Showing papers on "Superplasticity published in 1984"

Mechanism of Superplastic Flow in a Fine-Grained Ceramic Containing Some Liquid Phase

Abstract: Several results pertaining to large deformations at fast strain rates in a fine-grained ceramic material are described. Results for strain-rate, grain size, and temperature dependence of the flow stress are presented. They show that (a) ultrafine-grained ceramics are capable of high rates of deformation (about 10−4 to 10−4 s−1) at quite low stresses (1 to 20 MPa); (b) the mechanism of deformation is the enhanced rate of matter transport through the liquid phase segregated in the grain boundaries; (c) either uniaxial compression or tension tests may be used to determine the flow properties, except that a correction must be implemented for friction in the case of compression tests; and (d) microstructural changes can occur during deformation which influence the flow behavior. The ceramic is almost infinitely ductile in compression, whereas in tension elongations as large as W5% in one material, and more than 400% in another, were obtained. A model material, β-spodumene glass-ceramic, was used for this study but the results are likely to hold for other materials with equivalent microstructures, e.g., liquid-phase-sintered or hot-pressed materials such as the nitrogen ceramics

Computer simulation of a two-dimensional soap froth II. Analysis of results

Abstract: Further results are presented for a simulated two-dimensional soap froth. These include the dependence of the evolution of the system on the choice of initial conditions, and its mechanical properties. The results are set in the context of recent ideas on grain growth, superplastic deformation and the properties of emulsions.

Process for fabricating multi-alloy components

Abstract: A component, such as a turbine disk, made from a metal or metal alloy which has been processed to display superplastic properties at elevated temperatures, is diffusion bonded to a component or components, such as turbine blades, made from another metal or metal alloy, by disposing the components in a press with the surfaces to be bonded in mating contact. Moisture and oxygen are removed from between the surfaces. Heat and pressure are then applied, such as by forging at an elevated temperature or by hot isostatic pressing, to cause superplastic deformation of at least one of the components at the bonding surfaces. The heat and pressure are held sufficiently long to diffusion bond the surfaces. The new integral assembly is then heat treated to obtain desired properties. Consistently good diffusion bonds are achieved by this method.

Aluminum-lithium alloys II

Abstract: The topics covered in this volume include: methods of alloy preparation (including ingot casting, rapid solidification, and mechanical alloying); processing and alloying effects on microstructure and properties; superplastic deformation; and physical metallurgy fundamentals. Other topics discussed include: weldability; sodium and hydrogen effects on fracture; corrosion behavior (including general corrosion, stress corrosion, and high-temperature oxidation); and monotonic and cyclic properties at ambient and elevated temperatures. Attention is also given to the use of Al-Li alloys in aircraft structures.

Solid state bonding in superplastic Ti-6Al-4V

Abstract: The time taken to produce a bond between two sheets of superplastic Ti-6Al-4V has been calculated using existing theories of high temperature cavity growth and of sintering. The predicted conditions for bonding have been compared with those determined experimentally, and a theory based on the Chen and Argon model of creep cavity growth has been developed to take into account the effect of a material grain size which can be substantially smaller than either the cavity size or the original surface roughness. It has been found that high integrity bonds can be produced at 927°C with pressures between 0.7 and 2 MPa (100–300 lb in−2), and within a time scale of the order of 0.5–4 h.

Intercrystalline cracking, grain-boundary sliding, and delayed elasticity at high temperatures

Abstract: The hypothesis of an interrelation between grain-boundary sliding and delayed elasticity in polycrystalline materials at high homologous temperatures is used to investigate the conditions conducive to microcracking. It is known that a material may exhibit cracking activity on attaining a critical delayed-elastic strain corresponding to a critical grainboundary sliding displacement. Experimental data on ice at temperatures >0.9Tm are used to verify this concept. The new criterion is then extended to develop simple, selfconsistent equations describing the interdependence of stress, strain, time, temperature, and grain size in predicting the onset of structural degradation due to microcracking and hence possible failure by fracture or rupture. The merit of the theory lies in its ability to forecast explicitly a large number of commonly observed high-temperature phenomena, including superplasticity, brittle-ductile transition, and the stress and temperature dependence of the apparent activation energy for fracture. One derivation makes it clear that cracking occurs when a critical stress depending only on temperature (and independent of grain size) is exceeded. The near constancy of fracture strain in the quasi brittle range can also be predicted

Development of Udimet 720 for High Strength Disk Applications

Abstract: Udimet 720 is a cast-wrought nickel base superalloy which was originally used for land based gas turbine blades. Work is being done to develop this alloy for high strength disks applications. Hot tensile tests were preformed to define temperature and strain rate parameters for conventional, hot die and superplastic forging. This information was used to select forging parameters for disk type pancakes. Heat treatments were developed to produce mechanical properties suitable for disks. Microstructure and hot tensile properties are reported for various heat treatments. Tensile and stress rupture properties of U720 compare favorably to high strength P/M disk alloys. The production of fine grained U720 billet by use of the VADER (Vacuum Arc Double Electrode Remelt) process offers the potential to produce large disks from a cast-wrought high strength superalloy.

