Superior Superplastic Properties Research Articles

Superplastic deformation behavior of lamellar microstructure in a hydrogenated friction stir welded Ti-6Al-4V joint

It is rather difficult for Ti alloy welds to be superplastic formed due to their low superplasticity and high flow stress. In this study, the Ti-6Al-4V alloy after hydrogenation with 0.2 wt% hydrogen was subjected to friction stir welding (FSW). The superplastic behavior of the FSW joint was investigated. The stir zone with a fine lamellar microstructure exhibited a largest elongation of 660% at 825 °C and 3 × 10−3 s−1. Besides, the stir zone showed a similar superplasticity of close to 600% to the base material at 825 °C, 3 × 10−4−1 × 10−3 s−1. It is the first report on the superplasticity of FSW Ti alloy joints with hydrogenation. Compared to the FSW Ti-6Al-4V alloy joint without hydrogenation, the hydrogenated FSW joint showed superior superplastic properties, including a high elongation, less than a half flow stress and lower superplastic temperature. These superior superplastic properties were mainly attributed to the globularization of the lamellar microstructure and the high fraction of β phase induced by the hydrogen element. This study provides an effective way to reduce the difficulty of practical superplastic forming for Ti alloy welds, although a dehydrogenation process is still needed to avoid hydrogen embrittlement.

Principles of achieving superior superplastic properties in intermetallic alloys based on γ-TiAl+α2-Ti3Al

Superplastic behavior of the Ti-43.7Al-4.2Nb-0.5Mo-0.2B-0.2C alloy (at. %) in a fine grained condition consisted of ultrafine grains and remnant lamellar colonies has been investigated. The fine grained condition was produced via two-stage hot forging with decreasing the forging temperature in the temperature range of T = 1200-950 °C. The produced material was used for preparation of specimens for tensile testing. The tensile tests were performed at T = 800–1000 °C and ε′∼10−4-10−3 s−1 in air without any protection against oxidation. The tests showed incredible superplastic elongations (δ = 1270–2860%) at T = 900–1000 °C and enhanced values of the strain rate sensitivity coefficient, m > 0.3 at ε′∼10−4-10−3 s−1. The obtained superplastic properties and microstructural examination of the tensile strained specimens suggest that the grain boundary sliding was the predominant deformation mechanism during superplastic flow. Superior superplastic properties were attained owing to unusually slow dynamic grain growth during superplastic flow. This was supported by a high content of the α2-Ti3Al phase, high alloying with niobium and dynamic recrystallization/globularization during superplastic flow leading to transformation of remnant lamellar colonies into the fine grains. The activation energy was defined as Q ≈310 kJ/mol suggesting that the predominant deformation mechanism during superplastic flow was grain boundary sliding controlled by volume diffusion of aluminum and titanium in γ-TiAl and α2-Ti3Al. On basis of the performed study, the principles of achieving superior superplastic properties (δ>1000%) in intermetallic alloys based on γ-TiAl+α2-Ti3Al have been proposed.

Factors influencing superplasticity in the Ti-6Al-4V alloy processed by high-pressure torsion

Two different initial microstructures, martensitic and lamellar, were developed in a Ti-6Al-4V alloy to examine their effect on the high temperature mechanical properties and superplasticity after high-pressure torsion (HPT). Significant grain refinement was achieved in both conditions with grain sizes after HPT processing of ~30 and ~40 nm, respectively. The nanocrystalline alloy in both conditions was subjected to mechanical testing at 923–1073 K using strain rates in the range from 10−3 to 10−1 s−1. The martensitic and lamellar alloys exhibited excellent ductility at these high temperatures including superplastic elongations at 973 K with maximum elongations of 815% and 690%, respectively. The fcc phase was stable at elevated temperatures in the martensitic alloy and the results suggests the fcc phase may contribute to the superior superplastic properties of the martensitic alloy.

Achieving superior superplastic properties in fine grained intermetallic alloys based on γ-TiAl + α2‑Ti3Al

Achieving superior superplastic properties in fine grained intermetallic alloys based on γ-TiAl + α2‑Ti3Al

Thermal stability and superplastic behaviour of an Al-Mg-Sc alloy processed by ECAP and HPT at different temperatures

