Showing papers on "Superplasticity published in 1983"

Interpretation of superplastic flow in terms of a threshold stress

Abstract: In several recent experiments on the Zn-22% Al eutectoid and the Pb-62% Sn eutectic, a sigmoidal relationship between stress and strain rate is noted and the mechanical behaviour has been divided into three regions: low-stress region (region I), intermediatestress region (the superplastic region or region II), and high-stress region (region III). In region II, the stress exponent,n, is ≃ 2 and the apparent activation energy,Q, is close to grain-boundary diffusion,Qgb, but in both regions I and III the stress exponent and the activation energy increase (n > 2 andQ >Qgb). Analysis of the experimental data of the two superplastic alloys suggests that the transition in behaviour between region II and region I may not necessarily reflect a change in deformation process but can arise from the presence of a threshold stress which decreases strongly with increasing temperature. Based on consideration of various possible threshold stress processes during superplastic flow, it seems most likely that a threshold stress which depends strongly on temperature may result from impurity atom segregation at boundaries and their interaction with boundary dislocations.

Enhanced superplasticity and strength in modified Ti-6AI-4V alloys

Abstract: Although Ti-6A1-4V displays extensive superplasticity at 1200 K, lower superplastic forming temperatures are desirable. A study was conducted with the goal of modifying the composition of the Ti-6A1-4V alloy to lower the optimum superplastic forming temperature. Computer modeling results and previous experimental data suggested that additions to Ti-6A1-4V of beta-stabilizing elements which have high diffusivity in the beta-phase would permit lower superplastic forming temperatures. A series of modified alloys with 2 wt pct additions of Fe, Co, and Ni was prepared for experimental evaluation. The modified alloys achieved desirable microstructures for superplasticity at 1088 K,i.e., the grain size was approximately 5 µm and roughly equal volume fractions of the alpha- and beta-phases were present at the deformation temperature. The superplastic properties of the modified alloys were measured at 1088 K and 1144 K. The modified alloys produced values of flow stress, strain rate sensitivity, and total elongation at 1088 K approaching those of the base Ti-6A1-4V alloy at its standard superplastic forming temperature of 1200 K. In addition to lowering the superplastic forming temperature, the β-stabilizing additions also increased room temperature strength levels above those normally found for Ti-6A1-4V. Based on the room temperature and elevated temperature tensile properties, addition of selected beta-stabilizing elements to Ti-6A1-4V simultaneously raises resistance to deformation at room temperature and lowers resistance to deformation at elevated temperatures. This reversal in behavior is explained by considering the effect of beta-stabilizer additions on the deformation mechanisms at room temperature and at elevated temperatures.

Failure of superplastic alloys

Abstract: Although the subject of superplasticity has been extensively reviewed in recent years, one aspect – that of failure in tension – has received less attention than it deserves. In this assessment the factors influencing strain to failure in superplasticity are described, the current literature is reviewed, and directions for future research are suggested. Emphasis is placed on the dominating influence of cavitation damage in many superplastic materials – a feature which is attracting increasing interest because of its influence on the service behaviour of fabricated components.

Control of Superplastic Cavitation by Hydrostatic Pressure

Abstract: It has been shown that the application of hydrostatic gas pressures during superplastic deformation of fine grained 7475 Al can entirely prevent the intergranular cavitation normally encountered at atmospheric pressure. A critical ratio of hydrostatic pressure to flow stress may be defined for each superplastic forming condition above which virtually no cavitation occurs. In superplastic deformation conditions where intergranular cavitation plays a significant part in final tensile rupture, the superplastic ductility may be improved by the application of hydrostatic pressures. Similarly, detrimental effects of large superplastic strains on service properties may be reduced or eliminated by the application of suitable hydrostatic pressures during superplastic forming. In this case, superplastically formed material may have the same design allowables as conventional 7475 Al sheet.

Cavity growth and failure in superplastic alloys

Abstract: A model for the development of cavities in superplastic alloys is described, based on a previous publication, and is extended to predict strain to failure. It is demonstrated that variations in initial cavitation density within a test specimen can have a marked effect on strain to failure and the predictions of the model are shown to be in excellent agreement with experimental results obtained on the aluminium alloy 2004 (Supral).

The role of Coble creep and interface control in superplastic Sn-Pb alloys

Abstract: The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260μm. In Sn-2 wt% Pb with grain sizes larger than ∼ 50 μm, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses (∼ 0.1 MPa) and strain rates (∼ 10−10 sec−1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.

