, 2, 3 wt%) alloys were investigated after the multi-directional forging (MDF) process. Microstructural observations of the extruded materials revealed non-homogeneous structures with bimodal grain sizes, consisting of coarse elongated grains resulting from deformation, surrounded by fine grains formed by partial dynamic recrystallization. The grain sizes of the recrystallized regions in the Mg–1Gd, Mg–2Gd and Mg–3Gd alloys were 5.6, 3.9 and 3.1 μm, respectively. After six passes of the MDF process, the bimodal grain structure disappeared as a result of complete dynamic recrystallization and almost homogeneous microstructures with the grain sizes of 2.5, 2.2 and 1.9 μm were obtained in the Mg–1Gd, Mg–2Gd and Mg–3Gd alloys, respectively. The smaller fraction of the recrystallized region in the extruded Mg–3Gd alloy and the smaller grain size in the MDF processed condition imply that more pronounced recrystallization retardation occurs through solute drag mechanism at the highest Gd content. Superplasticity was assessed by measuring the strain rate sensitivity index (m-value), through shear punch testing (SPT) in a temperature range of 623–698 K at various strain rates. After six passes of MDF, peak m-values of 0.45 and 0.46 were obtained at 648 K for Mg–1Gd and Mg–2Gd alloys, respectively, while for the Mg–3Gd alloy the peak m-value of 0.48 was achieved at a lower temperature of 623 K. According to the m-value of about 0.5 and the corresponding activation energies of 101–114 kJ/mol, the governing superplastic deformation mechanism was determined as grain boundary sliding (GBS) controlled by grain boundary diffusion.

Ultra-fine-grained Zn-0.5Mn alloy processed by multi-pass hot extrusion: Grain refinement mechanism and room-temperature superplasticity

Abstract: Ultra-fine-grained Zn-0.5Mn alloy was prepared by multi-pass hot extrusion. Minimum grain size of the Zn-0.5Mn alloy was about 0.35 µm, it exhibited room temperature superplasticity. During multi-pass extrusion, the pinning force of MnZn13 rendered the subgrains stable and restrained grain growth. The ultra-fine grain was finally formed.

Determination of room-temperature superplastic asymmetry and anisotropy of Zn-0.8Ag alloy processed by ECAP

Abstract: Fine-grained Zn-0.8Ag alloy processed by equal channel angular pressing (ECAP) presents elongation over 650% and strain rate sensitivity 0.45 at room temperature. Examination along three orthogonal directions shows minimal superplastic anisotropy both under tension and compression. Measured tension-compression yield stress asymmetry indicates much easier grain boundary sliding under tension than compression.

Effect of Annealing Temperature on Microstructure and Mechanical Properties of a Severe Cold-Rolled FeCoCrNiMn High-Entropy Alloy

Abstract: An ultrafine-structured FeCoCrNiMn high-entropy alloy (HEA) was produced by severe cold rolling (SCR) with a 95 pct reduction, and the effect of annealing temperature over a wide range from 573 K to 1373 K (300 °C to 1100 °C) on the microstructure and mechanical properties of this SCRed HEA was systematically investigated. The results show that the microstructure of the single-phase FCC HEA after SCR is greatly refined and mainly comprises ultrafine crystalline, high-density dislocation cell and networks, which substantially improve the hardness and strength. The SCRed HEA starts to recrystallize at an annealing temperature of 773 K (500 °C) and is fully recrystallized at 973 K (700 °C) after 1 hours. The σ phases with a tetragonal structure form upon annealing at 773 K (500 °C), leading to a further enhancement in hardness and strength for the annealed HEA compared to the SCRed HEA. The average grain size remains below 500 nm when the SCRed HEA is annealed at 923 K (650 °C) for 1 hours, indicating that the ultrafine-structured HEA after SCR and subsequent annealing treatment possesses excellent microstructural thermal stability. The SCRed HEAs annealed at 923 K and 973 K (650 °C and 700 °C) exhibit excellent high strength–ductility combinations due to the formation of the fully recrystallized ultrafine-grained microstructure. The strain-rate sensitivity parameter (m value) with different grain sizes, from ultrafine to coarse, was evaluated by strain-rate jump tests. The m value of the HEA increases monotonically with increasing grain size and annealing temperature. Moreover, high-temperature tensile tests demonstrated that the annealed HEAs exhibit superplastic deformation behavior at 973 K and 1073 K (700 °C and 800 °C), and the formation of a Cr-rich phase is found during high-temperature deformation. It is believed that the plastic deformation facilitates the phase decomposition of fine-grained HEAs and grain boundary sliding is the key mechanism of high-temperature deformation.

