Showing papers on "Superplasticity published in 1999"

Low-temperature superplasticity in nanostructured nickel and metal alloys

Abstract: Superplasticity — the ability of a material to sustain large plastic deformation — has been demonstrated in a number of metallic, intermetallic and ceramic systems. Conditions considered necessary for superplasticity1 are a stable fine-grained microstructure and a temperature higher than 0.5 T m (where T m is the melting point of the matrix). Superplastic behaviour is of industrial interest, as it forms the basis of a fabrication method that canbeused to produce components having complex shapes from materials that are hard to machine, such as metal matrix composites and intermetallics. Use of superplastic forming may become even more widespread if lower deformation temperatures can be attained. Here we present observations of low-temperature superplasticity in nanocrystalline nickel, a nanocrystalline aluminium alloy (1420-Al), and nanocrystalline nickel aluminide (Ni3Al). The nanocrystalline nickel was found to be superplastic ata temperature 470 °C below that previously attained2: this corresponds to 0.36T m, the lowest normalized superplastic temperature reported for any crystalline material. The nanocrystalline Ni3Al was found to be superplastic at a temperature 450 °C below the superplastic temperature in the microcrystalline regime3.

Review Processing and mechanical properties of fine-grained magnesium alloys

Abstract: Magnesium alloys are promising light structural materials. The present paper focuses on fine-grained magnesium-based materials. Grain refinement is attained by hot working without additional treatments. Also, a very small grain size of less than 1 μm is obtained by equal channel angular extrusion. A good combination of high strength and high ductility at room temperature is attained by grain refinement. Furthermore, fine-grained magnesium-based materials exhibit superplastic behavior at high stain rates (≥10−1 s−1) or low temperatures (≤473 K). These point out the importance of grain refinement to process magnesium-based materials with excellent mechanical properties.

Thermal stability of ultrafine-grained aluminum in the presence of Mg and Zr additions

Abstract: Superplastic deformation occurs at high temperatures and requires the presence of a very small grain size. Experiments demonstrate that equal-channel angular (ECA) pressing is capable of introducing ultrafine grain sizes in pure Al, Al–Mg and Al–Zr alloys, with grain sizes lying in the sub-micrometer range for the alloys. It is shown by static annealing that these ultrafine grain sizes are not stable in pure Al and the Al–Mg alloys at elevated temperatures but in the Al–Zr alloys the grains remain small up to temperatures of ≈600 K.

Microstructural analysis on boundary sliding and its accommodation mode during superplastic deformation of Ti–6Al–4V alloy

Abstract: A study was carried out to investigate the mechanisms of superplastic deformation in two phase Ti-6Al–4V alloy. Tensile tests were carried out using fine (3 μm) and coarse (11 μm) grained microstructures at 600 and 900°C with a strain rate of 10−3s−1, and the quenched microstructures were analyzed by transmission electron microscopy. In the coarse grain microstructure, the planar arrays of dislocations were developed well in the α-phase both at 600 and 900°C. However, in the fine grain microstructure, dislocations were rarely observed in the α-phase at 600°C, while a few dislocations were observed near the α/β boundaries at 900°C. The results indicate that most of deformation is imposed on the β-phase in the fine grain microstructure, and on the α-phase as well as the β-phase in the coarse-grain microstructure. It is also considered that accommodation processes for phase/grain boundary sliding in the α and β phase are closely associated with the dislocation motion.

Stabilization and high strain-rate superplasticity of metallic supercooled liquid

