Showing papers on "Superplasticity published in 1979"
TL;DR: In this article, the deformation behavior of a superplastic Ti-6A1-4V alloy at 927°C has been characterized by means of constant strain-rate tensile tests up to large plastic strain.
Abstract: The deformation behavior of a superplastic Ti-6A1-4V alloy at 927°C has been characterized by means of constant strain-rate tensile tests up to large plastic strain. Significant hardening has been recorded in the course of deformation. Microstructural studies on deformed samples indicate the occurrence of simultaneous strain-rate induced grain growth, which explains nearly all of the hardening. A small amount of hardening may also be expected from grain elongation or grain clustering effects. As a result of concurrent grain growth, the strain-rate sensitivity is found to decrease with strain, thus indicating that stress-strain rate behavior determined initially may not be applicable after large amounts of plastic strain. The stressJstrain-rate data obtained from step strain-rate test for a variety of grain sizes, together with the grain growth kinetics plots, provide a means for developing a constitutive description for this material at large strains.
250 citations
TL;DR: In this paper, boundary sliding in the superplastic Pb-62% Sn eutectic alloy was investigated and it was shown that the contribution from sliding is high in the region II ( ϵ gbs ϵ t − 50 − 60%), where ϵgbs is the strain due to sliding and ϵl is the total strain.
121 citations
TL;DR: In this article, the microstructure of two-phase α β alloys such as Ti-6A1-4V was evaluated in terms of their fully characterized microstructures.
Abstract: The strong dependence of the superplastic behavior of metals and alloys on grain size has been demonstrated, and it is now well known that a fine grain size is normally a requirement for superplasticity. However, the microstructure of certain alloy systems such as Ti-6A1-4V cannot always be adequately characterized by a single parameter such as grain size. In two-phase α β alloys such as Ti-6A1-4V, other microstructural parameters such as volume fractions of the two phases, grain aspect ratio, grain size distribution and crystallographic texture may also influence superplasticity. For example, if “grain switching” is an important deformation mechanism in superplastic flow as suggested by Ashby and Verall, then factors such as grain aspect ratio and range of grain sizes would be expected to have an effect on superplastic behavior. In this study, these microstructural features were determined for several different heats of Ti-6Al-4V, and the corresponding superplastic properties were evaluated in terms of their fully characterized microstructure. The flow stress as a function of strain rate, strain rate sensitivity exponent (m) as a function of strain rate and total elongation on properties were found to be strongly influenced by microstructural parameters such as grain aspect ratios, grain size and grain size distribution.
104 citations
TL;DR: In this article, it was shown that superplastic properties of UHC steels with fine equiaxed ferrite or austenite grains (∼1 μm) stabilized by fine spheroidized cementite particles can be observed at normal forming rates.
Abstract: Ultrahigh carbon (UHC) steels have been investigated for their strength and ductility characteristics from 600 to 850°C. It has been shown that such UHC steels, in the carbon range 1.3 to 1.9 pct C, are superplastic when the microstructure consisted of fine equiaxed ferrite or austenite grains (∼1 μm) stabilized by fine spheroidized cementite particles. The flow stress-strain-rate relations obtained at various temperatures can be quantitatively described by the additive contributions of grain boundary (superplastic) creep and slip (lattice diffusion controlled) creep. It is predicted that superplastic characteristics should be observed at normal forming rates for the UHC steels if the grain size can be stabilized at 0.4 μm. The UHC steels were found to be readily rolled or forged at high strain-rates in the warm and hot range of temperatures even in the as-cast, coarse grained, condition.
98 citations
TL;DR: The steadystate stresses of lead-tin eutectic alloy (Pb-62Sn) samples of mean phase diameter from 5.5 to 9.9 μm were measured over a range of temperatures from 0° to 160°C and strain rates from 2.33 × 10−2 s−1.
83 citations
TL;DR: In this paper, a sound theoretical understanding of the dependence of the superplastic strain rate on temperature, grain size, and flow stress is presented, which can be of significant use in optimization of these variables, so that the material can be deformed at the highest possible forming rates and still benefit from the exceptionally large, neck-free ductility associated with super-plasticity.
