Showing papers on "Superplasticity published in 1980"

Microstructural and chemical transformations accompanying deformation of granite in a shear zone at Miéville, Switzerland; with implications for stress corrosion cracking and superplastic flow

Abstract: At Mieville, in the Aiguilles-Rouges Massif, granitic rocks of the basement are deformed into mylonites within a major subvertical shear zone. The ambient temperature during translation is estimated at 250° C±30° C from fluid inclusion filling temperatures in syntectonic microveins, from Δ 18O quartzilmenite of+15%, and from mineralogical criteria. Porphyroclasts of both oligoclase and orthoclase feldspar decrease from initial diameters of 20 mm and assume elliptical shapes during progressive deformation, due to recrystallisation of the margins to ultra-fine polygonal grains which extend out from the porphyroclasts in thin trails: the final stable grain size is <5 μ. The recrystallised feldspar has a composition of the parent porphyroclast,+albite, requiring relative gains of Na and losses of K+Ca compared to the precursor, and implying short range redistribution of the components during deformation. Decrease of free energy associated with the deformation catalysed change in feldspar composition, coupled with stored strain energy in the porphyroclasts may account for recrystallisation to a stable aggregate of ultrafine grain size. The long trails imply exceptionally high ductility, which, coupled with microstructural criteria, and admixture of quartz from neighbouring pure quartz aggregates by grain boundary sliding, is interpreted in terms of superplastic flow. Estimated temperatures of T/T m≈0.2 for the inferred superplastic deformation is lower by a factor of 2 than previously recorded for this flow michanism in silicates. The feldspar and quartz probably accomodated grain boundary sliding by intercrystalline diffusion. Biotite responds to deformation by bendgliding, kinking, and recrystallisation in mantles. The reaction of high-Ti parent grains to low-Ti biotite+Fe-muscovite+ ilmenite+chlorite is catalysed at all of these microstructural sites. Progressive deformation of the fine-grained products in the mantles is coupled with steady reaction to low-Fe muscovite+epidote+ sphene+rutile resulting in exceptionally ductile trails, as for the case of feldspar. Biotite grains have pervasive networks of nondisplacive intragranular fractures. At the fracture tips increase of the stress intensity has catalysed the reaction of high-Ti parent grains to low-Ti biotite+muscovite+ ilmenite which occupy the fractures. The fractures propagate and coalesce resulting in mechanical breakdown of the parent grains: these microstructures are believed to be examples of natural stress corrosion cracking. These features are also abundant in feldspar porphyroclasts where at fracture tips orthoclase→secondary orthoclase+albite, and oligoclase→secondary oligoclase+albite. Stress corrosion cracking may be significant in the steady state deformation of crustal rocks at low temperatures when intracrystalline plasticity is not generally dominant. Two way mass balance calculations utilising major and selected trace element data, reveal that deformation of the granite was essentially isochemical, involving average additions of <1 % H2O+CO2, at approximately constant specific gravity. The parameters Fe2+/∑Fe and δ18Owhole rock maintain relatively constant values across the shear zone, and this also implies limited participation of fluids in the deformation. Alkali elements and titanium display the largest percentage variation during progressive deformation, whereas SiO2, Al2O3, and P2O5, together with V, Ni, Cr,Y,Zr, and Nb remain relatively constant. All variations decrease at increasing states of deformation and this is interpreted in terms of mechanical mixing of chemical inhomogeneities of the granite precursor within the shear zone. Constraints imposed by variations in abundance of the relatively immobile elements imply that volume changes accompanying deformation in the shear zone were less than ±10%.

Suppression of Cavity Formation in Ceramics: Prospects for Superplasticity

Abstract: Ceramics exhibit macroscopic stress/strain rate relations that should lead to superplastic extension. However, premature fracture is normally encountered, due to the formation and growth of grain-boundary cavities. Thus, cavity nucleation and growth were analyzed in an attempt to identify microstructures and/or strain-rate regimes that would suppress cavity evolution and hence allow superplasticity. Analysis of cavity nucleation indicates that fine-grained materials devoid of grain-boundary amorphous phases and inclusions should sustain substantial deformation rates without nucleating cavities, especially if solid-solution additions that encourage rapid grain-boundary diffusion (while not excessively decreasing surface energy) are identified. The analysis of void growth indicates that high relative surface diffusivities are also desirable, indicating that alloy additions that do not depress (and probably enhance) the relative surface diffusivities must be selected.

Cavity growth in superplastic alloys

Abstract: A model is developed to describe the growth of voids in superplastic alloys. This is based on analyses by Hancock, Beere and Speight, and Edward and Ashby for the case of creep cavity growth controlled by plastic deformation of the matrix. It is shown that published data for the development of cavitation in several superplastic alloys fit the predictions of the model.