Abstract: Conventional bulk metallic materials have ordinarily been produced by the melting and solidification processes. The metallic liquid is unstable at temperatures below melting temperature and solidifies immediately into crystalline phases. Consequently, all bulk engineering alloys had been composed of a crystalline structure. Recently, the common concept has been exploded by the findings of the stabilization phenomenon of supercooled liquid for a number of alloys in Mg-, lanthanide-, Zr-, Ti-, Fe-, Co- and Pd–Cu-based systems. The alloys with the stabilized supercooled liquid state have three features in their alloy components, i.e. multi-component systems, significant atomic size ratios above 12%, and negative heats of mixing. The stabilization mechanism has also been investigated from experimental data of structure analyses and fundamental physical properties. The stabilization has enabled the production of bulk amorphous alloys in the thickness range of 1–100 mm by using various casting processes. The bulk amorphous Zr-based alloys exhibit high mechanical strength, high fracture toughness and good corrosion resistance. The stabilization also leads to the appearance of a wide supercooled liquid region before crystallization and enables the achievement of high-strain superplasticity through Newtonian flow in the supercooled liquid region. The Newtonian flow in the strain rate range just below the transition from Newtonian to non-Newtonian flow was also found to cause the suppression of crystallization of the supercooled liquid. In addition to the finding of the stabilization phenomenon, the clarification of the stabilization criteria of the supercooled liquid will lead to the future definite development of bulk amorphous alloys as basic science and engineering materials.

Superplasticity of Ultra-Fine Grained Al-Mg Alloy Produced by Accumulative Roll-Bonding

Abstract: Fully annealed Al-Mg alloy (5083) bulk sheets were highly strained up to a true strain of 4.0 at 473 K by the novel Accumulative Roll-Bonding (ARB) process, and superplastic behavior of the ARBed sheets was investigated at elevated temperatures. Just before tensile tests, usual recrystallization structure whose mean grain size was 10 μm formed at 573 and 673 K, while the ultra-fine grains with a mean grain size of 280 nm formed homogeneously at 473 K. The electron diffraction study in TEM showed that the ultra-fine grains are polycrystals with large misorientations to each other. The ARBed sheets exhibited conventional superplasticity above 573 K, where the maximum elongation was 430% with a m-value (strain rate sensitivity) of 0.43 at 673 K at a strain rate of 1.7 × 10 -3 s -1 . Further, the ARBed sheets showed large elongation up to 220% and large m-value over 0.3 even at 473 K, which is nearly half of the melting point of this alloy, at 10 -4 s -1 to 10 -3 s -1 . This large elongation at 473 K was concluded as low-temperature superplasticity caused by ultra-grain refining. The ARBed material kept ultra-fine grain size less than 1 μm even after low-temperature superplasticity at 473 K, so that the specimens largely deformed at 473 K still exhibited larger hardness than that of the fully recrystallized material.

Developing superplastic properties in an aluminum alloy through severe plastic deformation

Abstract: Equal-channel angular (ECA) pressing is a processing procedure which subjects a material to severe plastic deformation Tests were conducted on a commercial cast aluminum alloy to evaluate the properties associated with samples subjected to three different ECA pressing procedures The results show that all three procedures lead to an ultrafine microstructure and each procedure is capable of producing samples which exhibit high strain rate superplasticity Optimum superplastic properties were achieved in samples subjected to ECA pressing to a strain of ∼12 Under these conditions, the measured elongations to failure at a temperature of 673 K were 1210 and 950% at strain rates of 10−1 and 1 s−1, respectively

High-Strain-Rate Superplasticity due to Newtonian Viscous Flow in La 55 Al 25 Ni 20 Metallic Glass

Abstract: We have investigated the deformation behavior of a La 55 Al 25 Ni 20 (at%) metallic glass that has a wide supercooled liquid region of 72 K before crystallization. The glassy solid below the glass transition temperature exhibited non-Newtonian viscosity, and the supercooled liquid revealed a Newtonian viscosity that transferred to the non-Newtonian viscosity with increasing strain rate. The supercooled liquid exhibited a high-strain-rate superplasticity due to the Newtonian viscous flow that has a strain-rate sensitivity exponent (m value) of unity. The metallic glass exhibited large elongations of more than 1000% at strain rates ranging from 10 -4 to 10° s -1 and at relatively low temperatures of about 0.7 T m , and retained the ductile nature without crystallization even after the deformation. The maximum elongation to failure was about 1800% at a strain rate of 1.7 × 10 -1 s -1 and at 503 K (0.71 T m ) under a flow stress of about 40 MPa. The elongation was restricted by the transition to non-Newtonian viscosity and crystallization. We succeeded in establishing the constitutive formulation of the flow stress in the supercooled liquid region. Its formulation was expressed very well by a stretched exponential function σ flow = D e exp (H * /RT) [1-exp(E/{e exp(H ** /RT)} 0.82 )]. The superplasticity of the La 55 Al 25 Ni 20 metallic glass was superior to that of the Zr 65 Al 10 Ni 10 Cu 15 metallic glass. The metallic glass, moreover, had many advantages in the superplastic deformation, as compared with superplastic polycrystalline materials.

