Showing papers on "Superplasticity published in 1981"

Creep mechanism in Mg2GeO4: Effects of a phase transition

Abstract: We deformed polycrystalline Mg2GeO4 olivine and spinel in a Griggs-type solid medium apparatus. Flow of Mg2GeO4 olivine can be represented by = 6.5×l07 σ3.5 exp(−105(kcal/mol)/ RT) where σ (kbar) is the differential stress and (s−1) is the natural strain rate. For Mg2GeO4 spinel the flow law is = 2.6×104 σ2exp(−73(kcal/ mol)/RT). The low stress exponent and activation enthalpy coupled with fine grain size (3 μm) suggest that Mg2GeO4 spinel deformed by a superplastic mechanism. Flow parameters for the olivine phase suggest a dislocation creep mechanism. Comparison of theoretical superplastic flow laws for Mg2GeO4 olivine with the spinel phase data suggests that strain rates in Mg2GeO4 spinel are only about a factor of 3 lower than for Mg2GeO4 olivine of the same grain size. A similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion. Transformation from Mg2GeO4 olivine to spinel reduced grain size to approximately 3 μm. Thus we might expect a similar reduction in grain size in the earth's transition zone which could result in superplastic deformation of the transformed phase and cause a weak ‘decoupling’ zone at the transition boundary in the mantle. Superplasticity brought about by transformation-induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle.

Process for fabricating integrally bladed bimetallic rotors

Abstract: An integrally bladed bimetallic rotor for high temperature operation having airfoils with good stress rupture and creep strength and a hub with high tensile strength and good low cycle fatigue resistance is made from a preform of two concentric cylinders of different superalloy metals metallurgically bonded to each other at their interface, both materials being in such a condition that they exhibit superplastic behavior at a certain, controlled strain rate and temperature without incurring substantial grain growth. The metal of the outer cylinder is selected such that it has a lower gamma prime solvus temperature than the metal of the inner cylinder. The preform is isothermally forged at the controlled strain rate and temperature to form the airfoils from the alloy of the outer cylinder and to form the hub from the alloy of the inner cylinder. A sharp, reproducible interface is created between the two alloys. After forging, the rotor is heated to a temperature between the gamma prime solvus temperatures of the alloys and under conditions which cause full gamma prime solutioning of the airfoils up to the interface but only partial gamma prime solutioning of the rotor hub. The resulting airfoils have coarse grains and the hub has fine equiaxed grains.

Microstructural instabilities during superplastic forging of a nickel-base superalloy compact

Abstract: The high temperature flow behavior of a nickel-base superalloy powder compact, prepared by hot isostatic pressing has been examined by means of uniaxial compression testing in terms of the microstructures developed during plastic flow. The tests were done isothermally at 1050 and 1100 °C and at constant true strain rates between 10-5 s-1 and 1 s-1. The fine grained compact exhibits some degree of superplasticity which always increases with compressive flow as the grain structure is refined. The faster the rate of deformation, the finer is the grain size produced at high strains, when steady state conditions of flow appear to develop. By deforming to different strains at a given strain rate or into the steady state regimes at various strain rates, grain sizes in the range 1 to 5 pun were produced. By unloading and restraining the test pieces in situ, the effect of grain size on the onset of plastic flow has been examined and the yield stress observed to increase with grain size. It is shown that, in this material, hardening or softening occurs during flow depending on the size of the initial grains. The changes in microstructure and flow stress observed during deformation are analyzed and the potential offered for control of the microstructure during isothermal forging is discussed.

Effect of phase proportions on deformation and cavitation of superplastic ?/? brass

Abstract: Studies have been made, using metallographic and precision density techniques, of the deformation and cavitation behaviour during superplastic tensile straining at 873 K of three microduplexα/β brasses which, as a consequence of varying composition, contained varying proportions ofα andβ phases. It was observed that both strain-rate sensitivity and elongation-to-failure passed through a maximum when approximately equivolume proportions of the two phases were present. Cavitation, on the other hand, decreased rapidly as the volume fraction ofβ phase was increased. The cavitation behaviour was attributed to the relative abilities of the phases to accommodate grain boundary sliding. When a high proportion ofα phase is present accommodation is minimal and cavity nucleation. occurs readily. Evidence is presented to show that grain-boundary sliding plays a predominant role in cavity growth. When a high proportion ofβ phase is present accommodation is almost complete and cavity nucleation is minimal.