Abstract: The phenomenon of micro grain superplasticity, in which metals and alloys deform extensively at elevated temperatures under small stresses without risk of rupture, has been well documented over the past decade. Our under standing of the rate-controlling mechanisms for superplastic deformation processes in terms of range of stress, temperature, and strain rates is important not only for its intrinsic scientific value but also because of its significance in optimizing the processing parameters for forming opera tions (1). The strain rates for stiperplasticity are considerably lower than those for conventional hot-working processes. This factor has so far restricted the extensive adoption of superplastic forming operations. A sound theoretical understanding of the dependence of the superplastic strain rate on temperature, grain size, and flow stress will be of significant use in optimization of these variables, so that the material can be deformed at the highest possible forming rates and still benefit from the exceptionally large, neck-free ductility associated with superplasticity. Therefore, it is not surprising that the considerable attention given to this particular mode of deformation in recent years has resulted in two major advances: (a) several new models have been proposed for the per tinent deformation mechanism (2-6), and (b) more reliable new data (71 1, 17)have become available to examine the validity of the proposed models. Many of these advances were recently reviewed in excellent detail (12-15;
71 citations
70 citations
TL;DR: In this paper, it was shown that diffusion growth is favored at low total strains, and there is a transition to power-law growth at a critical cavity radius, which increases with decreasing strain-rate, so that there is also a transition from predominanly power-Law growth at high stress levels to predominantly diffusion growth at low stress levels.
Abstract: Cavity growth at high temperatures may be controlled by vacancy diffusion, giving cavities which are approximately spherical and randomly distributed, or by power-law creep, giving cavities which are elongated and aligned in the direction of the tensile stress. In general, diffusion growth is favored at low total strains, and there is a transition to power-law growth at a critical cavity radius,rc. The value ofrc increases with decreasing strain-rate, so that there is also a transition from predominanly power-law growth at high stress levels to predominantly diffusion growth at low stress levels. Both types of cavities have been observed in superplastic materials, but the diffusion growth rate may be enhanced if the cavity intersects a number of grain boundaries. The analysis is in good agreement with experimental results reported for three diffent superplastic materials.
58 citations
TL;DR: The superplastic behavior of PbSn eutectic alloy was investigated for grain sizes from 22 to 61 μm over a range of temperatures from 298 to 373 K and with strain rates from 10−7 to about 10−2 s−1 as discussed by the authors.
58 citations
TL;DR: In this paper, the deformation of the superplastic Zn-22% Al eutectoid is divisible into three distinct regions: region I, region II and region III.
Abstract: The mechanical behaviour of the superplastic Zn-22% Al eutectoid is divisible into three distinct regions. Experiments show the deformation is quasi-uniform at intermediate strain rates in region II, but neck formation is important at low strain rates in region I. Extensive cavitation occurs in regions I and II, but fracture in region I is due to necking. The results provide strong evidence for a decrease in the true value of the strain rate sensitivity in region I.
TL;DR: In this paper, a qualitative texture study using the crystallite orientation distribution function has been made on a superplastic Al-6Cu-0.3Zr alloy over a range of strain rates at 450°C.
TL;DR: In this article, the results of a mechanical property and TEM microstructural investigation of a superplastic Al-Cu-Zr alloy are reported, and the conventional 3 stage ln σvs ln ∈ curve is observed, but a high value ofm is retained to low strain-rates and suggests that any operative creep yield stress is low.
Abstract: The results of a mechanical property and TEM microstructural investigation of a superplastic Al-Cu-Zr alloy are reported. The conventional 3 stage ln σvs ln ∈ curve is observed, but a high value ofm is retained to low strain-rates and suggests that any operative creep yield stress is low. Marked mechanical property changes occur in the initial stages of deformation at the same time as dynamic recrystallization removes the initially directional microstructure of highly elongated grains. Following deformation at the higher strain-rates, Regions II and III, individual dislocations, dislocation networks, and low angle boundaries are observed. After deformation in Region I little evidence of any dislocation activity is seen. It is shown that two distinct recrystallization behaviors occur. In samples statically annealed for long periods recrystallization takes place in a discontinuous manner, high angle boundaries sweeping out from the ρ-particles, which act as nuclei. In the case of superplastically deformed samples the dynamic recrystallization is continuous in nature, and not related to the ρ-particles present. The current theories explaining the beneficial effects of ternary additions to Al-Zr alloys are examined in the light of the above observations.
TL;DR: In this paper, the hot deformation behavior of a cold-rolled Zr-bearing Al-Mn alloy and a commercial superplastic Al-Cu-Zr alloy has been investigated.