Diffusion bonding of superplastic 7075 aluminium alloy

Abstract: Diffusion bonding (DB) of superplastic 7075 Al alloy has been investigated for various temperatures, pressures and times, using a Gleeble 1500 test machine. After the removal of surface oxide, an organic solution was used to protect the surfaces prior to bonding. The strengths achieved after bonding were dependent on interface grain boundary migration and on grain growth during the bonding process. Under optimum conditions, bonds having parent metal shear strength and microstructure were obtained. The optimum temperatures for diffusion bonding, 510–520°C, corresponded with those at which the material exhibited optimum superplastic behavior. The characteristics and mechanisms of bonding are discussed.

Superplastic Sinter-Forging of Silicon Nitride with Anisotropic Microstructure Formation

Abstract: The microstructure and mechanical properties of silicon nitride, produced by a superplastic sinter-forging technique, were investigated. The obtained silicon nitride exhibited a highly anisotropic microstructure, where rod-shaped grains tended to be aligned perpendicular to the forging direction. A very high bending strength of 2108 MPa as well as a high fracture toughness of 8.3 MPa.m 1/2 were achieved when a stress was applied perpendicularly to the pressing direction. This very high strength was considered to be due to the reduced flaw size by the superplastic sinter-forging process and the steep R-curve behavior caused by the grain alignment.

An evaluation of superplasticity in aluminum-scandium alloys processed by equal-channel angular pressing

Abstract: An Al-0.2%Sc alloy and an Al-3%Mg-0.2%Sc alloy were subjected to equal-channel angular pressing to a strain of - 8 to produce grain sizes of ∼ 0.7 and 0.2 μm, respectively. Because of the presence of some areas of subgrain boundaries in the Al-3%Mg-0.2%Sc alloy, additional samples of this alloy were subjected to ECA pressing to a strain of ∼12. In both alloys, the grain sizes were reasonably stable up to annealing temperatures above 700 K. Tensile specimens were machined from the as-pressed samples for testing at elevated temperatures. The Al-3%Mg-0.2%Sc alloy pressed to a strain of ∼ 12 exhibited exceptional ductility with elongations up to a maximum of ∼ 1560% at 673 K at the high strain rate of 3.3 x 10 -2 S -1 . By contrast, the Al-0.2%Sc alloy failed under all conditions at elongations <400%. The difference in behavior between these two alloys is attributed to the nature of the deformation mechanism which serves to accommodate the superplastic process.

High temperature mechanical properties of Si3N4–MoSi2 and Si3N4–SiC composites with network structures of second phases

Abstract: Si 3 N 4 –MoSi 2 composites have been employed as ceramic glow plugs for an application to functional parts. The ceramic glow plugs were developed and mass-produced by DENSO Co. in Japan. The ceramic glow plugs are used to help start engines by heating them installed in the combustion chamber of the diesel engine. They can be heated at a higher heating rate, that is, the higher heat resistance than a metal glow plug with the result that the diesel engine can be started faster. To the purpose of improving the high temperature mechanical properties of Si 3 N 4 –MoSi 2 composites, the relationship between the composition and high temperature strength of the Si 3 N 4 –MoSi 2 composites were investigated at 1400°C. The 1400°C strength could be improved by controlling Y 2 O 3 contents. Si 3 N 4 –SiC composites with a three dimensional network structure of SiC particles has been investigated. The composites consist of a plurality of Si 3 N 4 grains surrounded by fine SiC particles which are discontinuously dispersed. It was found that the creep resistance and oxidation resistance as well as electrical and thermal properties of the composite can be considerably improved. Furthermore, the simultaneous improvement of both superplastic deformation and creep resistance by forming a network structure of Si 3 N 4 –SiC phase in the Si 3 N 4 –SiC composites was also examined at elevated temperatures. It was found that the superplastic deformation and creep resistance of the composites with the network structure of continuous Si 3 N 4 –SiC phase could been improved in one sintered body.