Showing papers on "Superplasticity published in 1996"
TL;DR: In this paper, the tensile properties of laminated metal composites (LMCs) are reviewed and several mechanisms responsible for the unique properties of LMCs have been proposed, such as fracture toughness, fatigue, and impact behaviour.
Abstract: Laminated metal composites (LMCs) are a unique form of composite material in which alternating metal or metal containing layers are bonded together with discrete interfaces. The mechanical properties of these materials are reviewed. The tensile properties at low and high temperatures are described. At low temperature, very high tensile strengths can be achieved in deposition processed laminates and very high tensile ductilities can be achieved in roll bonded laminates by layer thickness refinement. At high temperature, superplasticity has been observed and agrees with predictions from constitutive creep relations. Damage critical properties (such as fracture toughness, fatigue, and impact behaviour) and damping can be superior to those exhibited by the component materials. The damage critical properties are strongly influenced by local delaminations at layer interfaces. Mechanisms responsible for many of the unique properties of LMCs have been proposed. The influence of processing, laminate archit...
290 citations
TL;DR: In this paper, a new model, which was considered from the viewpoint of the accommodation mechanism by an accommodatin helper such as a liquid or glassy phase, was proposed in which superplasticity was critically controlled by the accommodation helper both to relax the stress concentration resulting from sliding at grain boundaries and/or interfaces and to limit the build up of internal cavitation and subsequent failure.
Abstract: High-strain-rate superplasticity (i.e., superplastic behavior at strain rates over 10-2s-1) has been observed in many meterials such as aluminum alloys and their matrix composites and it is associated with an ultra-fine grained stucture of less than about 3 μm. Its deformation mechanism appears to be different from that in conventional superplastic materials. Experimental investigations showed that a maximum elongation was attained at a temperature close to the partial melting temperature in many superplastic materials exhibiting high-strain-rate superplasticity. Recently, a new model, which was considered from the viewpoint of the accommodation mechanism by an accommodatin helper such as a liquid or glassy phase, was proposed in which superplasticity was critically controlled by the accommodation helper both to relax the stress concentration resulting from the sliding at grain boundaries and/or interfaces and to limit the build up of internal cavitation and subsequent failure. The critical conditions of the quantity and distribution of a liquid phase for optimizing superplastic deformation was discussed and then applied to consider the possibility of attaining high-strain-rate superplasticity in ceramic materials.
114 citations
TL;DR: In this article, an ultrafine silicon carbide powder with an average particle size of 90 nm was densified by hot-processing with the addition of Al2O3, Y2O 3, and CaO at 1750 °C.
Abstract: Ultrafine silicon carbide powder with an average particle size of 90 nm was densified by hot-processing with the addition of Al2O3, Y2O3, and CaO at 1750 °C. Silicon carbide nanoceramics with an average grain size of 110 nm were prepared by liquid phase sintering at low temperature. The materials showed superplastic deformation at a strain rate of 5.0 × 10-4/s at 1700 °C, which is the lowest temperature published. The microstructure and deformation behavior of materials from a submicrometer powder were also investigated as a reference.
94 citations
TL;DR: In this article, microstructural evolution during the cyclic cold-rolling and annealing process in an (α + γ) microduplex stainless steel, which consists of α subgrains and fine γ particles, has been studied in detail with the aim of clarifying the mechanism of dynamic continuous recrystallization.
93 citations
TL;DR: In this article, the microstructures of the adiabatic shear band (ASB) on the pure titanium side in the titanium/mild steel explosive cladding interface were investigated by means of OM, SEM and TEM.
84 citations
TL;DR: In this article, mechanisms-based constitutive equations are proposed for the high-temperature behaviour of a class of titanium alloys, for which the deformation mechanisms include diffusional creep, grain boundary sliding, dislocation creep and grain growth.
Abstract: Mechanisms-based constitutive equations are proposed for the high-temperature behaviour of a class of titanium alloys, for which the deformation mechanisms include diffusional creep, grain boundary sliding, dislocation creep and grain growth A computational procedure has been developed for the determination of the constitutive equations from a material database The constitutive equations and the procedure for their determination have been validated by modelling the behaviour of the titanium alloy Ti-6Al-4V at 927°CIt is shown that the procedure developed for the determination of the mechanisms-based constitutive equations can be used to identify the important deformation mechanisms in operation for particular stress, temperature and strain rate conditions For the case of the Ti-6Al-4V material, the procedure developed correctly predicts the material hardening due to grain growth and indicates that an additional hardening mechanism operates In addition, the procedure is able to identify grain
84 citations
TL;DR: In this paper, the deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C.
Abstract: Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10−5 s−1 to 1 × 10−2 s−1, with a maximum value of 0.65 at 5 × 10−4 s−1 and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C.
