Superplastic Deformation Mechanism Research Articles

The change in the fractal dimension of the shape of the cavities during the superplastic deformation of duralumin LY12CZ

Cavities have been widely observed during the superplastic deformation of metals and alloys, the superplasticity of metals and alloys generally being sensitive to cavity formation. However, the shape of the cavities is irregular during superplastic deformation, thus quantitative analysis of the shape of the cavities is one of the keys to the studying of the superplastic deformation mechanism. In this paper, the quantitative effects of process parameters on the fractal dimension, D f, of the shape of cavities during the superplastic deformation of duralumin LY12CZ (approximately corresponding to ASTM 2024) with and without the application of an external electric field is studied. The experimental results show that: (i) the fractal dimension of the shape of cavities varies with the change of process parameters (deformation temperature, T, true strain, ε, initial strain rate dot ε 0 ), both with and without the application of an external electric field; (ii) the fractal dimension of the shape of cavities also varies with the intensity of the electric field, E, under otherwise identical conditions. The results in this paper show that the improvement of superplasticity with the application of an external electric field is related to a lower value of the fractal dimension in the quasi-stable flow stage of superplastic deformation.

Superplastic deformation mechanisms of particulate reinforced aluminum matrix composites

Superplastic tensile tests of a SiC p reinforced 8090 aluminum alloy composite were carried out at strain rates ranging from 7.25 × 10 −4 to 3.46 × 10 −1 s −1 and at temperatures from 773 to 873 K. A maximum elongation of 300% was obtained at a strain rate of 1.83 × 10 −1 s −1 when tested at a temperature of 848 K which was slightly above the solidus temperature of the composite. Several important superplastic factors such as strain rate sensitivity, threshold stress and activation energy were calculated. The superplastic behavior of some particulate reinforced aluminum alloy based composites and several aluminum alloys was compared in view of the strain rate, the optimum testing temperature and the relationship between the flow stress and the strain rate. Considering the accommodation process for dislocation movements, the constitutive equations for superplastic deformation at temperatures above and below the solidus temperature of the composite were developed. Experimental results of the stress-strain relationship obtained at temperatures near to the solidus temperature of the composite compare well with the theoretical constitutive predictions.

Constitutive equations for superplastic deformation of SiC particulate reinforced aluminum alloys

Superplastic deformation mechanism in particulate reinforced aluminum alloy matrix composites was analyzed under the light of dislocation movements. The superplastic behavior of some particulate reinforced aluminum alloy based composites and the corresponding monolithic aluminum alloys was compared in view of strain rate, optimum testing temperature and the relationship between flow stress and strain rate. Under the consideration of the accommodation process for dislocation movements, the constitutive equations for superplastic deformation at temperatures below and above the solidus temperature of the composite were developed. To verify the theoretical predictions, superplastic tensile tests were conducted on the SiC p reinforced 8090 aluminum alloy composite. Experimental results of the stress-strain relationship obtained at temperatures near to the solidus temperature of the composite compare well with the theoretical constitutive predictions.

High strain rate superplasticity of a β-Si 3N 4 whisker reinforced pure aluminium composite made by squeeze casting

High strain rate superplasticity (HSRS) in ceramic whisker or particulate reinforced aluminum alloy composites is expected to offer an efficiently near-net shape forming technique to automobile, aerospace, and even semi-conductor industries, since the HSRS composites usually exhibit a total elongation of 250--600% at a high strain rate of about 0.1--10 s{sup {minus}1}. It is thought that primary deformation mechanism of the HSRS is grain boundary sliding since the composites have the fine grain size of 3{approximately}0.8 {micro}m. The purpose of this study is to develop a thermomechanical processing route to produce a fine microstructure and a HSRS in a {beta}-Si{sub 3}N{sub 4} whisker reinforced 99.99% pure aluminum composite fabricated by squeeze casting. In addition, superplastic deformation mechanism of the composite are also discussed.