Superplasticity of Ti-6Al-4V Titanium Alloy: Microstructure Evolution and Constitutive Modelling

Abstract: Determining a desirable strain rate-temperature range for superplasticity and elongation-to-failure are critical concerns during the prediction of superplastic forming processes in α + β titanium-based alloys. This paper studies the superplastic deformation behaviour and related microstructural evolution of conventionally processed sheets of Ti-6Al-4V alloy in a strain rate range of 10–5–10–2 s–1 and a temperature range of 750–900 °C. Thermo-Calc calculation and microstructural analysis of the as-annealed samples were done in order to determine the α/β ratio and the grain size of the phases prior to the superplastic deformation. The strain rate ranges, which corresponds to the superplastic behaviour with strain rate sensitivity index m ˃ 0.3, are identified by step-by-step decreasing strain rate tests for various temperatures. Results of the uniaxial isothermal tensile tests at a constant strain rate range of 3 × 10−4–3 × 10−3 s−1 and a temperature range of 800–900 °C are presented and discussed. The experimental stress-strain data are utilized to construct constitutive models, with the purpose of predicting the flow stress behaviour of this alloy. The cross-validation approach is used to examine the predictability of the constructed models. The models exhibit excellent approximation and predictability of the flow behaviour of the studied alloy. Strain-induced changes in the grain structure are investigated by scanning electron microscopy and electron backscattered diffraction. Particular attention is paid to the comparison between the deformation behaviour and the microstructural evolution at 825 °C and 875 °C. Maximum elongation-to-failure of 635% and low residual cavitation were observed after a strain of 1.8 at 1 × 10−3 s−1 and 825 °C. This temperature provides 23 ± 4% β phase and a highly stable grain structure of both phases. The optimum deformation temperature obtained for the studied alloy is 825 °C, which is considered a comparatively low deformation temperature for the studied Ti-6Al-4V alloy.

Superplasticity of metastable ultrafine-grained Ti 6242S alloy: Mechanical flow behavior and microstructural evolution

Abstract: Herein we quantitatively clarify the effects of grain size, β fraction, and morphology on the high-temperature deformation behavior of the Ti 6Al 2Sn 4Zr 2Mo–0.1Si alloy. For this purpose, five materials were subjected to high-temperature tensile deformation: the UFG1 specimen (having an equiaxed morphology with dα = 0.78 μm and Vβ = 2.8%), the UFG2 specimen (having an equiaxed morphology with dα = 0.99 μm and Vβ = 24.2%), the FG1 specimen (having an equiaxed morphology with dα = 2.65 μm and Vβ = 11.2%), the FG2 specimen (having an equiaxed morphology with dα = 4.12 μm and Vβ = 11.0%), and the STQ specimen (with an acicular α′ martensite morphology). The UFG1 specimen is produced by hot-rolling of the STQ specimen having an acicular α′ martensite microstructure at 750 °C. The UFG2 specimen is prepared by heat treatment of the UFG1 specimen at 400 °C. The FG1 specimen is as-received Ti 6242S alloy plate, and the FG2 specimen was prepared by heat treatment of the FG1 specimen at 900 °C. The UFG specimens exhibited higher ductility associated with frequent activation of superplasticity than the FG specimens owing to the effect of decreasing grain size. The STQ specimen exhibited higher ductility at 700 °C than the FG specimens. Quantitative analysis of the deformation mode according to internal-variable theory revealed much more grain boundary sliding in the UFG specimens. A comparison of the deformation behavior of the UFG1 and UFG2 specimens revealed excellent superplastic ductility in the UFG2 specimen at higher strain rates (10−3 and 10−2 s−1) and in the UFG1 specimen at lower strain rates (5 × 10−4 and 10−4 s−1). This behavior is ascribed mainly to different accommodation mechanisms during deformation of these specimens; dynamic β precipitation from supersaturated α microstructure occurred in the UFG1 specimen, whereas a decomposition process in which supersaturated β precipitates dissolve into the α phase was enhanced in the UFG2 specimen. In addition, the excess β precipitation observed in the UFG2 specimen led to enhanced α/β grain boundary sliding, resulting in further enhancement of the superplasticity.

