Showing papers on "Superplasticity published in 1986"

Superplasticity of yttria-stabilized tetragonal ZrO2 polycrystals

Abstract: The uniaxial tensile deformation behavior of 3 molpercent Y/sub 2/O/sub 3/-tetragonal ZrO/sub 2/ polycrystals was studied at temperatures up to 1500 C in ambient atmosphere. The tensile specimens which were elongated at constant displacement rates from 0.00011/s to 0.00055/s at 1450 C exhibited unusually large deformation. The superplastically deformed specimens retained a bending strength of 976 MPa at room temperature. Steady creep rates measured at a constant load in the range of relatively small strains were described by a stress component, n, of 2 and by an activation energy of 586 + or - 40 kJ/mol. Large deformations during the accelerating creep were accompanied by extensive formation of cavities. 19 references.

Method for diffusion bonding of alloys having low solubility oxides

Abstract: Diffusion bonding of surface layers of an alloy, such as an aluminum alloy, having surface oxides of low solubility in the alloy includes treating the surface layers to be bonded to remove existing surface oxide coatings, and diffusion bonding the surface layers to one another by placing the alloy to be bonded under sufficient pressure to cause disruption of the oxide coatings by localized surface deformation of the alloy, enhanced by a superplastic microstructure, without substantial deformation of the alloy, i.e., macroscopic deformation approaching zero percent, while heating the alloy in a non-oxidizing atmosphere for a time sufficient for diffusion bonding to occur. The alloy may be treated so that at least the surface layers thereof have a fine grain structure of the type associated with superplastic forming properties. Components may be formed by diffusion bonding and superplastic forming.

Superplastic flow in fine-grained alumina

Abstract: Dense polycrystalline alumina having an average grain size of 1.6 μm was deformed to true strains of up to 60% in simple uniaxial compression at 1693 K without significant cavitation. A strain rate of ∼2 × 10−4 s−1 was obtained at a uniaxial stress of ∼30 MPa. Such a reasonable flow stress at such high strain rates and a relatively low temperature raises a real possibility of isothermal superplastic forging of alumina ceramics. The mechanism of deformation was identified to be diffusion creep with grain boundaries as the dominant diffusion path. The specimens suffered considerable grain growth during deformation. Part of the grain growth resulted from time and temperature, but the majority was induced by deformation strain.

Investigation of the superplasticity of tin―lead eutectic by impression creep tests

Abstract: The superplasticity of the Pb-Sn eutectic was investigated in the temperature range of 283 to 340 K. The strain-rate sensitivity, the activation energy and the activation volume were determined by impression creep measurements at various strain rates and temperatures. The results obtained are in good agreement with the results of conventional unidirectional tensile tests, which show that the simple and economic impression creep test can lead to equivalent results with the conventional creep test.

Superplasticity in a thermomechanically processed High-Mg, Al-Mg alloy

Abstract: Superplastic elongations in excess of 400 pct have been observed in tension testing at 573 K (300 °C) and strain rate έ= 2 × 10-3 s-1 for a thermomechanically processed Al-10.2 pct Mg-0.52 pct Mn alloy. The thermomechanical processing consists of solution treatment and hot working, followed by extensive warm rolling at 573 K (300 °C), a temperature below the solvus for Mg in the alloy. This processing results in a fine subgrain structure in conjunction with refined and homogeneously distributed β(Al8Mg5) and MnAl6 precipitates. This structure does not statically recrystallize when annealed at 573 K (300 °C) but appears to recrystallize continuously during deformation at such a temperature and the resulting fine grain structure deforms with minimal cavitation. At temperatures above the Mg-solvus,e.g., 673 K (400 °C), recrystallization and growth occur readily resulting in rela tively coarser structures which deform superplastically but with extensive grain boundary sliding and cavitation.