80 citations
30 Apr 1996-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: A number of models have been proposed in order to describe the geometrical aspects of the progress of superplastic deformation as discussed by the authors, including cooperative grain boundary sliding (CGBS).
Abstract: A number of models have been proposed in order to describe the geometrical aspects of the progress of superplastic deformation. Geometrical models of superplastic flow considering sliding of individual grains and models of cooperative grain boundary sliding (CGBS) have been critically analyzed. Experimental evidence supporting models that treat CBGS as a sequential sliding of grains has been presented. This approach is further developed in terms of movement of cellular dislocations in two-phase materials.
74 citations
TL;DR: In this article, the possibility of attaining a superplastic forming capability at lower temperatures and/or faster strain rates by a reduction of the grain size into the submicrometer range was considered.
Abstract: High tensile ductility may be achieved in metallic alloys over a limited range of strain rates at elevated temperatures when the grain size is very small (typically less than ∼ 10 μm). These superplastic materials are often suitable for industrial forming operations. This paper considers the possibility of attaining a superplastic forming capability at lower temperatures and/or faster strain rates by a reduction of the grain size into the submicrometer range. Procedures are available for the fabrication of ultrafine-grained materials by imposing a very high plastic strain. These procedures are examined with reference to results obtained using Al-Mg solid solution alloys.
72 citations
TL;DR: In this paper, the authors concluded that the optimal structural superplasticity results from grain/interphase boundary sliding, and treated mesoscopic boundary sliding as a rate controlling mechanism for optimal super-plasticization.
Abstract: An assessment of the experimental findings leads to the conclusion that optimal structural superplasticity results from grain/interphase boundary sliding—diffusion coupled flow. An analysis of the boundary sliding process is presented first. By suggesting that both regions I and IIa (lower stress range of region II) of superplastic flow result from sliding—diffusion coupled flow, and—treating mesoscopic (cooperative) boundary sliding as the rate controlling mechanism for optimal superplasticity, the stress, temperature, and grain size dependences of the strain rate of deformation are predicted. The above equation is then related to the stress exponent n (the inverse of the strain rate sensitivity index m). An analysis for determining the true activation energy for the rate controlling process is presented. Expressions for the distribution of internal stresses arising from sliding and the boundary viscosity are derived, and the presence of an initial unsteady region, predicted in an earlier analysis, is sh...
59 citations
TL;DR: In this article, the internal mismatch between the transforming matrix and the non-transforming particulates was modeled by considering the internal superplastic strain, resulting in an increase in the strain to fracture.
TL;DR: In this paper, the authors describe the fabrication of a submicrometer grain size in the Zn-22% Al alloy by subjecting the samples to intense plastic straining in torsion under high pressure (~5 GPa) at room temperature.
Abstract: The Zn–22% Al eutectoid alloy is capable of exhibiting very high superplastic elongations, in excess of 2000% in tension, when the grain size is in the range of ~1–10 μm. This paper describes the fabrication of a submicrometer grain size in the Zn–22% Al alloy by subjecting the samples to intense plastic straining in torsion under high pressure (~5 GPa) at room temperature. Observations after straining revealed a heterogeneous microstructure with grain sizes in the range of ~0.1–0.5 μm. As a result of the low melting temperature of the alloy, the high internal stresses introduced by torsion straining are relaxed and the grain boundaries are close to an equilibrium configuration.
TL;DR: The degree of instability is determined by the size of grains, their shape coefficient which depends on the nature of an alloy and is equal to 1.1-1.5 after superplastic deformation as mentioned in this paper.
TL;DR: Grain structure and microstructure evolution during superplastic forming were studied on an unrecrystallized sheet of a modified 7050 super-plastic alloy as discussed by the authors, where a SEM-based local orientation technique was used to cover a large number of (sub)grain boundaries in combination with other metallographic techniques.
15 Apr 1996-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the superplastic properties of a 17 vol.% SiC particulate-reinforced ZK60A magnesium composite (ZK60/SiC/17p) have been studied.
Abstract: 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.
TL;DR: In this article, the microstructural evolution during superplastic deformation of a fine grain Al-4.7 pct Mg alloy (5083Al) has been studied quantitatively.
Abstract: The microstructural evolution during superplastic deformation of a fine grain Al-4.7 pct Mg alloy (5083Al) has been studied quantitatively. Starting from an average grain size of 7 µm, grain growth was monitored in this alloy both under static annealing and with concurrent superplastic deformation at a high test temperature of 550°C. Grain size was averaged from measurements taken in longitudinal, transverse, and thickness directions and was found to grow faster during concurrent superplastic deformation than for static annealing. A grain growth law based on an additive nature between time-based and strain-based growth behavior was used to quantify the dynamics of concurrent grain growth. The extent of void formation during deformation was quantified as the area fraction of voids on L-S planes. This void fraction, referred to as the cavity area percent, was recorded at several levels of strain for specimens deformed at two different strain rates. A constitutive equation incorporating this grain growth data into the stress-strain rate data, determined during the early part of deformation, was generated and utilized to model the superplastic tensile behavior. This model was used in an effort to predict the stress-strain curves in uniaxial tension under constant and variable strain rate conditions. Particular attention was paid to the effects of a rapid prestrain rate on the overall superplastic response and hardening characteristics of this alloy.