Hydrodynamical behaviour of a two-phase superplastic alloy: α/β brass

Abstract: Constitutive laws hitherto applied to two-phase superplastic alloys have not taken into account their two-phase nature. In order to determine the influence of each of the phases on the constitutive laws, α/β brass was studied for different phase fractions and at different temperatures. A constitutive law is found in which the strain rate is proportional to the strain rate of the softer β phase, the constant of proportionality being a rapidly decreasing function of the α-phase fraction. Thus analysed, the behaviour of this superplastic alloy resembles that of a two-phase fluid consisting of hard particles in a viscous non-Newtonian material. This similarity agrees with observations in superplastic materials, i.e. rotation of hard phases, differences in texture evolutions, and sliding along interfaces.

Mechanical Properties of Fine‐Grained Magnesium Oxide at Large Compressive Strains

Abstract: Fine-grained polycrystalline MgO specimens were deformed in compression at constant strain rates of ∼6.7×10−6 s−1 to 6.7×10−5 s−1 at 1173 to 1423 K. Both mechanical data and microstructural observations are distinctive of a diffusion-accommodated flow, where the grain-size parameter is of prime importance in enhancing plasticity. Under suitable experimental conditions, superplastic strain was achieved in specimens; no change was detected in the equiaxed-grain shape configuration. Finally, the effect of grain growth during deformation was considered; specimens of initial grain size ∼0.1 μm showed an apparent hardening which was quasi-linear with time.

Superplasticity: Mechanical and Structural Aspects, Environmental Effects, Fundamentals and Applications

Abstract: 1. Historical Introduction.- 1.1 First Observations of Structural Superplasticity.- 1.2 Early Observations of Environmental Superplasticity.- 1.3 The Revival of Interest in Structural Superplasticity.- 2. The Mechanics of Superplastic Deformation and the Assessment of Superplastic Behaviour.- 2.1 The Tensile Test.- 2.2 Constitutive Relations for Superplastic Materials.- 2.3 The Deformation Behaviour of Superplastic Materials.- 2.3.1 The tensile stress-strain curve.- 2.3.2 Tensile stress-strain rate variations.- 2.3.3 Compressive behaviour.- 2.3.4 Torsion testing.- 2.4 The Shape of the Deforming Specimen.- 2.5 Plastic Instability.- 2.5.1 The onset of necking.- 2.5.2 The geometry of neck formation.- 2.6 The Elongations of Rate-Sensitive Materials.- 2.7 The Strain-rate Sensitivity Index, m.- 2.7.1 Determination of m from the $${\sigma _t} - {\dot \varepsilon _t}$$ curve.- 2.7.2 Determination of m using change in strainrate method.- 2.7.3 Determination of m from stress-relaxation tests.- 2.7.4 Comparison of m values determined using different techniques.- 2.7.5 The physical significance of m value measurements.- 2.8 Strain-rate Sensitivity in Environmentally Superplastic Materials.- 2.9 Other Test Procedures for Studying Structural Superplasticity.- 2.9.1 Indentation tests.- 2.9.2 Measurement of internal stresses.- 2.9.3 Measurement of grain boundary shear.- 2.9.4 Damping characteristics.- 2.9.5 Miscellaneous tests.- 3 Structural Superplasticity - Experimental.- 3.1 The Mechanical Properties.- 3.1.1 Characteristics of deformation.- 3.1.2 Necessary conditions.- 3.1.3 Variables of deformation.- (a) Strain-rate.- (b) Strain.- (c) Strain-rate sensitivity index.- (d) Temperature.- 3.1.4 The influence of prior treatments and compositional variations.- 3.1.5 Elongation.- 3.1.6 Macroscopic characteristics: an assessment.- 3.2 Microstructural Studies.- 3.2.1 Qualitative metallography.- (a) Surface studies.- (b) Internal structure.- (c) P.Iarker experiments.- (d) Dynamic experiments.- 3.2.2 Quantitative metallography.- (a) Grain boundary sliding.- (b) Measurements of grain size and shape.- (c) Analysis of texture.- 3.2.3 Assessment of metallographic and topological features.- 3.3 Fracture.- 3.3.1 Macroscopic aspects.- 3.3.2 Microscopic aspects.- 3.3.3 Origin of cavitation.- (a) Phenomenological.- (b) Microstructural.- 4. Structural Superplasticity - Theoretical.- 4.1 Early Theories.- 4.2 Modern Theories.- 4.2.1 Rheological theories.- 4.2.2 Activation energy for superplastic flow.- 4.2.3 Atomistic theories.- (a) Diffusional flow mechanisms.- (b) Dislocation creep theories.- (c) Grain boundary deformation models.- (d) Multi-mechanisms.- 5. Environmental Superplasticity.- 5.1 Experimental Results.- 5.1.1 Macroscopic characteristics.- (a) Transformational plasticity.- (b) Temperature cycling.- (c) Neutron irradiation.- 5.1.2 Metallographic features.- 5.2 Theoretical Studies.- 5.2.1 Phenomenological approaches.- 5.2.2 Mechanistic models.- 5.2.3 Cavitation.- 6. Applications of Superplasticity.- 6.1 Forming Operations.- 6.1.1 Hydraulic bulging.- 6.1.2 Sheet thermoforming.- 6.1.3 Blow moulding.- 6.1.4 Deep drawing.- 6.1.5 Punch stretching.- 6.1.6. Forging and stamping.- 6.1.7 Extrusion.- 6.1.8 Dieless drawing.- 6.1.9 Powder metallurgy processes.- 6.2 Theoretical Analyses of Superplastic Forming Processes.- 6.2.1 Analysis of hydraulic bulging.- 6.2.2 Analysis of bulging into a 90 V-groove.- 6.2.3 Analysis of extrusion.- 6.2.4 Regression analyses.- 6.3. Practical Developments.- 6.3.1 Alloys.- 6.3.2 Design considerations.- 6.3.3 Tooling.- References.- Author Index.