Observations of grain boundary sliding during superplastic deformation

Abstract: Flat test specimens of a 10-micron grain size Al-33 pct Cu alloy were deformed in air over a range of strains at 450 °C (723 K) and at 10-5 strain per second, and examined by SEM. Direct observations and measurements were also made in a SEM during superplastic deformation of a 5-micron Pb-Sn eutectic alloy at room temperature at a strain rate of about 10-5 s-1. Extensive grain boundary sliding was noted. Microcreep curves were obtained between initially adjacent grains and showed that deformation is cyclical in nature and varies by several orders of magnitude locally from point to point. Many surface grains are observed to change neighbors many times, to undergo some rotation and tilt, to show little change in size or shape, and yet to avoid intergranular cracking. Superplastic deformation is a highly heterogeneous process, not unlike that observed in the creep of conventional types of alloys at elevated temperatures, but differing in the extent of plastic deformation free of intergranular cracking, combined with extensive recovery in the form of boundary migration.

Thermoforming of superplastic sheet in shaped dies

Abstract: Approximate theoretical solutions have been derived for the equations relating to thermoforming of superplastic materials by free bulging of circular and rectangular sheet, by bulging of circular sheet into conical and cylindrical dies, and by bulging of rectangular sheet into V-groove dies and between parallel walls. By defining the appropriate forming pressures such that deformation proceeds well within the superplastic-strain-rate range, these solutions can be used to predict thickness distributions in and forming times for the thermoformed parts. A number of experiments have been conducted, involving the bulging of superplastic eutectic Sn-38Pb sheet into dies of various shapes at room temperature, within the strain-rate range 0·01-0·2 min−1. These experimental results, together with other results for Zn-22Al eutectic alloy sheet formed at 250°C, taken from the literature, have been compared satisfactorily with the proposed theoretical solutions.

Grain boundary sliding model for superplastic deformation

Abstract: Comparison of the data for superplastic Pb–Sn eutectic with the existing theories on diffusion accommoda ted grain boundary sliding shows mixed agreement. In the superplastic region the observed deformation rates are about 102–103 times faster than Coble creep. In the present study, this discrepancy is resolved by a physically realistic three-dimensional grain rearrangement model for superplastic deformation. This model assumes that the ‘macroaccommodation’ during superplastic flow results in grain rearrangement due to grain boundary sliding and ‘microaccommodation’ at triple points is diffusional. The proposed model carries all the structural features of superplastic deformation and the constitutive equation predicts faster strain rates than the existing theories on diffusion accommodated grain boundary sliding. The model gives an activation energy equal to that for grain boundary diffusion. The model predicts the observed grain size dependence of the strain rate and also a threshold stress for s...

Method of eliminating core distortion in diffusion bonded and uperplastically formed structures

Abstract: Method for eliminating core distortion in diffusion bonded and superplastically formed structures or panels, wherein metal blanks or workpieces of metal, capable of diffusion bonding and superplastic forming such as a titanium alloy, preferably in the form of thin sheets of the order of 0.030 inch thick or less, are placed in contact with each other in a tooling apparatus or die, the sheets are joined at selected areas by diffusion bonding and are then expanded superplastically to form a desired panel or a truss core panel structure. In such method, the metal sheets are diffusion bonded at certain preselected areas under pressure and at elevated temperature, and the unbonded areas of the sheets are then expanded by superplastic forming into contact with the walls of the tool or die cavity, at elevated temperature e.g., 1600-1650° F. and an internal gas pressure e.g., about 300 psi, to form the desired panel. Following superplastic forming and cooling, the thin cell walls of the core panel are often distorted or crushed. As a feature of the invention, following superplastic forming and release of the internal gas pressure, and during cooling of the panel and while the metal panel is still at a temperature where the metal remains flexible or plastic, e.g., 1200°-1400° F., a low internal differential gas pressure, e.g., of about 3-7 psi, is introduced into the interior of the panel, expanding the panel out to its original form against the tool or die as a restraint, and straightening the thin core cell walls of the panel which were distorted following superplastic forming.

