Showing papers on "Superplasticity published in 1989"

Dynamic Grain Growth During Superplastic Deformation of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Abstract: Both static and dynamic grain growth were studied during superplastic deformation of fine-grained yttria-stabilized tetragonal zirconia. It was found that significant grain growth does not take place below 1300°C. Both static and dynamic growth were found to obey a similar equation of the form D3−D30=kt, where D and D0 are the instantaneous and initial grain sizes, respectively, t is the annealing time, and k is the kinetic constant for either static or dynamic grain growth. The activation energies were approximately 580 and 520 kJ/mol for static and dynamic grain growth, respectively.

Superplasticity in metals and ceramics

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Apparatus and a method for fabricating superplastically formed structures

Abstract: A superplastic forming apparatus is described in which a blank of superplastic material is formed onto a die located in a pressure cavity. The die, which is preferably made of ceramic material, is removable from the cavity and requires no special connections for gas or vacuums ducts and therefore is cheap to manufacture. The use of a separate die also allows accurate monitoring and control of the pressure on the die side of the superplastic blank.

Apparatus and method for superplastic forming

Abstract: A method and apparatus for the superplastic forming of a workpiece are disclosed. The invention comprises securing the workpiece within a die assembly located within an autoclave. The die assembly is of a type suitable for use in the superplastic fabrication of workpieces. The die assembly includes a lid, a die and first and second sealing mechanisms. The first sealing mechanism seals a first volume formed between the lid and the workpiece from a main volume of the autoclave. The second sealing mechanism is for sealing a second volume formed between the workpiece and the die from the main volume of the autoclave. The second volume has a backpressure, Pb. The first volume has a pressure Pf +Pb where Pf is the forming pressure for achieving superplastic forming of the workpiece. The main volume of the autoclave has a pressure, Pm. The back pressure, Pb is established so as to minimize grain boundary cavitation. A minimal pressure differential ΔP=Pm -Pb, is established which allows retention of the required sealing, while concomitantly providing Pf for the desired superplastic forming of the workpiece and eliminating the need to seal against Pb.

Control of cavitation during superplastic forming of high strength aluminium alloys

Abstract: This investigation deals with superplastic deformation and cavitation behaviour of a 7475 high strength aluminium alloy in uniaxial tension. Intergranular cavitation increases sharply with strain as a result of continuous nucleation, thus limiting the ductility to relatively small strains in the superplastic range and affecting the room temperature service properties. To reduce the overall cavitation, several processes have been carried out. They involve treatments of the material with or without superimposed hydrostatic pressure and they can be applied before, during, or after deformation. The results of these processes are presented and compared. It is concluded that cavitation can be significantly reduced and even eliminated. Such a result can increase the use of superplastic forming for industrial applications in aeronautics and aerospace technology.MST/985

Superplasticity in crystalline solids

Abstract: (1989). Superplasticity in crystalline solids. International Materials Reviews: Vol. 34, No. 1, pp. 312-312.

Domed structures and a method of making them by superplastic forming and diffusion bonding

Abstract: The present invention provides a method of making a domed structure using superplastic forming and diffusion bonding techniques. The dome is of a deep and/or irregular shape that cannot be achieved by creep forming. The method includes the step of forming a stack of two or more sheets made of superplastic material and the sheets of the stack being joined together in face to face contact by metallic bonds such that no sheet in the stack is bonded to more than one of its neighbouring sheets; the metallic bonds preferably lie on lines extending radially from and/or circumferentially around the said peak. The resulting stack is heated to superplastic termperatures and a pressure differential is established across the stack to form it into a domed shape. The resulting dome is placed in a mould and heated; an inert gas is injected between the sheets to inflate the stack superplastically thereby forming a space between the sheets which is divided into cells by webs formed from two of the said sheets that have been superplastically formed by the inflation process.

Microstructure, tensile properties and fracture behaviour of an Al-Cu-Li-Mg-Zr alloy 8090

Abstract: The addition of lithium to aluminium alloys has the potential for providing a class of high strength alloys with exceptional properties suitable for aerospace applications. One such candidate is 8090, a precipitation hardenable Al-Li-Cu-Mg alloy. Detailed optical microscopical observations were used to analyse the intrinsic microstructural features of the alloy. It is shown that microstructural characteristics have a pronounced influence on tensile properties and fracture behaviour of the alloy in the peak-aged, maximum strength condition. Tensile test results indicate that the alloy has property combinations comparable with other high strength commercial aluminium alloys. The elongation and reduction in area are higher in the transverse direction of the extruded plate. A change in fracture mode was observed with direction of testing. We rationalize such behaviour based on the grain structure of the material, and the nature, distribution and morphology of the second-phase particles. An attempt is made to discuss the kinetics of the fracture process in terms of several competing mechanistic effects involving intrinsic microstructural features, deformation characteristics of the matrix, brittleness of the grain boundary precipitates and grain boundary failure. The role of stress on particle fracture is highlighted.