Superplasticity in a 17 vol.% SiC particulate-reinforced ZK60A magnesium composite (ZK60/SiC/17p)

The superplastic characteristics of a 17 vol.% SiC particulate-reinforced ZK60A magnesium composite (ZK60/SiC/17p) have been studied. The composite has a fine microstructure (grain size ∼0.5 μm) and is stable at high temperatures. As a result of the fine microstructure, the composite exhibits superplasticity (with an elongation to failure of 350%) at very high strain rate (∼1 s −1). The superplastic properties of the composite, including elongation, strain rate sensitivity, activation energy, grain size dependence, and cavitation, were all characterized. From experimental results, i.e., the strain rate sensitivity value, the grain size dependence of strain rate, and the activation energy, the operative superplastic deformation mechanism in ZK60/SiC/17p is proposed. A comparison between the superplastic behavior of the composite and that of magnesium alloys, and particularly the unreinforced magnesium alloy counterpart of the composite, has also been made.

Whisker formation and the mechanism of superplastic deformation

The morphology and formation process of the whiskers which developed during the superplastic deformation of 7475 Al were studied as a function of temperature, strain rate and externally applied electric field. The whiskers were as fine as 0.1 μm and as long as 100 μm. Their density and length increased with test temperature and with decrease in strain rate, and with an applied electric field. The formation of the whiskers could not be explained by a dislocation or vacancy diffusion mechanism. Rather, their morphology and the high strain rates involved suggested that the mechanism is more likely viscous flow of a liquid-like substance originally at the grain boundaries. SEM observations revealed that the whiskers formed from flow lines which developed from grain boundary sliding. This strongly suggests that the mechanism of grain boundary sliding could also be viscous flow of a liquid-like, grain boundary material.

Superplasticity and solid state bonding of the TiAl intermetallic compound with micro- and submicrocrystalline structure

The use of superplastic phenomenon makes it possible to reduce considerably the temperature of solid state bonding of the TiAl intermetallic compound as compared with conventional diffusion bonding. At decreasing the superplastic deformation temperature due to the reduction of the grain size an adequate decrease of the temperature threshold of solid state weldability of the TiAl intermetallic compound was observed. In the TiAl intermetallic compound with submicrocrystalline grain size the formation of a sound solid state joint occurs in the process of deformation at the temperature t = 850 C and strain {var_epsilon} = 10%. Since the temperature-strain-rate conditions of solid state bonding and those of superplasticity coincide, it is assumed that the mechanism controlling the bonding process is grain boundary sliding, the main deformation mechanism of superplasticity.

A Superplastic Deformation Mechanism Based on Grain Boundary Dislocation Movement

A mechanism for superplastic deformation is developed based on experimental data and observations on a high strength Al-Zn-Mg-Cu alloy. In the model the main deformation mechanism is the slip and climb of dislocations along the plane of grain boundaries; the main accommodation process is the slip of dislocations within grains. The deformation rate is controlled by grain boundary diffusion. A relationship between stress and strain rate is deduced. The calculated stress-strain rate and strain rate sensitivity-strain rate curves are compared with the experimental ones. The whole deformation process is described. Some phenomenological features are discussed.

Nanocrystalline structure of non-equilibrium FeCu alloys obtained by severe plastic deformation under pressure

The nanocrystalline structure and non-equilibrium solid solutions of Fe-20 at% Cu and Fe-80 at% Cu have been observed after severe plastic deformation by shear under pressure. The structure of the deformed samples was studied by electron microscopy, X-ray diffraction and nuclear gamma resonance (NGR) methods. The coarse-grain FeCu alloys were transformed into the fine-grained state during straining up to ϵ = 5–6. Further strain up to ϵ = 7 can be considered as a cold forced superplastic deformation without change of the grain sizes. According to X-ray and NGR data, a redistribution of atoms and formation of non-equilibrium solid solutions are the result of strain from ϵ = 5–6 to ϵ = 7. Based on grain rolling, a mechanism of superplastic deformation has been proposed. Fast diffusion during cold plastic deformation is conditioned by the high stresses acting on the grain boundaries and the volumes of the fine grains. During deformation of the nanocrystalline FeCu alloys the volume diffusion coefficient are equal to about D v = 4.10 −16 cm 2/sec.