Development of forming limits for superplastic formed fine grain 7475 AI

Abstract: Forming limits in conventional sheet metal forming are given by strain levels obtainable prior to the onset of a localized neck or tear in the sheet. While the external appearance of such a neck is not observed in superplastic metals until strains become quite large, the formation of internal cavities could dictate the tolerable levels of strain in formed components. In this paper, these useful strain limits for a superplastic 7475 Al alloy have been explored. The approach used was to establish the influence of strain state (uniaxial, plane strain, and balanced biaxial) on the inception and growth characteristics of cavities and to correlate the extent of cavitation with material properties. Based on these data, it was then possible to establish strain states for which little or no loss in properties was observed, and thereby to define forming limits for superplastic forming this material. These results, coupled with comparisons against strains developed in actual parts as well as analytically predicted strains, show that a wide range of structural parts can be superplastically formed within the constraints of the recommended forming limits.

Ductility of the superplastic Pb-Sn eutectic at room temperature

Abstract: In the classic experiments of Pearson [1], conducted on the Bi -Sn and P b S n eutectic alloys in 1934, very high tensile elongations were obtained when test samples were pulled to failure under conditions of approximately constant stress. Fc, r example, the Bi -Sn alloy exhibited an elongation to failure of 1950% and the P b S n alloy pulled out to an elongation of 1505 %. It is surprising to note that, in general, very high elongations have not been attained in any of the many subsequent tests conducted on various superplastic alloys at room temperature. Tabulations of much of the available data are given in several reviews [2 -6 ] , and the maximum elongations reported at room temperature are usually < 1000% and invariably of the order of 213,0 to 500 %. Examples of these low elongations are provided by tests on the B i In eutectic [7], a Pb -Cd alloy [8], various Pb-T1 alloys [9], S n t wt% Bi [10], various dilute Zn-A1 alloys [11 -16 ] and the Z n 2 2 w t % A1 eutectoid [17]. In the P b 6 2 w t % Sn eutectic, the maximum elongations at room temperature are usually < 1000 % [ 18-21 ], although Avery and Backofen [22] used various heat treatments and were able to achieve a maximum elongation o f 1584%. It seems likely that the difficulty experienced in the more recent experiments in attaining very high elongations in the P b S n eutectic at room temperature is due to the use of commercial tensile testing machines operating at fairly fast strain rates. Inspection o f the early data indicates that the creep tests of Pearson [1 ] were conducted at a nominal strain rate of the order of 10 -6 sec -1 whereas the more recent tensile tests on the P b S n eutectic have been restricted to strain rates faster than about 10 ---4 sec -1 . T h u s , the present investigation was undertakel~ to examine this dichotomy in more detail by measuring the ductility of the P b S n eutectic alloy at room temperature over a range of strain rates down to < 10 -s sec -1. The P b 6 2 wt % Sn alloy was prepared from 99.999 % purity lead and 99.995 % purity tin by melting in air in a graphite crucible, chill-casting into an ingot having a thickness of 1.0 cm, and then rolling at room temperature to a final thickness of 0 .254cm. A spectrograp~c analysis revealed the following impurities in ppm: Ag 0.2, A1 0.5, Au 1, Bi 0.7, Ca 0.1, Cd 0.1, Cu2, Fe 1, In 1, Mg 0.2, Mn 0.1, Si 0.3 and T1 0.5. Tensile spedmens, having a gauge length of 1.27 cm and a gauge width of 0.64 cm, were cut from the sheet parallel to the rolling direction. Each specimen was annealed in air for 1 h, at 433 K to give an average spatial grain size, d (= 1.74 x mean linear intercept), of 6.1/~m. The specimens were tested at room temperature (298 + 2K) by pulling to fracture in an Instron machine operating at a constant rate of cross-head displacement; the tests were conducted at equivalent initial strain rates, ~, in the range from 6.6 x 10 -2 to 6.6 x 10 -6 sec -1. It should be noted that earlier work at a temperature of 413K, using the same alloy with a grain size of 6.9/~m, gave elongations o f up to > 4800% with an initial strain rate of about 10 -4 sec -1 [23]. Fig. 1 shows the calculated curves of the true stress, o, against true strain, e, for five selected initial strain rates each differeing by one order of magnitude. Each test was continued to fracture, and Fig. 2 shows the results plotted as AL/Lo(%) against the initial strain rate, ~, where AL is the total increase in length at the point of failure and Lo is the initial gauge length. These data show that the elongations are less than about 500% at initial strain rates above about 10 -4 sec ~1 in accordance with earlier reports [19-21 ], but there is a marked increase in the fracture elongations at strain rates below about 10 -4

The characteristic equation for superplastic flow

Abstract: The characteristic-area concept of steady-state flow is applied to superplasticity. It is shown that the parametric dependence of flow rate upon stress and grain size is satisfied if it is assumed that an interface reaction controls the diffusion fluxes which lead to strain development. The characteristic area is shown to be equal to the product of the grain diameter and the interfacial line-defect spacing.

