Showing papers on "Superplasticity published in 1982"

The mechanical properties of superplastic materials

Abstract: The relationship between stress and strain rate is often sigmoidal in superplastic materials, with a low strain rate sensitivity at low and high strain rates (regions I and III, respectively) and a high strain rate sensitivity at intermediate strain rates (region II) where the material exhibits optimal superplasticity This relationship is examined in detail, with reference both to the conflicting results reported for the Zn-22 pct Al eutectoid alloy and to the significance of the three regions of flow

Influences of material parameters and microstructure on superplastic forming

Abstract: The strain rate sensitivity of flow stress has long been recognized as an important factor in determining superplastic ductility, and its relationship to high tensile elongations is well understood from the mechanics point of view. However, the measurements of this parameter and other properties of superplastic materials are challenging, and quite varied results are observed from different test procedures used. In this paper a discussion of the various characterization methods is presented, and the relationship between the superplastic characteristics and the microstructure is brought forth. The applicability of the uniaxial superplastic properties on the biaxial deformation behavior as in the superplastic forming process is addressed, including a relationship of such properties to the selection of forming parameters.

Fracture processes in superplastic flow

Abstract: There are four distinct types of fracture in superplastic materials: failure by quasistable plastic flow, failure by necking, cavitation failure, and quasibrittle failure. The characteristics of these four types are described with reference to experimental examples. Maximum elongation occurs in a superplastic material when it pulls out to a fine wire in quasi stable flow. It is demonstrated that there are two basic requirements for this type of flow: (a) a suppression of localized (but not diffuse) necking, and (b) a suppression of significant cavity interlinkage (but not necessarily of cavity nucleation and growth).

The rate-controlling deformation mechanisms in superplasticity—a critical assessment

Abstract: The phenomenon of micrograin superplasticity is critically reviewed in the context of the dominant microstructural characteristics,i.e., grain boundary sliding and migration, grain rotation and rearrangement, and dislocation activity. Existing theoretical models consider the accommodation process for grain boundary sliding to be either purely diffusional or due to dislocation motion. The latter can be in the form of individual dislocations or dislocations in the pile-up arrays in the interior of the grains or in grain interfaces. The mechanical properties,i.e., stress, strain-rate, activation energy, threshold stress, and so forth, are compared with the prediction of these models. The extreme sensitivity of the activation energy for superplasticity (which usually equals that for grain boundary diffusion) to alloy or impurity content is emphasized. The very large influence to prior thermal and mechanical history of the specimens on the mechanical data is discussed in the context of alteration of the significant details of grain boundary substructure.

Microstructural aspects of superplasticity

Abstract: This short review summarizes the contribution of microstructural studies to the present understanding of the deformation mechanisms operating during superplastic deformation. A background summary is included that describes the major features of superplasticity, the relevant deformation theories, and the requirements for successful microstructural studies. However, most of the paper is concentrated on a discussion of the microstructural evidence in the literature on two types of material. The first is a representative group of modern commercial nickel, aluminum, and magnesium alloys. The second is the Zn-Al eutectoid, a member of the well characterized “classical” group of superplastic materials based upon eutectic or eutectoid compositions. The importance of dislocation motion at maximumm is emphasized, both as an accommodation mechanism for grain boundary sliding and directly in response to the applied stress. However, it is shown that, in IN100 and Supral 150, other processes occur simultaneously, including recrystallization and diffusion.

Microstructural observations of superplastic cavitation in fine grained 7475-Ai

Abstract: Intergranular cavitation is an important consideration in the successful development of a commercially viable superplastic forming process for the high strength aluminum alloy, 7475. This work examined the microstructural features involved in the initiation stages of cavity formation. The observations suggest that, with the optimum superplastic deformation conditions, cavity nucleation is generally the rate determining step in the overall development of cavitation with strain. Cavities do not generally form at even the largest of the common single phase inclusion particles unless forming conditions are such that the flow stress significantly increases. It appears that, as well as local stress concentrations, additional effects are required, such as temperature induced particle decohesion and internal gas evolution, in order that cavities may grow to stable sizes. Such conditions may exist at certain two phase inclusion particles in the 7475 Al alloy. Suitable modifications to the standard alloy processing may therefore be devised which result in even lower rates of cavitation at the optimum superplastic forming conditions.