Abstract: The hot-deformation behaviour of a cold-rolled Zr-bearing Al-Mn alloy and a commercial superplastic Al-Cu-Zr alloy has been investigated. During hot-deformation the subgrain size increased while the density of Al3Zr particles decreased with increasing strain. The decrease in particle density was caused by a discontinuous dissolution reaction at migrating sub-boundaries. This dissolution resulted in a subsequent reprecipitation of Al3Zr particles (the metastable phase). The strain-induced subgrain growth process has been classified as a continuous recrystallization reaction. In the superplastic alloy, grain growth ceased above a certain deformation level. The subgrain growth is interpreted as being a result of the strain-induced decrease in particle density. The kinetics has been discussed in terms of a two-stage model where the initial stage is associated with the discontinuous dissolution of the Al3Zr particles. During the later stage the effects of undissolved and reprecipitated particles have ...
Patent•
25 Jul 1979TL;DR: In this paper, an improvement to the method of making a structure by superplastic forming, wherein portions of a preform are expanded, in the super-plastic condition, against a forming member, is disclosed.
Abstract: An improvement to the method of making a structure by superplastic forming, wherein portions of a preform are expanded, in the superplastic condition, against a forming member, is disclosed. The improvement comprises the step, prior to expanding the preform, of providing a lower coefficient of friction between the portions of the preform to be expanded and the forming member than which exists between the portions of the preform where expansion is intended to be minimal and the forming member. The two values of coefficient of friction are obtained by applying first and second release coatings to the portions of the preform to be expanded and those where expansion is to be minimal respectively, with the first release coating having a coefficient of friction less than the second release coating. Alternately, the first and second release coatings can be applied to the forming member. The preferred first and second release coatings are boron nitride and yttria respectively.
TL;DR: In this article, superplastic flow of feldspar is inferred to have occurred in the deformation of a granite from Mieville, Aiguilles-Rouges massif, Switzerland.
TL;DR: In this article, the structure and properties of grain boundaries are considered during superplastic deformation on the basis of recent concepts of the interaction between lattice dislocations and grain boundaries.
Abstract: The structure and properties of grain boundaries are considered during superplastic deformation on the basis of recent concepts of the interaction between lattice dislocations and grain boundaries. It is shown that the boundaries contain both, lattice and grain boundary dislocations. The presence of these extraneous defects leads to an increase of grain boundary energy; there is an increase of the vacancy concentration and enhancement of the diffusion coefficient in the boundaries with non-equilibrium structure. The results of numerical estimation of dislocation density and of the change in diffusion parameters agree well with the established experimental data.
TL;DR: In this paper, the superplastic deformation behavior in two-phase AI-Cu alloys is explained by the assumption that grain boundary sliding is accommodated by slip in α-AI phase and diffusion in CuAl2 phase, and both phases are deformed at the same rate.
Abstract: Elevated temperature tension tests and metallographic studies have been made with nominally single-phase and two-phase AI-Cu alloys containing different volume fractions of CuAl2 phase, in order to gain a better understanding of superplastic deformation mechanism in a two-phase alloy. The superplastic deformation behaviour in these alloys may be explained quantitatively on the assumption that grain boundary sliding is accommodated by slip in α-AI phase and by diffusion in CuAl2 phase, and both phases are deformed at the same rate.
TL;DR: The feasibility of the solid-state bonding of similar and dissimilar ferrous materials over a wide range of carbon contents, below the eutectoid (A 1 ) transformation temperature has been demonstrated.
TL;DR: In this paper, the growth and reorientation of grain-boundary pores during superplastic deformation was studied and a model to explain their behaviour was proposed, showing that failure strains in compression of over 100% true strain are possible.
TL;DR: In this paper, superplastic structures have been developed in an as-cast ultrahigh carbon steel, containing 1.6% C, by heat treatment alone, and an optimum structure was found after about 10 cycles and strain rate sensitivities of about 0.45 and elongations to failure of over 300% have been achieved.
Abstract: Superplastic structures have been developed in an as-cast ultrahigh carbon steel, containing 1.6% C, by heat treatment alone. The heat treatment involves quenching from high temperature (1150° C) and then repeated cycling across the A1 transformation temperature (727° C). This changes the coarse, as-cast structure to an ultrafine ferrite structure containing fine spheroidized cementite particles. An optimum structure is found after about 10 cycles and strain rate sensitivities of about 0.45 and elongations to failure of over 300% have been achieved.