Investigation on grain growth and strain rate sensitivity of a superplastic microduplex steel at 1000°C

Abstract: A microduplex stainless steel (25.7 wt% Cr-6.6 wt% Ni) was investigated to examine grain growth during static annealing and superplastic deformation at 1000° C. The grain size at a constant strain rate of 1×10−4 sec−1 increases according to d∼8t 0.49 where d is the grain size and t is the time (in min) involved in deformation. Under the present test condition, the contribution of both static (time, t S) and dynamic (strain, e) annealing appear to be significant and can be expressed by d∞ 0.19e0.29. While the exponent of the first term is constant, the exponent of the second term may depend on the strain rate. Strain rate sensitivities were evaluated from differential strain rate tests for different initial grain sizes. Both strain rate sensitivity and grain size were noticed to increase with deformation.

Superplastic Forming Characterization of Titanium Alloys

Abstract: Generalized relations between metallurgical and process parameters for superplastic forming of titanium alloys were determined from a systematic characterization of the superplasticity indices for alpha-beta titanium alloys The flow stress at a constant strain rate of different alpha-beta alloys is uniquely related to grain size, beta transus temperature, and volume fractions of constituent phases at the test temperature The effects of different compositions and microstructural modifications are implicit in the ratio of the forming temperature T to the beta-transus temperature Tβ of a specific alloy For a fixed T/Tβ ratio, regardless of the alloy composition, the logarithm of flow stress is a function of the logarithm of the product of strain rate and the average grain size A constitutive equation for the stress, time, grain size, and temperature dependence of strain rate was obtained and the equation used to construct generalized, three-dimensional plots of the relation between flow stress, strain, strain-rate sensitivity, grain size, and temperature, with consideration given to the continuous change in alloy microstructure during superplastic deformation The applicability of the constitutive equation and three-dimensional plots for predicting superplastic strain rates was confirmed by constant-stress superplastic forming tests

Method for producing superplastic aluminum alloys

Abstract: A method of forming superplastic aluminum is disclosed. The prior need for overaging prior to rolling is eliminated by rolling the aluminum beginning at a hot rolling temperature and ending at a warm rolling temperature.

Ultra high carbon steel alloy and processing thereof

Abstract: An ultra high carbon steel having a composition of carbon in an amount of from about 0.8 weight percent up to the maximum solubility limit of carbon in austenite; silicon in an amount of from about 3 to about 7 weight percent; an effective amount of a stabilizing element acting to stabilize iron carbide against graphitization in the presence of silicon; and the balance iron. Preferably, the silicon is present in an amount of about 3 weight percent, and the stabilizing element is chromium. The ultra high carbon steel may be processed to a form suitable for subsequent superplastic forming by any technique which reduces its grain size to about 10 microns or less, and preferably to about 0.4 to about 2 microns. The silicon and the stabilizing element act to produce a stable iron carbide particle array to retain the fine grain size during superplastic processing, and to increase the eutectoid temperature so that superplastic processing may proceed at high strain rates and low stress levels at elevated temperature.

The effect of grain size on ductility in the superplastic Pb-Sn eutectic

Abstract: The transition to region II at ~ ~< 10-4 sec -1 is consistent with several sets of data showing that the strain rate sensitivity increases at a testing temperature of 298 K when the strain rate is reduced below about 10-4 sec -1 [10, 12-14, 17]. However, the measurements presented earlier [18] of the elongations to fracture were conducted with a grain size of 6.1pro, and detailed experiments on the superplastic Zn-22 wt % A1 eutectoid have shown that a reduction in grain size leads to the occurrence of region II at faster strain rates and thus to higher ductilities at the faster rates of strain [19]. Accordingly, the present experiments were conducted to investigate the elongations to failure of the Pb-Sn eutectic having the smallest grain size available after the fabrication procedure. The alloy was produced from 99.999% purity lead and 99.995% purity tin by melting in air in a graphite crucible, chill-casting into an ingot with a thickness of 1.0 cm, and rolling at room temperature to a sheet thickness of 0.254cm. The final material contained 38 wt % Pb, 62 wt % Sn, and the following minor impurities in ppm: Ag 0.2, A1 0.5, Au 1, Bi 0.7, Ca 0.1, Cd 0.1, Cu 2, Fe 1, In 1, Mg 0.2, Mn 0.1, Si 0.3 and T1 0.5. The average spatial grain size after fabrication, and without any pre-test annealing treatment, was d = 1.74 x E = 3.3 pm, where /~ is the value of the mean linear intercept. The accuracy of these, and subsequent grain size measurements, was estimated as + 10%. Tensile specimens were cut parallel to the rolling direction with a gauge length of 1.27 cm and a gauge width of 0.64 cm. Each specimen was tested at room temperature (298-+ 2K), without a prior annealing treatment, by pulling to failure in an Instron machine operating at a constant rate of crosshead displacement: the equivalent initial strain rates were in the range from 6.6 x 10 -s to