Advanced processing methods for titanium

Abstract: Among the topics discussed are recent advancements in Ti near-net shape technology, vacuum hot pressing of large, near-net shape spar fittings, Ti alloy sheet forming, superplastic forming/diffusion bonding of Ti alloys, the influence of H additions on the high temperature superplasticity of Ti alloys, the effects of processing on the fatigue life of Ti-6Al-4V castings, and the laser welding of a Ti alloy. Also considered are aspects of speed machining of Ti, the microstructures and mechanical properties of laser-welded Ti alloys, and the failure characteristics of cutting tools for Ti alloy machining at high speeds.

Cavitation and cavity growth during superplastic flow in microduplex Cu-Zn-Ni alloys

Abstract: A study has been made of cavity growth during superplastic tensile deformation of two microduplex α/β nickel-silvers, one a Cu-Zn-Ni alloy and the other a Cu-Zn-Ni-Mn alloy. For cavities with radii of >0.5 /gmm, measured growth rates were found to be in good agreement with values calculated on the assumption that cavity growth was controlled by viscous flow of the matrix. For smaller cavity sizes a diffusional growth mechanism could predominate. Metallography revealed that cavity morphology changed with strain in a manner consistent with diffusion-controlled growth at small sizes, and matrix deformation controlled growth at intermediate and large cavity sizes. Density studies showed that the overall level of cavitation was independent of both strain rate and temperature, and was influenced only by strain.

Superplasticity in Rapidly Solidified White Cast Irons

Abstract: Superplastic properties of three different composition white cast irons were investigated in the temperature range of 630 to 725 °C. Fine structures consisting of 1 to 2 μm ferrite grains were developed in these materials by consolidation of rapidly solidified powders at intermediate temperatures below the A1 critical temperature. Tensile elongations of 1410 pct were found for a 3.0 pct C + 1.5 pct Cr white cast iron, 940 pct for a 3.0 pct C white cast iron, and 480 pct for a 2.4 pct C white cast iron when tested at 700 °C and at a strain rate of 1 pct per minute. The superplastic white cast irons exhibited a high strain rate sensitivity exponent,m, of 0.5 and activation energies for plastic flow were found to be nearly equal to the activation energy for grain boundary self-diffusion in iron. These observations are in agreement with the creep behavior of superplastic materials controlled by grain boundary diffusion.

Effect of grain size on cavitation in superplastic Zn-Al eutectoid

Abstract: The effect of initial grain size on cavitation during superplastic deformation in two commercially available Zn-Al eutectoid alloys has been studied using metallography and precision density measurements. Cavitation was found to be minimal for initial grain sizes below about 5 μm. Superplastic deformation caused grain growth in both alloys under all testing conditions, and when the grain size exceeded about 8 μm a significant level of cavitation was produced. The grain size and extent of cavitation increased with increasing strain along the specimen gauge length, with cavities concentrated in regions adjacent to the fracture tip. Although never very large, the cross-sectional area at fracture increased with increasing levels of cavitation. It was concluded that cavitation in Zn-Al eutectoid results from incomplete accommodation of grain-boundary sliding when excessive grain growth leads to restricted grain-boundary diffusion and/or to restricted grain-boundary migration.

Structural metal matrix composite and method for making same

Abstract: A composite is produced by placing a reinforcement between foils of a superplastic metal alloy to provide stack. The stack is then heated to a temperature at which the metal alloy exhibits its superplastic properties, and pressure is applied to the heated stack. This causes the foils to flow around the reinforcement and diffusion bond together in the solid state. A structural composite is thus formed comprising a reinforcement dispersed throughout a matrix of superplastic metal alloy.

Superplastic Behavior of the Sn-Pb Eutectic in the As-Worked State

Abstract: Superplastic behavior of the Sn-Pb eutectic was studied in the as-worked state by mechanically working the cast material to varying degrees. The flow behavior was explored in the temperature range of 298 to 443 K and metallographic observations of longitudinal and transverse sections were made. The microstructure was inhomogeneous for low degrees of working, while at high reductions the grains were nonequiaxed. There was strain softening or hardening up to some strain depending on the test conditions. The grains became more equiaxed and coarser in size with increasing deformation. Due to these changes in microstructure and the nonunique stress-strain rate relation, the as-worked material is not suitable from the viewpoint of assessing the mechanisms of superplastic flow based on steady state. Strain dependency of flow stress was also observed in specimens specially processed to obtain equiaxed grains in the as-worked state.