Cavitation and fracture in the superplastic Al-33% Cu eutectic alloy

Abstract: Experiments were conducted on the Al-33% Cu eutectic alloy in both an annealed and an as-extruded condition. For both conditions, the relationship between flow stress and strain rate is sigmoidal with maximum ductilities occurring at intermediate strain rates in the superplastic Region II. Specimens fail by necking at the faster strain rates in Region III, but the severity of necking is reduced with decreasing strain rate and the necks are very diffuse in Region II. There is extensive internal cavitation in the fractured specimens, especially at lower strain rates and in the vicinity of the fracture tip. It was observed that cavities form preferentially on theα-θ interphase boundaries. It is shown by calculation that the observed change from small rounded cavities to large cavities elongated along the tensile axis is reasonably consistent with the theories of cavity growth in fine-grained superplastic alloys

Modelling of superplastic bulge forming of domes

Abstract: A model for the bulge forming of domes, which is based on the observation of stress-related hole growth, has been developed. The variation in dome thickness with time (and height) has been analytically expressed as a function of the strain rate sensitivity parameter m, the grain size d and the substructure-related constant in the constitutive equation for superplasticity. Both constant-pressure and constant-apex-strain-rate forming can be simulated using the model. The predictions of the analysis show good agreement with experiment, with an improvement being obtained over previous models.

Simulation of static and deformation-enhanced grain growth effects on superplastic ductility

Abstract: The growth of a localized geometric inhomogeneity in a uniaxial test section is simulated through a computer model in which both static(i.e., grain growth in the absence of deformation) and deformation-enhanced grain growth processes are incorporated. The grain growth kinetics are incorporated through a constitutive equation based on the general mechanistic model of superplastic flow, which incorporates a threshold stress as well power-law creep and, therefore, intrinsically results inm varying with strain rate. Results of the simulation indicate a strong effect of the type of test condition, constant strain rate or constant extension rate, on the rate of neck growth ifm varies with strain rate. It is also shown that static grain growth can have a different effect on the neck growth than deformation-enhanced grain growth.

Strain hardening during superplastic deformation of A1-7475 alloy

Abstract: Strain hardening dominates the deformation process in fine-grained A1-7475 alloy in the temperature range 400 to 515 °C. It is shown that anomalously low stress exponents are obtained as a result of strain hardening in strain-rate-change tests. In order to measure stress exponents in a quasi-steady state condition, the samples must be initially deformed at a relatively high stress (≍ 10 MPa) to a relatively high strain (≍ 0.5) before initiating a strain-rate-change test. Such a procedure revealed that a stress exponent about equal to two and an activation energy (141 kJ/mole) nearly equal to the activation energy for lattice diffusion are obtained. The results are interpreted in terms of a model involving grain boundary sliding accommodated by slip following the Gifkins' “core and mantle” concept. It is proposed that strain hardening is associated with the development of a boundary-dislocation structure in the mantle region in a manner similar to the development of subgrains in the core of a grain when slip is the principal deformation mode.

Advances in superplasticity and in superplastic materials.

Abstract: Superplasticity and superplastic materials have gained considerable attention in recent years. The numberof publications in these fields is increasing rapidly attesting to the current strong interest in these subjects. It can be stated that the technology of makingcomplexshapedcomponentsby gas pressure techniques through use of superplastic sheet has comeot age. Onthe other hand, bulk superplastic forming is still is its infancy Considerable progress has beenmade in understanding the processes occurring in fine structure superplasticity and in internal stress superplasticity. Besides the traditional superplastic alloys based on nickel, titanium and zinc, newalloys have been developed which included microduplex stainless steel, aluminum-lithium and aluminum-magnesiumalloys, mechanically alloyed aluminum, whisker and particle reinforced metal matrix composites, aluminumbronze and yttria-stabilized zirconia ceramic polycrystals.

Nickel alum1nides - new advanced alloys

Abstract: New advanced alloys based on Ni Al intermetallic compounds have been introduced. This paper deals with the potential applications for the new alloys, the status of their processing technology, property data, and the commercial sources for the material. The processing technology includes conventional techniques, innovative near-net-shape techniques, and powder metallurgy and superplastic forming. Tensile and creep properties, as influenced by processing techniques, microstructure, and cold work, are presented.

The cavitation and fracture characteristics of a superplastic AlCuLiZr alloy

Abstract: An AlCuLiZr alloy processed to recrystallize statically exhibits superplastic characteristics at a testing temperature of 723 K, with an optimum elongation to failure of 560% at a strain rate of 3 × 10 −4 s −1 . Under optimum conditions the alloy exhibits diffuse necking, whereas at strain rates higher and lower than the optimum value there is an increasing tendency for sharp necking. It is shown that the optimum superplastic elongation is obtained by a suitable combination of the strain rate sensitivity and the strain-hardening coefficient. The alloy exhibits considerable cavitation under all experimental conditions, with cavitation being restricted to regions near the fracture tip in specimens which failed with a sharp neck, and cavitation being more uniformly distributed in specimens with a diffuse neck. Cavities are nucleated during superplastic deformation at coarse AlFeCu particles, and cavity growth occurs predominantly by a power-law mechanism. It is shown that surface diffusion at elevated temperatures may lead to the observation of rounded cavities, even when cavity growth has occurred by a power-law growth mechanism.