Dynamic Evolution of Microstructures in Superplastic Ni<SUB>3</SUB>Al

High temperature deformation and microstructural evolution were studied in tension using single phase polycrystalline Ni 3 Al doped with boron. The samples with grain sizes from 3.0 to 18 μm were rapidly H 2 gas-quenched immediately after straining in order to examine deformed structures and to elucidate the mechanism of superplastic deformation. A stress peak appears at a strain of less than 0.1 more remarkably with increasing initial grain size, and with increasing strain rate or decreasing temperature. Dynamic recrystallization (DRX) initiates along prior grain boundaries at near the peak stress and develops toward grain interiors on further deformation, leading to the evolution of necklace structures. A rapid decrease in flow stress after the peak is mainly due to grain boundary sliding at interfaces between DRX grains. In contrast, work hardening takes place at low and moderate strains for samples with smaller initial grain sizes, when they are deformed at lower strain rates or higher temperatures. Under these conditions, grain boundary sliding occurs predominantly accompanied with grain coarsening due to DRX. Superplastic elongation appears in specimens exhibiting no significant behavior of work softening or work hardening. It is concluded that work softening or hardening behavior depends sensitively on the relative difference between the initial grain size and the stable DRX grain size evolved at high strains, and high temperature deformation of Ni 3 Al can be controlled by grain boundary sliding assisted by DRX.

Experimental and Numerical Investigations of Superplastic Deformation Mechanisms

Experimental and Numerical Investigations of Superplastic Deformation Mechanisms

High strain rate superplasticity of AlN particulate reinforced aluminium alloy composites

Ceramic whisker or particulate reinforced aluminium alloy composites have a great potential for automobile engineering components, aerospace structures, semi-conductor packaging and so on, because of the composites ability to exhibit a high specific elastic modulus and specific tensile strength, excellent wear resistance and heat resistance, low thermal expansion and good dimensional stability. A serious problem involving practical application of ceramic whisker or particulate reinforced aluminium alloy composites is due to the low tensile ductility, fracture toughness at room temperature and, also, their hardness qualities that make it difficult to deform by conventional forming processing and machining by ordinary tools. It has been found, however, that aluminium alloy composites reinforced by SiC or Si[sub 3]N[sub 4] whiskers or particulates produce superplasticity at a high strain rate of about 0.1s[sup [minus]1]. Superplastic deformation mechanisms of the ceramic whisker or particulate reinforced aluminium alloy composites are fine grain boundary sliding, interfacial sliding at a liquid phase and dynamic recrystallization. An AlN particulate reinforced aluminium alloy composite exhibits a high elastic modulus and a high thermal conductivity, and their thermal expansion is similar to silicon in that the AlN particulate reinforced aluminum alloy composite is expected to apply to semi-conductor packaging in the aerospacemore » structure. In addition, if the composite could produce superplasticity at high strain rates, the market of aerospace application for superplastic composites could be expanded. The purpose of this study is to make clear if an AlN particulate reinforced aluminium alloy composite can produce superplasticity at high strain rate and the superplastic characteristics.« less

The neighbour-switching mechanism of superplastic deformation: The constitutive relationship and deformation-enhanced grain growth

Abstract Grain-boundary rearrangements for superplastic deformation are modelled with cellular dislocation glide and climb in a two-dimensional cellular array. These processes account for the inhomogeneous nature of the microstructural flow. The constitutive relationship is written to consider cellular dislocation motion here. It is analysed and found to predict the nonlinear viscosity (i.e. strain-rate sensitivity) generally exhibited by superplastic materials, if the mobile cellular dislocation density increases with increasing strain rate (or flow stress), as Morral and Ashby suggest. Two models of deformation-enhanced grain growth based on cellular dislocation climb are developed, and semiquantitative predictions are compared with the behaviour exhibited by single-phase (Sn-1% Bi) and quasi-single-phase (A1 alloy 7475) materials.

Superplastic deformation mechanism accommodated by the liquid phase in metal matrix composites

Abstract The accommodation processes for superplastic metal matrix composites are required to release the stress concentration near matrix-reinforcement interfaces because of stress concentration caused by interfacial sliding. An optimum superplastic elongation was found at the temperature where local melting of interfaces was confirmed by in situ transmission electron microscopy observation. It is postulated that superplastic flow in metal matrix composites is controlled by a grain-boundary sliding mechanism accommodated with relaxing the stress concentration by an isolated liquid phase at matrix-reinforcement interfaces.