Superplasticity in a multi-directionally forged Mg–Li–Zn alloy

Abstract: Superplastic behavior of the Mg–8Li–1Zn alloy, processed by extrusion at 573 K followed by multi-directional forging (MDF) at 423 K, was studied using shear punch testing (SPT) of miniature specimens at various temperatures and strain rates. Microstructural analysis indicated moderate grain refinement, reducing the gran size from 8 μm in the initial as-extruded condition to 4 μm after 8 passes of MDF. Processing by MDF not only refined the grain structure but also provided a more homogeneous microstructure and a larger fraction of high-angle grain boundaries. SPT was performed at shear strain rates in the range of 3.3 × 10−3–1.3 × 10−1 s−1 and temperatures in the range of 498–573 K. The strain rate sensitivity (SRS) index (m-value), obtained from the slope of the central region of the sigmoidal shear stress–shear strain rate curves, was as high as 0.51 for the MDF processed material at 548 K. This is in contrast to the as-extruded material that exhibited a linear behavior with a maximum m-value of only 0.29 at the same temperature. For the MDF processed material, the activation energy of 61 kJ mol−1, which is close to 67 kJ mol−1 for the grain boundary diffusion in β-Li, and the high m-value of 0.51 suggest that the prevailing deformation mechanism is grain boundary sliding (GBS) controlled by grain boundary diffusion.

Microstructural evolution and superplastic deformation mechanisms of as-rolled 2A97 alloy at low-temperature

Abstract: The microstructure and texture evolutions of 2A97 Al Li alloy with initial unrecrystallized structure during superplastic deformation were investigated. A total elongation of 300% was obtained at 390 °C at an initial strain rate of 3 × 10−3 s−1. The initial banded structure gradually transformed into a recrystallized structure, characterized by equiaxed grains, random boundary misorientation distribution and a weak texture at high strains. The true strain-stress curve exhibited three stages: work hardening (stage I), steady-state (stage II), and deformation instability regions (stage III). The corresponding deformation mechanisms varied at different stages. The strain rate sensitivity index (m) remained constant of 0.35 and the texture density increased with strain at stage I, where the dislocation creep and subgrain rotation were the dominant mechanisms. During stage II, the m-value increased to 0.44, and the texture density decreased drastically with strain. The combination of grain boundary sliding (GBS) and dislocation creeep could explain the behavior of the alloy. Grain growth and cavitation resulted in the decrease in m-value at stage III, where GBS was the main deformation mechanism.

Superplastic Deformation and Dynamic Recrystallization of a Novel Disc Superalloy GH4151

Abstract: The superplastic deformation of a hot-extruded GH4151 billet was investigated by means of tensile tests with the strain rates of 10-4 s-1, 5 × 10-4 s-1 and 10-3 s-1 and at temperatures at 1060 °C, 1080 °C and 1100 °C. The superplastic deformation of the GH4151 alloy was reported here for the first time. The results reveal that the uniform fine-grained GH4151 alloy exhibited an excellent superplasticity and high strain rate sensitivity (exceeded 0.5) under all experimental conditions. It was found that the increase of strain rate resulted in an increased average activation energy for superplastic deformation. A maximum elongation of 760.4% was determined at a temperature of 1080 °C and strain rate of 10-3 s-1. The average activation energy under different conditions suggested that the superplastic deformation with 1 × 10-4 s-1 in this experiment is mainly deemed as the grain boundary sliding controlled by grain boundary diffusion. However, with a higher stain rate of 5 × 10-4 s-1 and 1 × 10-3 s-1, the superplastic deformation is considered to be grain boundary sliding controlled by lattice diffusion. Based on the systematically microstructural examination using optical microscope (OM), SEM, electron backscatter diffraction (EBSD) and TEM techniques, the failure and dynamic recrystallization (DRX) nucleation mechanisms were proposed. The dominant nucleation mechanism of dynamic recrystallization (DRX) is the bulging of original grain boundaries, which is the typical feature of discontinuous dynamic recrystallization (DDRX), and continuous dynamic recrystallization (CDRX) is merely an assistant mechanism of DRX. The main contributions of DRX on superplasticity elongation were derived from its grain refinement process.