TL;DR: In this article, the authors report on the kinetics of transformation of a eutectoid Zn-AI alloy containing additions of Cu, Mg and Ca. The alloy possesses excellent superplasticity at elevated temperatures and it has a relatively high strength at ambient temperature (∼345 MPa).
Abstract: We report herein on the kinetics of transformation of a eutectoid Zn-AI alloy containing additions of Cu, Mg and Ca. The alloy possesses excellent superplasticity at elevated temperatures, and it has a relatively high strength at ambient temperature (∼345 MPa). TTT curves for the alloy are presented, and the corresponding microstructures obtained at the various transformation temperatures are reported. Also, the results of Jominy endquenched tests are reported and the corresponding continuous cooling kinetics are compared to the isothermal kinetics. The alloy was observed to decompose by two distinct mechanisms, depending upon the degree of supercooling. At temperatures just below the eutectoid, it decomposes into a lamellar microstructure, whereas at larger undercooling it decomposes into a coherent two-phase mixture. The interlamellar spacing and colony size are reported as a function of transformation temperature, and shown to follow expected trends. Since neither the lamellar nor coherent microstructure is superplastic, thermomechanical methods of producing a superplastic structure are discussed.
TL;DR: In this article, superplastic behavior in hot rolled AI-Si eutectic was investigated, for a grain size of 18 μm, over a range of strain rates, temperatures, and compositions.
Abstract: Superplastic behaviour in hot rolled AI-Si eutectic was investigated, for a grain size of 18 μm, over a range of strain rates, temperatures, and compositions. The AI-Si eutectic was successful in producing superplasticity with over 300% elongation. Two dominant superplasticity mechanisms seem to operate: a dislocation climb at high strain rates and b grain boundary sliding with diffusional accommodation at intermediate and low strain rate. The calculations of strain rate and activation energy support this contention. In addition, the contribution of the imposed strain to the total ductility and to the strain rate sensitivity was also studied. The results indicate that superplasticity in eutectic AI-Si is severely restricted by structural coarsening at large strains, particularly at the lower strain rates.
TL;DR: In this article, the anisotropic properties of superplastic Al6Cu-0.3Zr alloy have been investigated and it has been shown that mechanical anisotropy measurements can not only indicate a change in the dominant deformation mode, but that they can also assess the varying role of crystallographic slip in a deformation process.
01 Jan 1979
TL;DR: The mechanisms of hot deformation with either superplasticity, dynamic recovery, or dynamic recrystallization are certainly distinct having different ranges of permitted strain rate and temperature, different microstructural preconditions and different strain rate sensitivities.
Abstract: The mechanisms of hot deformation with either superplasticity, or dynamic recovery, or dynamic recrystallization are certainly distinct having different ranges of permitted strain rate and temperature, different microstructural preconditions and different strain rate sensitivities. The phenomena are similar in so far as an approximately steady state deformation can be established for each with no strain hardening and with a dynamically stable microstructure being, respectively, fine grains of two phases, recovered subgrains, and recrystallized grains containing a range of substructures. Such deformations proceed at comparatively low flow stresses and with high ductilities particularly comparable in compression or torsion modes of deformation.
TL;DR: In this article, serrated flow in the austenitic state of 300 grade commercial marageing steel has been investigated in the strain rate range from 4.17 × 10−5 to 1.67 × 10 −2 S−1 and at temperatures between 325 and 700°C.
Abstract: Serrated flow in the austenitic state of 300 grade commercial marageing steel has been investigated in the strain rate range from 4.17 × 10−5 to 1.67 × 10−2 S−1 and at temperatures between 325 and 700°C. The strain exponent (m + β) was found to be 1.5, which is in agreement with Saada's model of vacancy generation and which also suggests that the mobile dislocation density remains constant during plastic deformation at various strain rates. The solute responsible for the onset of serrations was found to be Mo atoms from the activation energy for solute migration and from the identification of precipitates by transmission electron microscopy. In the serrated flow region higher work-hardening, lower ductility, and negative strain rate sensitivity were also observed. At higher temperatures, where serrations were absent, a very large amount of total elongation was observed which may be attributed to superplasticity.