Evaluation of microstructural instability during the differential strain rate test of a superplastic alloy

Abstract: Stress (σ)-strain rate (\(\dot \varepsilon \)) data of banded and elongated grain microstructures of the Pb-Sn eutectic alloy were analysed over 298 to 443 K to evaluate microstructural instability during differential strain rate tests in the superplastic region. With reference to a stable equiaxed microstructure exhibiting uniqueσ-\(\dot \varepsilon \) relation, banded structure is more susceptible to strain hardening while the elongated grain microstructure exhibits either strain softening or strain hardening depending on the test temperature. This flow behaviour is considered in terms of a change in grain size, represented by the cube root of the grain volume. Activation energy for grain growth calculated from the differential strain rate test data indicates that the activation energy depends on strain rate and type of microstructure.

Superplastic al alloy

Abstract: PURPOSE: To provide a titled alloy having an exceptionally outstanding superplastic characteristic by limiting the compsn. of an Al alloy contg. Mg and ≥1 kind among Mn, Cr and Zr and further contg. Cu. CONSTITUTION: A superplastic Al alloy has the compsn. (by wt%) contg. 3.5W 6% Mg, contg. ≥1 kind among 0.1W1% Mn, 0.05W0.35% Cr and 0.03W0.25% Zr, further contg. 0.12W2% Cu and consisting of the balance Al and unavoidable impurities. The crystal grains are fined by the effect of accelerating recrystallization and the movement and slip of the grain boundary are accelerated by the addition of Cu. As a result, the alloy exhibits the much better superplastic characteristic. The above-mentioned Al alloy is obtd. by subjecting the slab after casting to a homogenization treatment in which the slab is held at about 450W 530°C for about 1W48hr, hot rolling at about 250W530°C and about ≥30% draft, cold working at about ≥40% reduction ratio, etc. to a final plate thickness. COPYRIGHT: (C)1984,JPO&Japio

High temperature compressive behaviour of as-cast Al-Cu alloys of varying composition

Abstract: Al-Cu alloys with different copper contents ranging from 10 to 45 wt% were deformed in compression in the as-cast condition. Measurements of the strain-rate sensitivity parameterm using the crosshead speed jump technique show that alloys of composition close to the eutectic become superplastic after about 25% strain, withm increasing from about 0.2 at the beginning of deformation to about 0.5 or more, depending on composition. This transition to the superplastic state is accompanied by an important decrease of the flow stress during compression and it is associated by the breakdown of the initially lamellar structure of the eutectic phase in the highly deformed regions of the specimen. Moreover for the alloy with 37 wt% copper, the transition corresponds also to the degeneration of the dendritic primary phase and this alloy shows a particularly high value ofm and a low steady state flow stress in spite of the large volume fraction of the hard CuAl2 compound. Alloys of composition far away from the eutectic do not exhibit superplastic behaviour during compression, at least in the range of strain rate investigated and this is due to the large grain size and the important grain growth that occurs during deformation.

The influence of prestrain on ductility in the superplastic Pb-Sn eutectic alloy

Abstract: Experiments were conducted to investigate the effect of prestraining on the ductility of the superplastic Pb-62 wt % Sn eutectic alloy at room temperature It is shown that prestraining at a fast strain rate in region III leads to a decrease in the elongation to failure on subsequent testing at a slower rate in region II This decrease is due to the development of strain inhomogeneities during the prestrain There is a further decrease in elongation if a holding time is inserted between the prestrain in region III and the subsequent testing in region II It is shown also that prestraining at a slow strain rate leads to a sharp increase in the elongation to failure on subsequent testing at a fast strain rate The reasons for these trends are discussed with reference to the occurrence of grain growth and the uniformity of strain