Superplastic Deformation Mechanisms in Powder Metallurgically Processed Al-5Mg-2.2Mn Alloy.

The tensile behavior of a powder metallurgically processed Al-5mass%Mg-2.2mass%Mn alloy with a fully recrystallized fine grained structure of 3 μm in size was characterized at strain rates between 10-4 and 2 s-1 at temperatures from 748 to 823 K. This alloy exhibited superplasticity at high strain rates around 10-2 s-1. A maximum elongation of 570 % was obtained at a constant strain rate of 6x10-3 s-1 at 823 K. An analysis of the threshold stress indicated that the true stress exponent is 2 for all testing temperatures and the true activation energy for superplastic flow is equal to that for lattice diffusion in aluminum. It is postulated that superplastic flow in the P/M Al-Mg-Mn alloy is controlled by a grain boundary sliding mechanism accommodated by dislocation climb or glide controlled by lattice diffusion.

Superplastic behaviour of Ti-48at.%Al two-phase titanium aluminide compacts made from mechanically alloyed powder

An HIP compact of MA-processed powder having a nominal composition of Ti-48at.% Al was produced. The compact consisted of a large amount of TiAl(λ) and a small amount of Ti 3Al ( α 2), in a completely ultra-fine equiaxed grain structure. This two-phase compact showed typical superplastic deformation behaviour. A maximum elongation of 550% was obtained. A strain exponent, n = 2, and grain size exponent, p = 2, were determined from the results of a strain-rate-change test and a creep test at constant initial stress using samples having various grain sizes, respectively. The activation energy for creep, Q c at constant stress was calculated to be 350 kJ/mole. It is concluded that the superplastic deformation mechanism of the material under study is grain boundary sliding controlled by lattice diffusion in the TiAl phase.

Influence of Amorphous Grain Boundary Phases on the Superplastic Behavior of 3‐mol%‐Yttria‐Stabilized Tetragonal Zirconia Polycrystals (3Y‐TZP)

Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y‐TZP in order to determine their influence on the superplastic behavior between 1200° and 1300°C and on the room‐temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y‐TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y‐TZP with 5 wt% glass, but the room‐temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y‐TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y2O3 and ZrO2, but the strain rate did not scale inversely with the viscosity of the silicate phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y‐TZP with and without glassy grain boundary phases.

Superplastic Deformation Mechanisms at High Strain Rates in Mechanically Alloyed Aluminum IN905XL.

High strain-rate superplasticity was obtained in the mechanically alloyed aluminum IN905XL alloy. Introducing a threshold stress into the rate equation, the true values of n and Q established. It is postulated that superplastic flow in the IN905XL alloy is controlled by a grain boundary sliding mechanism accommodated by dislocation climb or glide controlled by grain boundary diffusion.

Superplastic behavior of regular α2 and super α2 titanium aluminides

Superplasticity in two titanium aluminides, regular α 2 and super α 2 were studied at temperatures ranging from 920 to 1040 °C and strain rates from 2 × 10 −5 to 5 × 10 −3 s −1. Results show that superplastic elongations of over 500% can be obtained in this group of intermetallic materials. Dynamic grain growth and changes in volume fraction of the phases were observed in both alloys. The regular α 2 alloy failed mainly by intergranular separation whereas transgranular fracture with extensive ductility occurred in the super α 2 alloy. Possible mechanisms of superplastic deformation of these intermetallic alloys are also proposed.

Analyses of the Superplastic Behavior in Super α<SUB>2</SUB> Titanium Aluminide and γ′ Nickel Aluminide

Experimental results on superplasticity in a titanium aluminide, Ti 3 Al, have been reported. Elongations of over 500% can be obtained in this intermetallic alloy. Preliminary results on microstructural evolution of the Ti 3 Al alloy show that dynamic grain grown occurs and the volume fraction of α 2 phase decrease during superplastic deformation. The superplastic properties of Ti 3 Al are compared with those of another aluminide, Ni 3 Al, in terms of the effects of testing temperature and strain rate. Possible mechanisms of superplastic deformation of these intermeticallic alloys are also proposed