Influence of Hydrostatic Pressure and Humidity on Superplastic Ductility of Two β‐Spodumene Glass‐Ceramics

Abstract: The extent of superplastic deformation in two lithium aluminosilicate glass-ceramics was found to be limited either by intergranular cavitation or by the initiation and growth of surface cracks. Tensile tests were performed under a superimposed hydrostatic pressure to distinguish between the two failure mechanisms. Two materials, one with a high flow stress and the other with a low flow stress, were studied. Cavitation was substantially suppressed by hydrostatic pressure in both materials. The strain-to-fracture, however, was enhanced only in the material with the lower flow stress. (In at least one instance the tensile ductility became almost infinite.) In the material with the higher flow stress, failure was caused by the initiation and propagation of cracks from the surface. The crack-propagation mode of failure was found to be sensitive to humidity, raising the possibility of stress-corrosion cracking at high temperatures in glass-ceramic materials.

Superplastic deformation of strongly textured Ti-6 Al-4 V: Part 1 Stress and strain anisotropy

Abstract: Etude entre 880 et 928 C. La superplasticite anisotropique de la deformation depend de la microstructure alignee et non de la texture de la phase initiale α. Apres une deformation (∼ 0,9) superplastique, la deformation devient isotropique

Process modeling for superplastic forming of metal sheets

Abstract: Methods for developing particular models which accurately predict the material, process and design parameters for superplastic forming of metal parts under optimum conditions consistent with the desired properties in the finished part Generalized control equations are developed which enable designers to develop consistency of the forming parameters for materials from different lots The invention permits control of superplastic forming in accordance with particular raw material microstructure and the effects of time, temperature, strain and strain rate during superplastic forming on the material microstructure, and establishes the relationship between the microstructure and the properties of the parts being formed These methods applied to the specific area of superplastic forming of metal sheets exemplify a more generalized method of conducting systematic research to derive the maximum information from a minimum number of experiments involving available data

Superplastic aluminium alloy sheets

Abstract: Sheets of high strength superplastic Al--Zn--Mg alloys or Al--Cu alloys which having a maximum grain size below 30 μm on average are described. These alloy sheets are obtained by a method which is characterized by keeping a homogenized and hot worked ingot of the alloy at two different temperatures for sufficient times whereby a fine grain structure is formed in the alloy to impart superplasticity to the alloy.

Method for superplastic forming and diffusion bonding Y shaped support structures

Abstract: A method of forming Y shaped support structures by diffusion bonding and/or superplastic forming wherein layers of metal capable of diffusion bonding and/or superplastic forming are placed in a sandwich-like assembly having a top, bottom and intermediate layers, the layers having been selectively coated with stop-off. The sandwich assembly is subjected to sufficient pressure and heat to diffusion bond and superplastically form the sandwich assembly into a Y shaped stringer or support structure. The layers and/or a reinforcing member are selectively coated with stop-off so as to form at least two Y shaped support structures.

Production of metallic articles

Abstract: Alloys having a composition suitable for superplastic deformation usually require heat treatment after casting and mechanical working in order to produce in the alloy the necessary fineness of grain stucture to permit such deformation to occur. It has now been found that some such alloys including in particular ranges of aluminum alloys containing zirconium (or Nb, Ta or Ni) may be heated to a superplastic forming temperature and non-superplastically deformed at that temperature to induce dynamic recrystallisation and simultaneously produce a fine recrystallised grain structure and superplastic deformation.

Effect of cavitation on post-deformation tensile properties of a superplastic copper-base alloy

Abstract: The effects of cavitation, introduced during superplastic tensile flow, on post-deformation tensile properties have been studied for a microduplex Cu-Zn-Ni alloy. Increasing levels of cavitation of up to 4 percent by volume caused a progressive reduction in tensile strength, while ductility was decreased to a substantially greater extent. Increasing the volume fraction of cavities also changed the appearance of the fracture surface from cup and cone to macroscopically brittle. A post deformation annealing treatment reduced the level of cavitation and caused an increase in room temperature ductility, but the proportion of cavities removed during annealing became progressively less as cavitation was increased.