The influence of thermomechanical processing variables on superplasticity in a High-Mg, Al-Mg alloy

Abstract: The superplastic response of an Al-10.2 pct Mg-0.52 pct Mn alloy, warm rolled at 573 K (300 °C), may be enhanced by annealing at a lower temperature, 473 K (200 °C), prior to subsequent stressstrain testing at a warm temperature,e.g., 573 K (300 °C). This enhancement is attributed to recovery and precipitation during annealing effectively retarding continuous recrystallization and growth during subsequent deformation. The finer resultant structure more readily sustains superplastic flow processes. The warm-rolled structure, a refined subgrain structure in conjunction with fine β (Mg5Al8) and MnAl6 precipitates, will statically recrystallize only upon heating to a temperature above the Mg solvus, such as 713 K (440 °C). Such recrystallization results in a relatively fine-grained material but also suppresses warm-temperature superplasticity; deformation at 573 K (300 °C) takes place by dislocation flow in the recrystallized material. At temperatures above the Mg solvus, both warm rolled and recrystallized materials exhibit superplastic elongations but cavitate extensively as a result of deforming by boundary sliding.

Superplastic deformation characteristics of microduplex Ti–6Al–4V alloy

Abstract: A study has been made of the superplastic tensile behaviour of a microduplex Ti–6AI–4V alloy in the temperature range 760–940°C. Maximum superplastic elongations were obtained for a temperature of 880°C at which the volume fraction of β–phase was approximately 0·4. At temperatures below about 850°C, the main microstructural changes which occurred involved dynamic grain refinement and cavitation. The cavities, which occurred at α/α and α/β boundaries, interlinked parallel to the tensile axis at high strains. However, the volume fractions of cavities observed, although not quantitatively assessed, were always relatively small and had no significant effect on the fracture behaviour of tensile specimens, all of which pulled down to a fine point or chisel edge at failure. At temperatures greater than 850°C, no cavities were observed except near the fracture surface, and the most important structural change was grain growth. At all the temperatures examined, the structural changes which occurred during...

The development of cavity growth maps for superplastic materials

Abstract: Superplastic alloys usually deform to very large extents but excessive cavitation can lead to premature cavitation failure in these materials. Several mechanisms can contribute to the growth of cavities during superplastic deformation although, generally, there is only one mechanism that controls cavity growth. Cavity growth mechanisms of relevance to superplastic materials are analysed in detail and the possible transitions in rate-controlling cavity growth mechanisms are considered. The contribution of lattice diffusion to the diffusional growth of cavities is included in the overall analysis of cavity growth. Cavity growth maps are constructed to show the dominant cavity growth mechanisms under different experimental conditions. Equations are developed to predict the appropriate transitions in cavity growth mechanisms with increasing cavity radii. Finally, it is demonstrated that the predictions of the cavity growth maps are consistent with the experimental results in several superplastic materials.

Superplastic deformation characteristics of two microduplex titanium alloys

Abstract: A comparison of the superplastic deformation behaviour of Ti-6Al-4V (wt%) between 760 and 940‡ C and Ti-6Al-2Sn-4Zr-2Mo between 820 and 970‡ C has been carried out on sheet materials possessing similar as-received microstructures. High tensile elongations were obtained with maximum values being recorded at 880‡ C for Ti-6Al-4V (Ti-6/4) and at 940‡ C for Ti-6Al-2Sn-4Zr-2Mo (Ti-6/2/4/2), under which conditions both alloys possessed aΒ phase proportion of approximately 0.40. For a given deformation temperature the Ti-6/4 alloy had a slightly lower flow stress than the Ti-6/2/4/2, and this was attributed to the lowerΒ phase proportion in the latter alloy. However, at the respective optimum deformation temperatures the Ti-6/2/4/2 alloy had the lower flow stress. The results show that suitably processed Ti-6/2/4/2 alloy is capable of withstanding substantial superplastic strains at relatively low flow stresses, although the optimum deformation temperature is higher for this alloy than for Ti-6/4 material possessing a similar microstructure.

Effect of strain rate sensitivity and cavity growth rate on failure of superplastic material

Abstract: Necking development and fracture strain of superplastic material under tensile load are analysed by introducing a model of cavity growth into the long wavelength approximation analysis which can describe the external neck development of specimens during deformation. The results show that both strain rate sensitivity m and cavity growth rate η have an important influence on the fracture strain of superplastic material. According to these results, a fracture diagram is presented in m–η coordinates, which is divided into three: a region in which material fails by macroscopic external necking, a region where cavity growth is predominant leading to fracture without pronounced external necking, and an intermediate region where both fracture modes occur. The prediction of fracture strain for various superplastic alloys exhibiting cavity growth during deformation is in good agreement with experimental results. The present analysis thus enables quantitative prediction of the effects of both strain rate se...

