Showing papers on "Superplasticity published in 2000"

Superplastic extensibility of nanocrystalline copper at room temperature

Abstract: A bulk nanocrystalline (nc) pure copper with high purity and high density was synthesized by electrodeposition. An extreme extensibility (elongation exceeds 5000%) without a strain hardening effect was observed when the nc copper specimen was rolled at room temperature. Microstructure analysis suggests that the superplastic extensibility of the nc copper originates from a deformation mechanism dominated by grain boundary activities rather than lattice dislocation, which is also supported by tensile creep studies at room temperature. This behavior demonstrates new possibilities for scientific and technological advancements with nc materials.

Hot working of commercial Ti–6Al–4V with an equiaxed α–β microstructure: materials modeling considerations

Abstract: The hot deformation behavior of Ti–6Al–4V with an equiaxed α–β preform microstructure is modeled in the temperature range 750–1100°C and strain rate range 0.0003–100 s−1, for obtaining processing windows and achieving microstructural control during hot working. For this purpose, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain. The map exhibited two domains: (i) the domain in the α–β phase field is identified to represent fine-grained superplasticity and the peak efficiency of power dissipation occurred at about 825°C/0.0003 s−1. At this temperature, the hot ductility exhibited a sharp peak indicating that the superplasticity process is very sensitive to temperature. The α grain size increased exponentially with increase in temperature in this domain and the variation is similar to the increase in the β volume fraction in this alloy. At the temperature of peak ductility, the volume fraction of β is about 20%, suggesting that sliding of α–α interfaces is primarily responsible for superplasticity while the β phase present at the grain boundary triple junctions restricts grain growth. The apparent activation energy estimated in the α–β superplasticity domain is about 330 kJ mol−1, which is much higher than that for self diffusion in α-titanium. (ii) In the β phase field, the alloy exhibits dynamic recrystallization and the variation of grain size with temperature and strain rate could be correlated with the Zener–Hollomon parameter. The apparent activation energy in this domain is estimated to be 210 kJ mol−1, which is close to that for self diffusion in β. At temperatures around the transus, a ductility peak with unusually high ductility has been observed, which has been attributed to the occurrence of transient superplasticity of β in view of its fine grain size. The material exhibited flow instabilities at strain rates higher than about 1 s−1 and these are manifested as adiabatic shear bands in the α–β regime.

Electroplasticity in metals and ceramics

Abstract: The influence of an electric field or corresponding current on the plastic deformation of metals and ceramics is reviewed. Regarding metals, the following are considered: (a) the effects of high density electric current pulse on the flow stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temperatures. The major effect of the current pulses was to reduce the thermal component of the flow stress. This resulted from the combined action of an electron wind force, a decrease in the activation enthalpy for plastic deformation and an increase in the pre-exponential, the last making the largest contribution. Besides giving a reduction in the flow stress during superplastic deformation, an external electric field reduced cavitation and grain growth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the charged surface. In the case of ceramics, the effects of an internal electric field on the plastic deformation of polycrystalline NaCl at 0.28–0.75TM and on the superplasticity of fine-grained oxides (MgO, Al2O3 and ZrO2) at T>0.5TM are considered. Regarding NaCl, at T≤0.5TM an electric field E≥10 kV cm−1 is needed to enhance dislocation mobility in single crystals. However, a field of only 1 kV cm−1 significantly reduced the flow stress in polycrystals, which is concluded to result from an enhancement of cross slip. At T>0.5TM, there occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the fine-grained oxides at T>0.5TM, an internal electric field E≤0.3 kV cm−1 gave an appreciable, reversible, reduction in the flow stress by an enhancement of the rate-controlling diffusion process. Limited work suggests that a field may also retard grain growth and cavitation in ceramics.

Microstructural modeling of metadynamic recrystallization in hot working of IN 718 superalloy

Abstract: The hot deformation behavior of IN 718 superalloy has been characterized in the temperature range 900–1100°C and strain rate range 0.001–1.0 s−1 using compression tests on process annealed material, with a view to obtain a correlation between grain size and the process parameters. At a strain rate of 0.001 s−1, the material exhibits dynamic recrystallization (DRx) at 975°C and superplasticity at 1100°C, while metadynamic recrystallization (MDRx) occurs in the temperature range 950–1100°C and strain rate range 0.01–1.0 s−1. Unlike in the DRx domain, the grain size (d) variation in the MDRx regime could not be correlated with the standard Zener–Hollomon (Z) parameter due to strong thermal effects during cooling after hot deformation. However, it follows an equation of the type d=cexp(−Q/RT), where c, p and R are constants, Q the activation energy for MDRx and T the temperature. The value of p is very low (0.028) and the apparent activation energy is about 275 kJ mole−1, which is very close to that for self-diffusion in pure nickel. The data obtained from several investigators are in agreement with this equation. Such an equation combines the mild dynamic effect in MDRx with a stronger post-deformation cooling effect and may be used for predicting the grain size of IN 718 hot forged in the MDRx regime.