Characterization of Fine-Grained Superplastic Aluminum Alloys

Abstract: This paper summarizes the results of some recent research on a thermomechanical method of refining the grain size in precipitation hardenable aluminum alloys and illustrates the infuence of grain refinement on several material properties. Grain refinement is achieved by deliberately introducing a large number of nucleation sites for recrystallization and by controlling grain growth after recrystallization. Recrystallization to a relatively small and equiaxed grain size has been achieved in a number of commercial aluminum alloys using these concepts. The influence of the fine recrystallized grain size on such properties as superplastic deformation, room temperature tensile properties, fatigue life, and exfoliation corrosion resistance is discussed. The results show that refinement to a grain size of 8–14 µm is sufficient to develop extensive superplasticity and to yield a small increase in tensile properties in alloys such as 7075 and 7475.

Transient titanium alloys

Abstract: Transient titanium alloys having the composition A-XH, where A is titanium or a titanium alloy, such as, Ti-8Al-1Mo-1V, Ti-5Al-2.5Sn, Ti-6Al-4V, or Ti-6Al-2Sn-4Zr-2Mo; where H represents hydrogen and where X is a weight percent value between about 0.01 and 0.5%. The alloys may be produced by heating the base titanium or titanium alloy under pressure in a non-flammable atmosphere containing hydrogen. The alloys containing hydrogen may be formed by superplastic forming techniques. After forming, the original titanium or titanium alloy can be restored by heating the formed part or structure, under vacuum, driving out the hydrogen from the alloy. Parts and structures formed from the transient alloy, and restored, retain the strength and structural integrity of the base alloy.

Cavitation and cavity sintering during compressive deformation of a superplastic microduplex Cu–Zn–Ni alloy

Abstract: A superplastic microduplex α–β nickel silver alloy has been deformed in tension and compression. Cavitation occurred during superplastic tensile flow, but not during homogeneous compressive deformation. In those regions of the compression specimens which were subjected to barrelling and transient tensile stress, however, a measurable level of cavitation was detected. Application of compressive stress caused cavities introduced during prior tensile deformation to sinter and the degree of sintering increased with increasing compressive strain, but was relatively independent of strain rate. The closure rate of cavities was found to be consistent with a viscous mechanism of sintering and was much greater than the closure rate predicted by stress-directed vacancy diffusion.

Superplastic Formability Testing

Abstract: Incremental strain rate, constant strain rate, constant stress, and cone forming tests were systematically evaluated to determine the superplastic formability of titanium alloys. The need for characterizing the strain and time dependences of superplasticity indices and the inadequacy of incremental strain-rate tests for predicting superplastic forming rates were demonstrated. A simplified biaxial conical-die forming test for superplastic forming evaluation of sheet material at elevated temperatures was developed.

Process for producing superplastic aluminum alloy strips

Abstract: Process for producing superplastic aluminum alloy strips comprising continuously casting and rolling a molten aluminum alloy containing 4.0 to 6.0% (by weight) of magnesium, 0.4 to 1.5% (by weight) of manganese, 0.05 to 0.2% (by weight) of chromium and less than 0.50% (by weight) of silicon, thereby preparing a cast strip of 3 to 20 mm in thickness, after subjecting the cast strip to annealing treatment at a temperature of 420° to 530° C., subjecting the annealed strip to the former step of cold rolling and intermediate annealing and then subjecting the intermediately annealed strip to the latter step of cold rolling until the reduction ratio reaches to a value of not less than 60%.

Inhomogeneous Deformation of Some Aluminium Alloys at Elevated Temperature

Abstract: The microstructures developed during high temperature deformation of an Al-Mg alloy have been compared with those formed in pure aluminium, and a two-phase Al-Si alloy. The variation in subgrain misorientation across the grains has been studied by SEM, and is found to be most significant in the Al-Mg alloy. The origin of scalloped grain boundaries has been investigated, and related to the local deformation in the grain boundary region, and the substructure. It is shown that although microstructures formed during deformation may suggest the occurrence of some dynamic recrystallisation in these alloys, this conclusion would not be justified.