Superplastic aluminum alloys containing scandium

Abstract: Superplastic forming of aluminum work stock is improved by including therein about 0.05% to about 10% or 15% scandium. In preferred practices, soluble elements such as magnesium are also included in the aluminum alloy.

Superplastic creep of fine-grained BaTiO3 in a reducing environment

Abstract: Commercially available BaTiO3 powders were hot-pressed 15 min at 1250★C to yield dense (≥99.5% of theoretical), fine-grained (∼0.45-pun average) poly-crystals. These were creep tested under compressive 5- to 60-MPa applied stresses at 1150★ to 1250★C. True strains reaching -0.50 were achieved without significant damage or structural change in the material. The observed flow behavior and microstructures are consistent with siructural superplastit. ity being the dominant deformation mode under the present testing conditions.

Superplastic Forming and Diffusion Bonding of Titanium Alloys

Abstract: New and advanced fabrication methods for titanium components are emerging today to replace age-old fabrication processes and reduce component cost. Superplastic forming and diffusion bonding are two such advanced fabrication technologies which when applied individually or in combination can provide significant cost and weight benefits and a rather broad manufacturing technology base. This paper briefly reviews the state of understanding of the science and technology of super plastic forming of titanium alloys, and their diffusion bonding capability. Emphasis has been placed on the metallurgy of superplastic flow in two phase titanium alloys, the microstructural and external factors which influence this behaviour.

Y2O3-安定化ZrO2/Al2O3複合体の圧縮変形

Abstract: Compressive deformation of 80wt% ZrO2 (3mol% Y2O3)/20wt% Al2O3 composite was studied in air at temperatures from 1400° to 1550°C Extraordinary large deformation (up to -1.8) was observed at strain rates from 10-3 to 10-4s-1. The stress exponent was from 1.2 to 2 and activation energy was 620±40kJ/mol. It was observed that the Al2O3 aggregate evolved during the deformation. This phenomenon was explained by the grain rearrangement model as a consequence of grain switching doring the superplastic flow.

Mechanism of plastic void growth during superplastic flow

Abstract: Existing models for plastic hole growth have been re-evaluated in terms of their applicability to superplastic flow. The Cocks and Ashby model is modified to include more properly the effect of void shape, and the effect of simultaneous strain hardening is also discussed. Some new experimental data on the growth of artificial holes drilled in a sheet sample of Coronze 638 are presented and compared with data on the development of general cavitation damage in the same material without artificial holes. The difference between the two sets of results is explained in terms of a coalescence effect during general damage. Cavitation rates are found to be independent of strain in both sets of experiments. This result is explained by including the effect of strain hardening (brought about by strain enhanced grain growth) in the models.MST/461

Advanced High-Temperature Alloys: Processing and Properties

Abstract: Achievements in high-temperature metallurgy, solidification, and metals processing are highlighted in 16 conference papers. The first section is on solidification. It discusses direct casting of coilable ferrous alloy strips, metallurgical advances in investment casting technology, and the development of single-crystal superalloy turbine blades. The interface of rapidly solidified materials and particle metallurgy is presented by atomization models and mechanisms. Also covered in this second section are rapidly cast crystalline thin sheet materials and mechanical alloying for preparing superalloys. Another section looks into advanced mechanical processing. It reviews the role of hot isostatic pressing, advances in superplastic materials, and thermomechanical processing of Inconel 718 and its effects on properties. The final section deals with the trends and needs of high-temperature materials, superalloys in 2001, titanium aluminides as future turbine materials and creep damage. The information available through these proceedings will give the reader an updated look at high-temperature materials.

Superplastically formable aluminum alloy and composite material

Abstract: Superplastically formable aluminum alloys and composite materials are prepared from rapidly solidified, coarse aluminum powder of a precipitation hardenable alloy, processed to have a low oxide and contaminant content. The powder is mixed, together with reinforcement in the case of the composite material, and then consolidated and extruded at a high extrusion ratio to promote microstructural uniformity and to break up the surface oxide present on the particles. The extrusion is then thermomechanically processed to impart a recrystallized fine-grain aluminum microstructure which is suitable for use in superplastic forming. The unreinforced powder alloy exhibits uniform elongations of over 800 percent at a strain rate of 2×10-4 per second, and a composite having 0.10 volume fraction silicon carbide reinforcement exhibits uniform elongations of over 450 percent at the same strain rate.