Hot deformation and microstructural damage mechanisms in extra-low interstitial (ELI) grade Ti–6Al–4V

Abstract: The hot deformation behavior of extra-low interstitial (ELI) grade Ti–6Al–4V alloy with Widmanstatten preform microstructure over wide temperature (750–1100oC) and strain rate ranges (0.001–100 s−1) has been studied with the help of processing maps. In the lower temperature and strain rate regime (850–950°C and 0.001–0.1 s−1), globularization of the lamellar structure occurs while at higher temperatures (980–1100oC) the β phase exhibits large-grained superplasticity. The tensile ductility reaches peak values under conditions corresponding to these two processes. A dip in ductility occurs at the β transus and is attributed to a possible nucleation of voids within prior β grains. At lower temperatures and strain rates below about 0.1 s−1, cracking at the prior β grain boundaries occurs under mixed mode conditions. At strain rates higher than 1 s−1 and temperatures lower than about 950oC, the material exhibits a wide regime of flow instabilities. On the basis of these results, a temperature–strain rate window for hot working this material without microstructural defects is identified.

Microstructural and mechanical characterization of a superplastic 6xxx aluminum alloy

Abstract: A new thermomechanical process has been developed which produced a fine grain structure in an Al–Mg–Si–Cu alloy. The alloy under investigation falls within the composition limits of both 6013 and 6111. The refined microstructure has an average grain diameter of approximately 10 μm and an average aspect ratio near 1.6. Superplasticity was investigated using ambient pressure, uniaxial tensile tests and cone tests with backpressure. The refined material exhibits superplasticity above 500°C. Uniaxial tests indicated a strain rate sensitivity of 0.5 at 540°C, where the elongation reached 375% for a flow stress of 680 psi (4.7 MPa). Cone tests revealed excellent overall formability, and the suppression of cavitation with backpressure. The effects of strain on grain size and porosity were determined.

Microstructural evolution and superplasticity of rolled Mg-9Al-1Zn

Abstract: Microstructural evolution and superplasticity of a Mg-9Al-1Zn alloy rolled at 673 K were investigated at 573 K and 1.5×10−3 s−1. The grain size of the as-rolled Mg alloy was 39.5 μm. However, the grain size of the specimen deformed to a true strain of 0.6 was 9.1 μm. The grain refinement was attributed to dynamically continuous recrystallization during an initial stage of tensile test. Stabilization of subgrain boundaries by fine particles and stimulation of continuous recrystallization by prior warm-deformation were not needed to attain dynamically continuous recrystallization in the Mg alloy. As a result of the grain refinement, the rolled Mg alloy exhibited superplastic behavior.

Particle-stimulated nucleation of recrystallization for grain-size control and superplasticity in an Al-Mg-Si-Cu alloy

Abstract: Grain refinement of 6xxx aluminum alloys for superplasticity through particle-stimulated nucleation of recrystallization (PSN) has been a difficult task in the past due to the inhomogeneous nature of the precipitate distributions produced by traditional overaging heat treatment methods. Stretching prior to aging does not alleviate the problem. A new approach has been developed, wherein the interfaces of deformation bands caused by severe rolling were exploited as heterogeneous nucleation sites for precipitates in an Al–Mg–Si–Cu alloy. (US and International patents are pending.) This approach, combined with a two-step low–high heat treatment resulted in a homogeneous distribution of globular precipitates near 1 μm in diameter. In contrast to the globular shape, the plate-shape morphology was observed in the absence of pre-age deformation. Subsequent rolling and recrystallization resulted in a fine, uniform, equiaxed grain structure with an average grain diameter of 10 μm. The grain structure was weakly textured, statically stable, and superplastic above 500°C. A maximum strain rate sensitivity of 0.5 was achieved, with a corresponding maximum elongation of 375%.