Superplastic forming and diffusion bonding of Inconel 718

Abstract: This paper presents the results of a program designed to delineate the capabilities of Inconel 718 for superplastic forming (SPF) and diffusion bonding (DB) as low cost, weight saving processes for fabricating rocket engine parts. Elevated tem- perature total elongation and step-strain rate tests indicated that specially processed, fine-grained Inconel 718 behaved sufficiently superplastically to indicate the potential for the SPF of parts of con- siderable complexity. This was further confirmed with the SPF of a model part and a section of an actual rocket engine part. 718 can be diffusion bonded using a nickel interface layer to form high shear strength bonds. It was shown that Inconel

Effect of strain, strain rate and temperature on cavity size distribution in a superplastic copper-base alloy

Abstract: The distribution of cavity sizes in a microduplex α/β copper-zinc-nickel-manganese alloy (a nickel-silver) subjected to superplastic tensile straining has been examined as a function of strain, temperature and strain rate using quantitative optical metallography. The number of cavities that became optically visible increased throughout straining, but the rate at which they became visible decreased at higher strains. The distributions of cavity sizes in specimens deformed to the same strain at different temperatures or strain rates were essentially identical. The size distribution data were fully consistent with the observations that for a given strain the overall volume of cavities formed in the alloy was independent of temperature and strain rate. Growth for all cavity sizes is dominated by matrix plastic flow. The insensitivity of void volume and cavity size distribution to strain rate and temperature, and hence stress, reflects the resolution of the density and metallographic techniques. While higher stresses will lead to a wider range of initial cavity sizes by lowering the critical nucleus size, cavities at the lower end of the size range will not grow sufficiently to become optically resolvable or to produce a density differential compared with a specimen deformed to the same extent at a lower stress.

Superplastic aluminum alloy

Abstract: This invention relates to a superplastic aluminum alloy comprising Mg, Cu, and at least one of Mn, Cr and Zr. The alloy has an elongation substantially of from 330% to 800% and can be readily produced.

Superplastic co alloy and its manufacture

Abstract: PURPOSE:To manufacture a novel superplastic Co alloy by precipitating lump and granular carbide having a specified grain size in the matrix of a Co alloy having a specified grain size CONSTITUTION:The composition of a molten Co alloy is composed of, by weight, 15-40% Cr, 015-1% C and >40% Co The molten Co alloy is solidified at >=10 K/sec cooling rate to regulate the secondary dendrite arm spacing to <=10mum The resulting Co alloy is aged at a prescribed temp to precipitate carbide on the surface The composition of the molten Co alloy may be composed of, by weight, 015-1% C, 15-40% Cr, 3-15% W and/or Mo, <1% B, 0-20% Ni, 0-1% Nb, 0-1% Zr, 0-1% Ta, 0-3% Ti, 0-3% Al and the balance Co

Modelling of cavitation in two phase superplastic alloys under uniaxial stress systems

Abstract: Superplastic materials are characterized by their ability to undergo exceptionally large deformation under low stresses. Many alloys, however, are prone to cavitation, which leads to premature failure even though deformation is within the superplastic regime. A model is presented which analyses the evolution of pre-existing cavities in two phase alloys subjected to uniaxial stress systems. It is seen that this model adequately describes the cavitation phenomenon of α–β brasses and microduplex Cu-Zn-Ni alloys in the tensile test. Closure of cavities in the compression test may also be described by this model. The results obtained thus enable the formation of an overall view of the behaviour of a two phase superplastic alloy subjected to the uniaxial tension test, and the difference in cavity evolution of different materials.

Manufacture of superplastic aluminum alloy

Abstract: PURPOSE: To manufacture and Al alloy excellent in superplastic elongation by successively subjecting an Al-alloy ingot with a specific composition to homogenizing heat treatment, hot working, heating and holding, cooling, cold working, and hot working under specific conditions. CONSTITUTION: The Al-alloy ingot consisting of, by weight, 3W8% Zn, 0.5W3% Mg, further 1 or ≥2 kinds selected from the group consisting of ≤3% Cu, 0.05W2.0% Mn, 0.05W2.0% Cr, 0.05W0.5% Zr, 0.05W0.5% V, and ≤0.15% Ti, and the balance Al with impurities (where Fe and Si contents are regulated to ≤0.15%, respectively) is subjected to homogenizing heat treatment at 300W550°C for 6W48hr. The above ingot is hot-worked to the prescribed thickness at 300W550°C, which is subjected to single-stage or two-stage heating and holding at 350W550°C, to cooling at ≥30°C/hr cooling rate, and then to cold working at ≥at least 30% draft, followed by hot working at 20W40% draft at 300W400°C. In this way, the Al alloy of Al-Zn-Mg type having ≥about 600% superplastic elongation can be obtained. COPYRIGHT: (C)1988,JPO&Japio