Precipitation behavior and high strain rate superplasticity in a novel fine-grained aluminum based alloy

Abstract: The development of aluminum alloys with superior mechanical properties and high strain rate superplasticity is an important prerequisite for advancing applications of superplastic blow-forming technology. This study focuses on the effect of Ce and Fe additions on the microstructural parameters and tensile properties of a superplastic Al-4.8%Mg-0.6%Mn-0.15%Cr (AA5083-type) alloy. The studied alloy exhibits a bimodal particle size distribution with coarse crystallization origin inclusions and fine secondary precipitates. Both the coarse and fine particles lead to grain refinement. The coarse Ce-, Fe-, and Mn-rich intermetallic particles of crystallization origin provide evidence of a particle-stimulated nucleation effect. The semi-coherent Mn-enriched compact-shaped dispersoids with an Ashby-Brown contrast, a quasicrystalline structure, and a mean size of 38 nm are precipitated after low-temperature homogenization and exhibit a strong Zener pinning effect. The proposed thermomechanical treatment results in the development of a grain size of 4 μm and an ultimate tensile strength of 340 MPa in the recrystallized sheet. The superplastic deformation behavior in a strain rate range of 1 × 10−2 to 1 × 10−1 s−1 and the associated strain-induced changes in the grain structure and mechanical properties of the developed alloy are presented and discussed.

The Effect of Isothermal Multi-Directional Forging on the Grain Structure, Superplasticity, and Mechanical Properties of the Conventional Al–Mg-Based Alloy

Abstract: The current study observed a grain structure evolution in the central part and periphery of the sample of an Al–Mg–Mn-based alloy during isothermal multidirectional forging (IMF) at 350 °C with a cumulative strain of 2.1–6.3 and a strain per pass of 0.7. A bimodal grain size distribution with areas of fine and coarse grains was observed after IMF and subsequent annealing. The grain structure, mechanical properties, and superplastic behavior of the samples subjected to IMF with a cumulative strain of 6.3 and the samples exposed to IMF with subsequent cold rolling were compared to the samples exposed to a simple thermo-mechanical treatment. The micro-shear bands were formed inside original grains after the first three passes. The fraction of recrystallized grains increased and the mean size decreased with an increasing cumulative strain from 2.1 to 6.3. Significant improvements of mechanical properties and superplasticity were observed due to the formation of a homogenous fine grain structure 4.8 µm in size after treatment including IMF and subsequent cold rolling.

Thermoplastic deformation behavior of a Fe-based bulk metallic glass within the supercooled liquid region

Abstract: Fe41Cr15Co7Mo14C12B9Y2 (Fe-B9) bulk metallic glass (BMG) rods with high glass forming ability and large supercooled liquid (SCL) region were fabricated by arc melting and suction casting. The amorphous state of these Fe-B9 BMG rods was ascertained by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The thermoplastic deformation behavior of these BMG rods was studied by using the hot compression test at different temperatures (873 K, 883 K, 893 K, and 903 K in the SCL region) and strain rates (1 × 10−3–5 × 10−2 s−1). The results of the hot compression test reveal that the flow stress of Fe-B9 BMG reduces systematically with increasing temperature and decreasing strain rate. Strain sensitivity exponent (m) values of the Fe-B9 BMG were calculated to be about 0.36–0.59 in the SCL region, indicating that Fe-B9 BMG possesses superplasticity. Overall, the optimum working conditions of thermoplastic forming for Fe-B9 BMG can be achieved by compressively deforming the sample with a constant strain rate of 2.5 × 10−3 s−1 at a temperature from 873 to 883 K.

The Deformation Characteristics, Fracture Behavior and Strengthening-Toughening Mechanisms of Laminated Metal Composites: A Review

Abstract: Multilayer metal composites have great application prospects in automobiles, ships, aircraft and other manufacturing industries, which reveal their superior strength, toughness, ductility, fatigue lifetime, superplasticity and formability. This paper presents the various mechanical properties, deformation characteristics and strengthening–toughening mechanisms of laminated metal matrix composites during the loading and deformation process, and that super-high mechanical properties can be obtained by adjusting the fabrication process and structure parameters. In the macroscale, the interface bonding status and layer thickness can effectively affect the fracture, impact toughness and tensile fracture elongation of laminated metal matrix composites, and the ductility and toughness cannot be fitting to the rule of mixture (ROM). However, the elastic properties, yield strength and ultimate strength basically follow the rule of mixture. In the microscale, the mechanical properties, deformation characteristics, fracture behavior and toughening mechanisms of laminated composites reveal the obvious size effect.