An Al-3%Mg-0.2%Sc alloy was processed by ECAP and HPT at different temperatures. Afterwards, samples subjected to 10 turns of HPT at 300 and 450 K, 8 passes of ECAP at 300 K and 10 passes of ECAP at 600 K were annealed for 1 hour at 523 K and their mechanical properties and microstructure were examined using microhardness measurements and EBSD analysis. In addition, tensile specimens with similar dimensions were machined from the HPT and ECAP-processed materials and further tensile tested at 523 K. The results demonstrate that the Al alloy processed by HPT at 450 K exhibits higher microhardness values (~138 Hv) and a smaller average grain size (~0.28 µm) after annealing at 523 K among all SPD processing conditions. Accordingly, the material subjected to HPT at an elevated temperature displays superior superplastic properties such that an elongation of ~1020 % was attained after testing at 523 K at 1.0 × 10−3 s−1. Furthermore, detailed EBSD analysis revealed a significant fraction of low-angle grain boundaries (LAGBs > 33 %) in the ECAP-processed material after annealing which may be responsible for the inferior superplastic behaviour by comparison with the HPT-processed samples (LAGBs < 13 %) tested at 523 K.

Processing, Superplastic Properties and Friction Stir Welding of Fine-Grained AZ31, AZ91, AE42 and QE22 Magnesium Alloys

The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und QE22 magnesium alloys. The optimal over-aging temperature was determined to be 300 °C in the case of AZ91 and AE42 alloys and 350 °C for QE22 alloy. After optimized TMT, the average grain sizes were 13.5 µm (AE42), 11.1 µm (AZ91) and 1.9 µm (QE22). The QE22 alloy exhibited the superior superplastic properties, with maximum elongation to failure 750 % and strain rate sensitivity parameter m=0.73. The Friction Stir Welding showed that the original base material grain structure of the alloys AZ31 and AZ91 replaced by ultrafine grains in the stir zone. The purpose of the present paper is to present the results of the grain refinement in magnesium alloys by thermo mechanical treatment and stir welding.

Superplasticity and superplastic forming of Mg–Al–Zn alloy sheets fabricated by strip casting method

Superplastic properties and formability of the AZ31 sheet processed by strip casting and subsequent warm rolling were examined. Microstructure of the AZ31 sheet with thickness of 1.3 mm was uniform and composed of equiaxed grains with an average size of 6.6 μm. The sheet exhibited excellent superplasticity with a maximum elongation of 800% at 673 K and 2 × 10−4 s−1. A 250 mm × 250 mm size panel with complicated embossing patterns could be successfully formed into near net shape by gas pressure against a single female die. Small grain size and slow grain growth during deformation resulted in the superior superplastic properties at 673 K. The governing deformation mechanisms in any given strain-rate and temperature ranges could be predicted by the deformation mechanism maps for Mg–Al–Zn alloys.

Achieving high strain rate superplasticity in an Al–Li–Mg alloy through equal channel angular extrusion

An Al–Li–Mg–Sc–Zr alloy was fabricated by an ingot metallurgy technique and subjected to intense plastic straining through equal channel angular extrusion at three different temperatures, 240, 325 and 400°C. The superplastic properties and microstructure evolution of the alloy were examined in tension in the temperature interval 250–500°C at strain rates ranging from 1·4 × 10−5 to 1·4 s−1. Superior superplastic properties (elongation to failure of 3000% with a corresponding strain rate sensitivity coefficient m of ∼0·6) were attained at 450°C and ɛ=1·4 × 10−2 s−1 in samples processed at 400°C to a total strain of ∼16. The alloy in this state had an average grain size of ∼2·6 μm and the recrystallisation fraction was about 90%. It was shown that the highest superplastic ductility appears in samples with more uniform microstructure containing the highest portion of high angle boundaries. It was established that the uniformity of structure and its stability under superplastic deformation is more important for achieving superior elongation to failure than the grain size if the latter varies from 0·5 to 2·6 μm.

Dynamic Recrystallization and Grain Growth in a ZK60 Magnesium Alloy Sheet Produced by Isothermal Rolling

A ZK60 magnesium alloy was subjected to isothermal rolling (IR) at 275 and 300°C. This processing resulted in grain refinement through dynamic recrystallization (DRX) at both temperatures. The recrystallized volume fractions of 82 and 95% and average sizes of fine grains of 2.5 and 3.7 µm were achieved after IR at 275 and 300°C, respectively. It was shown that the ultrafine-grained structure produced by DRX at 300°C exhibited higher stability under following static and dynamic annealing than that produced at 275°C. This fact was attributed with the formation of a less constrained DRX structure at higher temperature of IR. As a result, the sheet produced from the ZK60 alloy at 300°C showed superior superplastic properties. Conversely, it was not feasible to enhance the superplastic properties in the ultrafine-grained alloy produced at 275°C because significant grain growth occurred during further processing of the as-rolled alloy.

Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 µm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10−5 to 1.4 s−1 in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10−2 s−1. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10−2 s−1 with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10−1 s−1, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.

High strain rate superplasticity in a commercial Al–Mg–Sc alloy

It was shown that an Al–5.7%Mg–0.32%Sc–0.3%Mn alloy subjected to severe plastic deformation through equal-channel angular extrusion exhibits superior superplastic properties in the temperature range of 250–500 °C at strain rates ranging from 1.4 × 10 −5 to 1.4 s −1 with a maximum elongation-to-failure of 2000% recorded at 450 °C and an initial strain rate of 5.6 × 10 −2 s −1.

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10-6 to 2.8 × 10-2 s-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ϵ = 2.8 × 10-4 s-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.

High Strain Rate Superplasticity in an Al-Li-Mg Alloy Subjected to Equal-Channel Angular Extrusion

The superplastic behavior of an Al–4.1%Mg–2.0%Li–0.16%Sc–0.07%Zr alloy (1421 Al) subjected to intense plastic straining by equalchannel angular extrusion (ECAE) was studied in the temperature interval 250–450 ◦ C at strain rates ranging from 1.4 × 10 −5 to 1.4 s −1 . The grain size after ECAE was about 0.8 µm and the fraction of high angle boundaries was about 80 pct. The highest elongation of 1850% without failure appeared at a temperature of 400 ◦ C and initial strain rate of 1.4 × 10 −2 s −1 with corresponding coefficient of the strain rate sensitivity of 0.6. It was shown that the ECAE processed 1421 Al exhibits superior superplastic properties in the temperature range 300–450 ◦ C with the strain rate sensitivity higher than 0.4. Microstructural evolution and cavitation during high strain rate superplastic deformation were examined .

High strain rate superplasticity in the fine-grained duplex stainless steel Fe-22Cr-5Ni-3Mo-0.3N

The fine-grained duplex stainless steel Fe-22Cr-5Ni-3Mo-0.3N consisting of α- and γ-Fe(Cr,Ni,Mo) solid solutions exhibits structural superplasticity at deformation temperatures of 900 to 1050°C. The equiaxed microstructure with an average grain size of d α,γ = 3 μm was produced by thermomechanical processing. This steel shows also superior superplastic properties at high strain rates up to e 5.10 -2 s -1 . Maximum strain rate exponents of m = 0.5 and elongations to failure of more than 800% were achieved. The superplastic deformability (m > 0.3) of this steel in a wide strain rate range enables near net shape deep drawing or blow forming of parts with complex shape applying low flow stresses. A deformation model is presented to describe the superplastic behaviour at high strain rates. Grain and interphase boundary sliding is accommodated by sequential steps of dislocation glide and climb. The maximum m-value of about 0.5 and an activation energy of 260 kJ/mol, which is comparable to that of self diffusion of iron in γ-Fe (270 kJ/mol), and high dislocation densities indicate that dislocation climb in the slightly solid solution hardened γ-Fe phase (solid solution class II type of material) is the rate controlling step for superplastic flow.

Dislocation-creep-controlled superplasticity at high strain rates in the ultrafine-grained quasi-eutectoid Ti10Co4Al alloy

The ultrafine-grained two-phase Ti10Co4Al alloy, consisting of α-Ti(Al) solid solution with a fine dispersion of 23 vol.% of the intermetallic Ti 2Co compound, exhibits superior superplastic properties at relatively low deformation temperatures between 650 and 750 °C and high strain rates up to ε ̇ ≈ 5 × 10 −2, s −1 . Maximum m values of about 0.5 and elongations of more than 1000% were achieved. The activation energy of the rate-controlling deformation mechanism at high strain rates ( ε ̇ ⩾ 10 −3, s −1 ) is in good agreement with that reported for lattice self-diffusion of titanium in α-Ti. A grain size exponent of about p = 2 was determined at high strain rates which decreases in the regime of power-law creep. A deformation model is presented to explain the superplastic behaviour of this alloy at high strain rates. This model, proposed by Fukuyo et al., is based on grain boundary sliding, accommodated by sequential steps of dislocation glide and climb. A general flow equation for dislocation-creep-controlled superplasticity involving dislocation glide competing with climb processes is discussed. In the slightly solid-solution-hardened α-Ti(Co, Al) matrix (solid solution class II) dislocation climb becomes the rate-controlling step. The maximum m value, the grain size exponent and the activation energy for superplastic flow of Ti10Co4Al are related to the predictions of solid solution class II alloys in the above model.