Delta-alpha bond/superplastic forming method of fabricating titanium structures and the structures resulting therefrom

Abstract: Method of manufacturing structures from sheets of material having superplasticity as well as diffusion bonding capabilities wherein a combination of diffusion bonding and superplastic forming is performed concurrently with the use of low pressures and a simple heat furnace. The material to be formed and bonded is placed between and sealed in tooling constructed of materials having different degrees of thermal expansion. A low pressure from an external source is applied between the sheets of material to be formed during heating. Forming shims positioned within the tooling provide areas of diffusion bonding when the applied pressure and differential of tooling expansion force the sheets of material together at the shim locations.

The influence of texture on superplasticity of the titanium alloy VT 6

Abstract: This paper describes a specific influence of crystailographic texture on tensile properties of the titanium alloy VT6 under superplastic flow conditions. The texture effect has been examined on alloy states with identical microstructures but with different preferred grain orientations. Tests over wide temperature and strain-rate intervals have shown that the formation of a strong texture as a result of pretreatment of a titanium alloy leads to a decrease in flow stress, an increase in plasticity and a shift of the optimum strain-rate region to higher rates. Data on the effect of texture on the anisotropy of properties of the VT6 alloy are also presented. The present results suggest that the influence of preferred orientation on the characteristics of superplastic flow is a general phenomenon for finegrained superplastic materials.

A possible mantle instability due to superplastic deformation associated with phase transitions

Abstract: Laboratory deformation studies of metals and some oxides indicate that these solids are easily deformed while undergoing a phase transformation. This superplastic effect, if it occurs for mantle silicates, may result in an instability of mantle flow. The instability is illustrated by considering a sheared layer of viscous fluid containing a phase transition. With a simple idealization of the phase transition weakening effect, an oscillatory fluid motion which cycles material through the phase transition is shown to reduce the rate of viscous dissipation associated with shearing the fluid layer. Some possible consequences for mantle flow include the partial decoupling of mantle layers at phase transitions and short wavelength free air gravity anomalies due to induced normal stress variations at the base of the lithosphere.

Method for fabricating superplastically formed/diffusion bonded aluminum or aluminum alloy structures

Abstract: A method for fabricating superplastically formed/diffusion bonded structures wherein metal blanks of aluminum or an aluminum alloy having an oxide surface layer are joined at selected areas by diffusion bonding and expanded superplastically to form a desired sandwich or integrally stiffened structure. According to such method, a suitable mask is applied to selected areas of the aluminum or aluminum alloy blanks. The assembly is then placed in a vacuum chamber and at a suitable low pressure and suitable voltage a glow discharge is created between the metal blank or blanks functioning as cathode or sputtering target, and an anode to selectively remove the oxide from the metal blanks and deposit the oxide debris on the anode. After the oxide layers have been removed from the surfaces of the blanks to be bonded, such surfaces are brought into intimate contact within the sputtering chamber or in an area equipped with heated platens, and while preventing reoxidation of the cleaned surface areas of the metal blanks, such blanks are subjected to press pressure and heat. The cleaned oxide-free areas bond, while the remaining areas, protected by the natural oxide and covered by the masks, do not bond. Such masks can be removed after sputtering and either prior to or during die bonding. If desired, the aluminum or aluminum alloy blanks can be initially anodized, to provide a thicker bond resistant oxide layer. The die bonded metal blanks are then subjected to superplastic forming in known manner to fabricate an integrally stiffened or sandwich structure.

Compressive creep of alloys of the ZrC-ZrB2 and TiC-TiB2 systems

Abstract: A study was made of the compressive creep of two-phase alloys of the ZrC-ZrB2 and TiC-TiB2 systems at temperatures of 1700–2420°C and stresses of 5–30 MPa. In the ZrC-ZrB2 system two-phase alloys in a wide range of carbide phase concentrations — from 20 to 70 mole% are characterized by a creep rate exceeding by one to two orders the creep rates of their individual components, while in the TiC-TiB2 system the highest creep rate is exhibited by alloys with carbide contents of 30–50 mole %. The effect of contamination with tungsten carbide on the creep of alloys of the systems investigated was determined. In TiC-TiB2 alloys it is possible for coherent phase boundaries to form and for superplastic creep phenomena to manifest themselves, involving also the operation of threshold mechanisms of plastic deformation.

Superplasticity in a tool steel

Abstract: A 01 tool steel has been studied for its superplastic characteristics for three different structures: (a) a coarse spheroidized-annealed structure; (b) a structure of fine spheroidized cementite pa...