TL;DR: 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 as discussed by the authors.
TL;DR: In this paper, the superplastic properties of Mg2Sip composites were investigated by constant-strain-rate tensile tests for MgSip[sbnd]-Zn composites.
Abstract: The superplastic properties have been investigated by constant-strain-rate tensile tests for Mg2Sip[sbnd](Mg[sbnd]Zn) and Mg2Sip[sbnd](Mg[sbnd]Al) composites (where Mg2Sip represents Mg2Si particles). The composites exhibited superplastic behaviour at a high strain rate of 10−1s−1. It is noted that the high-strain-rate superplasticity was attained in a solid state including no liquid phase for the Mg2Sip[sbnd](Mg[sbnd]Zn). This is because the stress concentrations around Mg2Si particles could be sufficiently relaxed by diffusional flow and/or diffusion-controlled dislocation movement. The mechanical properties in a solid state for the magnesium matrix composites were roughly in agreement with those for magnesium alloys, taking into consideration a threshold stress and an increase in the shear modulus due to the presence of hard particles. Calculations based on the critical strain rate for cavity nucleation suggested that magnesium-based materials have high potential for high-strain-rate superplas...
TL;DR: In this article, the authors investigated recrystallization kinetics at different temperatures and correlating the grain sizes with particle sizes and volume fractions, and found that the addition of particle-forming elements of Mn and Zr resulted in a substantial reduction in the recrystization kinetic behavior, but complete suppression of static subgrain stabilization was not observed.
Abstract: The 5083 Al alloy (Al-4.75Mg-0.8Mn) holds potential for superplastic forming (SPF), but slow rates of forming limit its use for many applications. Higher strain rates are believed possible through the development of finer grained microstructures or stabilized subgrain structures. Grain sizes after recrystallization and recrystallization characteristics are known to be dependent on the amount and distribution of second-phase particles in the matrix. In this study, the concentration and sizes of such particles were varied by additions of particle-forming elements of Mn and Zr and by modifications of the rolling and aging schedules (thermomechanical processing (TMP)). The investigation involved studying recrystallization kinetics at different temperatures and correlating the grain sizes with particle sizes and volume fractions. The addition of Mn and Zr, for the composition ranges and TMP methods studied, resulted in a substantial reduction of the recrystallization kinetics, but complete suppression of static recrystallization (or subgrain stabilization) was not observed. However, statically recrystallized grain sizes as small as 6 μm were achieved.
TL;DR: In this paper, the superplasticity behavior of Fe-28Al alloys has been investigated by tensile testing, optical microscopy and transmission electron microscopy, and the maximum strain rate sensitivity index m was found to be 05 and the largest elongation reached 620%.
TL;DR: In this article, the alloys Mg-6Si and Mg−4Si-4Zn (wt-%) were extruded at 573 K and the mechanical properties investigated by tensile testing at strain rates from 2 × 10−5 to 2 ×10−2s−1 and at temperatures in the range 673-773 K.
Abstract: The alloys Mg–6Si and Mg–4Si–4Zn (wt-%) were extruded at 573 K and the mechanical properties investigated by tensile testing at strain rates from 2 × 10−5 to 2 × 10−2s−1 and at temperatures in the range 673–773 K. The Mg–Si alloy behaved as M type at any given strain rate. The Mg–Si–Zn alloy behaved as A type at strain rates above 2 × 10−4s−1. The activation energy for the deformation process was higher than those for lattice self diffusion and for the chemical interdiffusion of Zn in Mg. This is probably related to the cross-slip of dislocations. For the Mg–Si–Zn alloy, superplastic behaviour was found at strain rates below 2 × 10−4s−1. The dominant superplastic deformation process is likely to be grain boundary diffusion controlled grain boundary sliding.MST/3192
TL;DR: In this paper, the beneficial effects of heat treatment at high temperature for several hours before deformation on cavitation behavior of a superplastically deformed 7475 alloy were investigated.
TL;DR: In this article, a Zn-22 wt-% Al alloy was superplastically deformed in the optimal range to obtain experimental evidence based on scanning electron and transmission electron microscopy for mesoscopic grain/interphase boundary sliding and plane interface formation that facilitates the cooperative boundary sliding process.