Method for superplastic forming and diffusion bonding complex continuous structures

Abstract: FORMING METHOD OF STRUCTURAL UNIT USING SUPERPLASTIC FORMING AND BONDING INCLUDES RELEASE: THE SELECTION OF TWO COUCHES2, 4 CAPABLE OF MATERIAL CONNECTION WITH THE RELEASE AND SUPERPLASTIC FORMING THE COATING SELECTIVE WITH REGARD TO STOP AT LEAST ONE FILM SURFACES SUCH THAT FACE IS A WAY TO FACE THE SURFACES6 8 COATED MATERIAL CAN STOP FORMING REINFORCEMENT BODIES ARISING FROM PREDETERMINED LOCATIONS, THE APPLICATION SELECTIVE MATERIAL STOP FORMING A COATING ON AT LEAST ONE OF SURFACES12 14 COUCHE10 A WAY TO AVOID THE FORMATION OF ADJACENT AREAS OF WEAKNESS TO BODIES OF REINFORCING RAISED, THE FORMATION OF CONSTRUCTION SANDWICH SUCH LAYERS AND SUBMISSION OF THAT SANDWICH CONSTRUCTION STEPS TO LINK BROADCAST AND FORMING SUPERPLASTIC.

Effect of heat treatment on superplastic response of powder-consolidated nickel-base superalloy IN 100

Abstract: Superplastic deformation of samples of the powder-consolidated nickel-base superalloy IN 100 was carried out at 1311 K Before deformation the specimens were annealed at 1473 K (above the γ′-solvus) to reduce the dislocation density since the material had initially been consolidated in the range 1348–1388 K (below the γ′-solvus) and consisted of a partially recrystallized structure of γ- and γ′-grains After annealing the matrix was dislocation free and contained an almost uniform distribution of γ′-cubes Specimen microstructures were examined after deformation in the strain rate range 3 × 10−5 to 3 × 10−3 s−1 After testing at the higher strain rates a high dislocation density was observed within the grains and extensive grain-boundary cavitation had occurred At the slowest strain rate, cavitation was not so severe and the overall dislocation density was much lower and varied from grain to grain The γ′/γ morphology changed significantly during superplastic deformation The microstructural cha

Method of producing superplastic aluminum sheet

Abstract: The use of a sequentially applied warm working/cold working procedure in the conventional steps of preparing heat treatable superplastic alloys yields material which demonstrates substantially equiaxed fine grain structure and improved superplastic forming characteristics.

Superplastic alloys formed by electrodeposition

Abstract: There are provided superplastic alloys formed by electrodeposition of the alloy onto a cathode from an electrolyte containing a first metal ion, which is iron, nickel or cobalt, and a second constituent different from the first, which is iron, nickel, cobalt, tungsten or molybdenum, or a colloidal dispersoid. The products formed are fine-grain deposits free of intergranular embrittling films, and exhibit grain boundary flow at a superplastic temperature below a recrystallization temperature of the deposit. Nickel-cobalt alloys are preferred, and are deposited from halide-free sulfamate baths, with care being taken to eliminate all anode oxides from the system. In a complex structure, the approximate initial hardware contour is formed by electrodeposition, and the final structure formed by superplastic forming.

Microstructural changes during superplastic deformation of powder-consolidated nickel-base superalloy IN–100

Abstract: Changes in the microstructure occurring during superplastic deformation of the powder-consolidated nickel-base superalloy IN–100 at 1311 K are reported. The initial microstructure was shown to contain a range of recovered and recrystallized grains together with some warm-worked unrecrystallized grains of high dislocation density. Heating to the test temperature and holding produced no changes in microstructure other than dissolution of M23C6 precipitates. Prolonged annealing at the test temperature without deformation led principally both to a reduction in the volume fraction of warm-worked areas and removal of some dislocations from recrystallized grains. Deformation in the high strainrate regime (region III) produced grain refinement associated with a marked increase in dislocation density and some cavitation. Some randomization of the distribution of MC carbides on the prior particle boundaries occurred which implied that substantial grain-boundary sliding had taken place. In the regime of opti...