Fundamentals of Superplasticity and Its Application

Abstract: The term superplasticity has been used to describe extraordinary elongations (several hundred percent) obtained during tensile deformation of polycrystalline materials. Mostly a scientific curiosity fifteen years ago, superplastic materials are now being used in a number of industrial applications, including near-net-shape forming and solid state welding. Two types of superplastic flow have been observed: internal stress superplasticity and fine structure superplasticity. Both rely on one common characteristic: a high sensitivity of the flow stress to the strain rate. This is a necessary but not sufficient condition for superplasticity. The various structural prerequisites for fine structure superplasticity are evolving rapidly and the phenomenology of superplastic flow is well documented. This knowledge, coupled with advances made in understanding normal plastic flow in crystalline solids (diffusion controlled dislocation creep), permits predicting methods for enhancing and optimizing superplasticity in materials.

Superplastic behaviour of powder-consolidated nickel-base superalloy IN-100

Abstract: The superplastic behaviour of the powder-consolidated nickel-base superalloy IN-l00 has been determined in the temperature range 1193–1311 K and over a range of strain rates from ∼ 10−5 to ∼ 10−1 S−1. The results showed that characteristically three regimes of behaviour occurred. At low strain rates the strain-rate sensitivity was low and work hardening occurred throughout the tests. At intermediate and high strain rates the strain-rate sensitivity index m was >0.6 and work softening was observed. At the highest strain rates m fell to ∼0.2. The activation energy of deformation was high and greater than that expected for volume diffusion. At high strain rates the deformation appeared to be controlled by dislocation creep. It was not possible to identify unequivocally the deformation mechanism in the superplastic regime although the evidence favoured a combination of grain-boundary sliding and Coble creep.

A microstructural examination of the flow behaviour of a superplastic copper alloy

Abstract: There are important differences in the microstructures of specimens of a superplastic copper alloy deformed in the three regions of flow associated with superplasticity. There is very extensive dislocation activity at high strain rates in Region III, whereas at intermediate and low strain rates in Regions II and I the dislocation density is low and many of the grains appear to be dislocation-free. Measurements show that grain-boundary sliding is important in Region II but decreases in magnitude in the less superplastic Regions I and III.

Thinning control in superplastic metal forming

Abstract: Flat metal stock is contoured into a shaped blank prior to being superplastically formed into a finished part. The stock is shaped in steps.

On the uniqueness of stress-strain rate relation for superplastic flow of the Pb-Sn eutectic

Abstract: The uniqueness of stress-strain rate relation in superplastic deformation of the Pb-Sn eutectic alloy for different grain sizes and test temperatures has been explored after subjecting the deformation processed alloy to a standard tensile prestrain. About 30 pct elongation at 421 K was found to eliminate the mechanical and microstructural instabilities that are characteristic of the as-worked material. After this prestraining, the flow stress was uniquely dependent on strain rate for a given grain size and test temperature, irrespective of the cross head speed, the path through which a particular strain rate is achieved, or the strain.

Induced or constrained superplastic laminates for forming

Abstract: A metal laminate possessing high strength characteristics as well as good ductility and malleability, comprising one or more layers of a superplastic material metallurgically bonded to one or more layers of a non-super plastic material; and the method of preparing the laminate to induce plastic or super plastic characteristics to the nonsuper plastic material.

Methods of enhancing superplastic formability of aluminum alloys by alleviating cavitation

Abstract: Disclosed is a method of forming metallic materials exhibiting the phenomenon of superplasticity, particularly high strength aluminum alloys, by the technique of superplastic forming by focing an alloy blank into a die cavity using opposing fluid pressures, the pressure on the die side of the blank initially balancing the pressure on the opposite blank side, the die side pressure being reduced as the other pressure is maintained constant or increased to deform the blank at a combined high pressure whereby precision molding occurs as the blank engages the die intimately and cavitation in the blank is reduced or alleviated.

Superplasticity of commercial aluminum alloys

Abstract: Superplasticity of aluminium alloys was first studied in the Al-33%Cu eutectic alloy [5.1]. After an appropriate treatment, i.e., hot processing, the alloy acquires a microduplex structure and a relative elongation of 2000% at 500°C. Later, superplasticity was observed in other eutectic and near-eutectic aluminium-based alloys, viz., Al-Si, Al-Cu-Mg, etc. [5.1]1.

Forming metal articles

Abstract: A method of forming metal articles comprises placing a pair of sheet members 1, 2 or superplastically deformable material in opposed, spaced apart relationship, (conveniently by means of a reinforced member 3), heating them to the temperature range within which superplastic deformation can take place, and urging portions of the sheets toward each other until they lie one against the other thereby to effect bonding. The bonding is preferably by means of diffusion bonding.