Abstract: A Zn–22 wt-% Al alloy was superplastically deformed in the optimal range to obtain experimental evidence based on scanning electron and transmission electron microscopy for mesoscopic grain/interphase boundary sliding and plane interface formation that facilitates the cooperative boundary sliding process. It was seen that such localised shear surfaces had formed in the entire specimen bulk in an interconnected manner. At boundaries that formed part of a cooperative shear surface the triple junctions had migrated to a near 1800 orientation. Mesoscopic grain/interphase boundary sliding appeared to be accommodated by local boundary migration brought about by diffusion at intercrystalline boundaries and lattice dislocation motion at interphase interfaces in accordance with the requirements of the model presented in Part 1.MST/3075
TL;DR: In this paper, charge-compensating dopants, Ti4+ and Mn2+, have been introduced to achieve low-temperature superplasticity in alumina, which significantly enhance the diffusion and deformation processes during sintering and forming.
Abstract: To achieve low-temperature superplasticity in alumina, we have introduced charge-compensating dopants, Ti4+ and Mn2+, which jointly have a high solubility and significantly enhance the diffusion and deformation processes during sintering and forming. Zirconia as a second-phase pinning agent has also been incorporated to impart microstructural stability against static and dynamic grain growth. The superplastic alumina obtained can be shape-formed under biaxial tension to 100% engineering strain at temperatures below 1300°C. Deformation characteristics of this alumina at temperatures from 1200° to 1400°C and at strain rates from 4 × 10-6 to 3 × 10-3/s are described. The origin of enhanced kinetics is attributed to the formation and dissociation of dopant-defect complexes.
TL;DR: A general equation for the creep rate as a function of these factors, as well as the elastic modulus and a diffusion coefficient, is used to compare models as discussed by the authors, and the models give different exponenonen
Abstract: Mathematical models that have been proposed for creep in ceramics are described Emphasis is on models involving grain boundary motion (sliding or flow) In Lifshitz models the crystalline grains elongate with strain; the elongation results from diffusion, slip, or solution and precipitation In Rachinger models the grains do not elongate during creep The sliding strain can be accommodated by viscous flow of a glassy phase at the grain boundaries, or if there is no boundary glass by diffusion or slip in superplastic models Sliding of a glass-free boundary can result in cavitation, cracking, or formation of boundary dislocations or triple point folds Most models of ceramic creep at high temperatures predict a steady state (stage II) creep rate that depends on the applied stress, grain size, and temperature A general equation for the creep rate as a function of these factors, as well as the elastic modulus and a diffusion coefficient, is used to compare models The models give different exponen
TL;DR: In this article, the activation energy for superplastic flow in a variety of Si 3 N 4 Al composites exhibiting high-strain-rate super-plasticity was analyzed.
TL;DR: In this paper, the authors review the literature on the numerical simulation of superplastic forming (SPF) and present a finite element method of solution to SPF simulation within the context of the standard flow formulation.
TL;DR: In this article, solid-state diffusion bonding of gamma-based titanium aluminides for aerospace applications was carried out with different bonding parameters within the superplastic temperature and pressure range.
Abstract: Solid-state diffusion bonding in conjunction with superplastic forming is a potential candidate for producing complex structural components from gamma-based titanium aluminides for aerospace applications. Solidstate diffusion bonding of TiAl was carried out with different bonding parameters within the superplastic temperature range. Defect-free sound bonds were achieved within the temperature range of 925–1,100 °C and the pressure range of 20–40 MPa. Microtensile tests were carried out to evaluate room-temperature tensile properties of the bonds for comparison of the bonding parameters.
TL;DR: In this article, a detailed study of the superplastic behavior of 4 mol% yttria-stabilized zirconia polycrystals as a function of the stress was presented, where the observed variation in the apparent creep parameters n and p were rationalized in terms of the presence of a threshold stress below which grain boundary sliding cannot operate.
TL;DR: In this paper, the hot deformation characteristics of a newly typed, high strength magnesium-based alloy, Mg-8.3 wt.% Al−8.1 wt% Ga alloy, produced by rapidly solidified and powder metallurgy method have been investigated.
Abstract: The hot deformation characteristics of a newly typed, high strength magnesium-based alloy, Mg–8.3 wt.% Al–8.1 wt.% Ga alloy, produced by rapidly solidified and powder metallurgy method have been investigated. Tensile tests were carried out at a temperature range from 523 to 623 K and a strain rate range from 10 −4 to 1 s −1 . Superplastic characteristics were found and, especially, a maximum elongation-to-failure of 1080% was obtained at 573 K and at a relatively high strain rate of 10 −2 s −1 . Because of the presence of fine microstructures at high temperatures, the optimum superplastic strain rate of the Mg–Al–Ga alloy was higher than that of the reported conventional superplastic aluminum and magnesium alloys.