The effect of strain rate, temperature and texture on anisotropic deformation in Ti-6Al-4V

Abstract: The effect of temperature and strain rate on the transition between grain-boundary sliding and slip-controlled deformation in Ti-6AI-4V at elevated temperatures has been determined by measuring the strain anisotropy. The anisotropy below the transition was dependent on texture, but above the transition was dependent on contiguousα-phase.

Superplastic ferrous duplex-phase alloy and a hot working method therefor

Abstract: A superplastic hot working method for a duplex-phase, nitrogen-containing ferrous alloy and stainless steel, and a superplastic duplex-phase ferrous alloy are disclosed. The ferrous alloy comprises: at least one of Si and Mn in an amount of not less than 0.5% and not less than 1.7%, respectively; and N: at least 0.01% in solid solution, wherein Si eq and Mn eq which are defined as: Si eq=Si+(2/3)(Cr+Mo), and Mn eq=Mn+2 Ni+60 C+50 N, satisfy the formula: (5/6)(Si eq)-15/2≦Mn eq≦(11/5)(Si eq)-77/5, and its superplastic hot working is carried out by deforming the alloy heated to 700°-1200° C. at a strain rate of 1×10-6 S-1 to 1×100 S-1. In another aspect, superplastic hot working of a duplex-phase stainless steel comprising Cr: 10.0-35.0%, Ni: 2.0-18.0%, Mo: 0-6.0%, and N: 0.005-0.3% and having the values of Si eq and Mn eq as above is carried out by deforming the steel at a strain rate of from 1×10-6 S-1 to 1×101 S-1 after heating to a temperature of at least 700° C. and at most 100° C. below the temperature at which the steel transforms into a single ferrite phase, preferably in a non-oxidizing nitrogen atmosphere.

Superplastic forging of zirconia ceramics

Abstract: The invention relates to producing relatively flaw free zirconia ceramic shapes requiring little or no matching by superplastic forging of a fully sintered or partially sintered preform. The preform is prepared by providing a starting powder comprising by weight from about 70% to about 100% zirconia and from about 0 to about 30% of stabilizing additives, e.g. commerically available TZ-3Y powder, cold pressing to obtain a preform of green density ranging from about 30% of theoretical density to about 75% of theoretical density and partially or fully sintering at atmospheric pressure, e.g. in argon or air at a temperature ranging from about 900° C. to about 1900° C. The superplastic forging is carried out by isothermally pressing the partially or fully sintered preform into substantially final shape utilizing a temperature and strain rate such that surface cracks do not occur, e.g. utilizing a temperature of 1310° C., a substantially constant displacement rate based on an initial strain rate of 7×10-3 seconds -1 and open die or closed die processing in a graphite or metal die.

Superplastic deformation of Al-Li-Cu-Mg alloy sheet

Abstract: Al-2.5 Li-1.2 Cu-0.6 Mg-0.12 Zr (wt%) alloy sheet was cold-rolled, solution heat-treated for 20 min at 510° C, prestrained by 3% and superplastically deformed at 450 to 540° C at strain rates between 1×10−4 and 2.8×10−1 sec−1. The maximum elongation obtained was 300%. Significant cavitation occurred above about 0.5 strain at a rate (void volume/unit strain) of 4% at 540° C and 6% at 500° C. The onset of cavitation coincided with a reduction in the room-temperature tensile properties after reheat-treatment. During annealing at 500 to 540° C, grain coarsening near the sheet surface was associated with magnesium and lithium depletion. Superplastic deformation produced a fine equiaxed microstructure by dynamic recrystallization.

Pressing contoured shapes

Abstract: A process for forming superplastically deformable sheet metal into contoured shapes, especially complex saddle shapes having compound curvature, utilises the technique of pressure forming in combination with a forming tool with the sheet metal in condition for superplastic deformation. A preform of superplastically deformable sheet metal is first formed and then sealed to a pressure box. A forming tool is engaged with the preform face external of the pressure box and, under conditions for superplastic deformation, the sheet metal is pressure-formed into conformity with the forming tool. Apparatus for performing the process is also disclosed.