Application of superplasticity in commercial magnesium alloy for fabrication of structural components

Abstract: An investigation of the superplastic characteristics of magnesium alloys with several grain sizes revealed that grain boundary sliding took place more easily with grain refinement. The required grain size for high strain rate superplastic forming was estimated to be ∼2 μm. The required grain structure could be obtained by several procedures, hot extrusion with a high extrusion ratio, severe plastic deformation via equal channel angular extrusion, consolidation of machined chip, and/or powder metallurgy processing of rapidly solidified powders, on a laboratory scale. The processing route of hot extrusion was selected in this study. An experimental study of superplastic press forming was conducted for a commercially extruded ZK60 alloy. The fabricated product did not essentially contain macroscopic defects, i.e. cracks or cavities. From an examination of tensile characteristics, it was found that the post-formed alloy exhibited higher strength and higher ductility compared with some conventional cast magnesium alloys, aluminium alloys, and steels. The experimental results support the possibility of using superplastically formed magnesium to produce structural components.

Formation of a submicrocrystalline structure in TiAl and Ti3Al intermetallics by hot working

Abstract: A method based on initiation of dynamic recrystallization (DRX) during hot working has been developed to produce a submicrocrystalline (SMC) structure (d < 1 µm) in massive work-pieces of hard-to-deform materials, like titanium aluminides, The method involves continuous grain refinement due to dynamic recrystallization at a decreasing temperature. A microstructure with a grain size of 0.1 to 0.4 µm and no porosity was produced in different TiAl and Ti3Al based alloys. Partial disordering was detected in a Ti3Al alloy with the SMC structure. The grain refinement hardened the intermetallic alloys at room temperature (RT). In a fully ordered Ti3Al alloy RT ductility increased when the grain size decreased, while the ductility of a partially disordered SMC Ti3Al and TiAl alloys was close to zero.

Equal-channel angular pressing of an Al-6061 metal matrix composite

Abstract: An Al-6061 metal matrix composite, reinforced with 10 vol % Al2O3 particulates, was subjected to equal-channel angular (ECA) pressing at room temperature to a total strain of ∼5 It is shown that the intense plastic straining introduced by ECA pressing reduces the grain size from ∼35 μm to ∼1 μm and this leads to an increase in the microhardness measured at room temperature Inspection revealed some limit cracking of the larger Al2O3 particulates as a consequence of the ECA pressing Tensile testing after ECA pressing gave a maximum ductility of ∼235% at a temperature of 853 K when testing at strain rates from ∼10−4 to ∼10−3 s−1 It is suggested that high strain rate superplasticity is not achieved in this material after ECA pressing due to the presence of relatively large Al2O3 particulates

Influence of scandium on superplastic ductilities in an Al–Mg–Sc alloy

Abstract: Ultrafine grain sizes, of the order of approximately 0.2 ?m, may be introduced into Al-Mg-Sc alloys by subjecting the material to severe plastic deformation through the process of equal-channel angular pressing (ECAP). Experiments were conducted to evaluate the influence of the solution treatment temperature on the ductility of an Al-3% Mg-0.2% Sc alloy after ECAP. The results show the highest ductilities are achieved when the solution treatment temperature is within the narrow range of approximately 878 to about 883 K, immediately below the temperature associated with the onset of partial melting. These high temperatures serve to maximize the amount of scandium in solid solution and this leads, on subsequent heating, to an extensive precipitation of fine secondary Al3Sc particles which inhibit grain growth at the higher temperatures. Conversely, solution treatments at temperatures below approximately 878 K give less Sc in solid solution within the matrix and the precipitation of the Al3Sc particles is then insufficient to retain a uniform ultrafine microstructure.

Morphology and structure of various phases at the bonding interface of Al/steel formed by explosive welding

Abstract: The bonding interface of explosively-welded aluminium and steel in three explosive conditions have been investigated by using scanning electron microscopy, transmission electron microscopy, electron diffraction and electron probe microanalysis methods. The results show that all the interfaces have the shape of waves with curled front formed by process of superplasticity and some discontinuous reacted zones. They consist of amorphous and nano sized crystals and quasi-crystals as well as the compounds such as AlFe, Al2Fe, Al3Fe and Al6Fe with various shapes. The basal steel crystal near the interface has structure of martensite and perlite crystals which are deformed by the process of superplasticity. The size of reacted zone becomes large with increasing amount of explosive charge powder and separation of the driver Al plate from the basal steel plate.