Transformational Superplasticity of Bi2WO6 and Bi2MoO6

Abstract: Transformational Superplasticity was studied in the compounds Bi2WO6 and Bi2MoO6. The magnitudes of transitional strain are related to the (Tt/TmP), s of the phase transitions and are proportional to the externally applied stresses. Strain-rate sensitivities were similar, 0.85 and 0.86; however, the Bi2WO6 exhibited a strain-axis intercept and the Bi2MoO6 a stress-axis intercept. The grain-size effect present in the Bi2WO6 supports an accommodated grain-boundary sliding mechanism for the superplastic deformation process.

Titanium base alloy for superplastic forming

Abstract: A titanium base alloy with improved superplastic properties is provided. The alloy has 6% Al and from 1.5 to 2.5% of a beta-stabilizing element which has high diffusivity in titanium, namely Co, Fe, Cr, or Ni. In a preferred embodiment, the alloy is a Ti-6Al-4V type alloy modified by the addition of about 2% Fe.

Flow behaviour of Mar M200 powder compacts during isothermal forging

Abstract: The high-temperature deformation behaviour of Mar M200 superalloy powder compacts, pressed below the γ′ solvus, has been examined. Compression tests were carried out at temperatures between 950 and 1200°C and at constant true-strain rates between 10−1 and 10−5 S−1. Superplastic behaviour was found at all the temperatures used and at strain rates below 10−2 S−1. The strain rate sensitivities at low strain rates are in the range 0.4–0.6 while at higher strain rates the exponents fall to between 0.1 and 0.3. In each range the exponent increases with temperature, indicating higher degrees of plasticity. Variations in strain-rate sensitivities are related to the various deformation mechanisms which are thought to contribute to flow. The effects of powder particle mesh size and size distribution on the flow properties of the compacts are also considered. Coarser mesh sizes produce the highest peak flow stresses at all strain rates and temperatures investigated. Compacts made predominantly from fine mesh...

Determination of strain rate sensitivity index in superplastic Ti-6Al-4V

Abstract: * Rocky Flats specification for a weldable grade of beryllium, 1.5 pct Be maximum, similar to Brush Wellman S-100 and average grain size less than 35/Lm. 4. S. Beitscher: RFP-1205, Rockwell International, Rocky Flats, Golden, CO, December 1 l, 1968. 5. Evaluation Test Methods for Beryllium, Materials Advisory Board, Division of Engineering, National Academy of Sciences, National Research Council, Washington, DC, MAB-205M, Committee on Beryllium Metallurgy, T. L. Johnston, Chairman, March, 1966.

Method of forming a superplastic material

Abstract: 1. Method for shaping a piece from a blank (11, 12) which is placed inside a mold (1) comprising an imprint (7, 8) of the piece to be obtained and which is subjected to conditions in which its constituent material becomes superplastic said shaping resulting from a difference of pressure applied between the two faces of said blank (11, 12) so that the latter undergoes a plastic deformation to adopt the shape of said imprint (7, 8), characterized in that the said pressure difference is applied by a multitude of separate successive impulses.

Composition for fabricating superplastically formed/diffusion bonded structures

Abstract: A method for fabricating superplastically formed/diffusion bonded structures wherein metal blanks of a titanium alloy are joined at selected areas by diffusion bonding and expanded superplastically to form a desired sandwich or integrally stiffened structure. In such method, the metal blanks are treated in selected areas with a "stopoff" material to prevent bonding at those areas during diffusion bonding and to permit forming or shaping at the same areas during superplastic forming. An improved stopoff compound is provided for this purpose, in the form of yttria of relatively coarse particle size, coarser than 5 microns, in a suitable volatilizable vehicle. Such stopoff compound is inert to reactive metals such as titanium at the high diffusion bonding temperatures, and permits relatively low breakthrough pressure-time product during superplastic forming, thereby preventing excessive strain or rupture of the metal through non-superplastic deformation.

温度範囲4.2～523Kでの超塑性Al-Zn共析合金の力学的特性に対する粒径の影響

Abstract: Tension tests at 4.2 to 523K were carried out at different strain rates on the alloys having various grain sizes from 0.87 to 3.89μm. Some empirical relations are found among tensile stress σuy, strain rate e, temperature T and grain size L. A relation σuy-σj∝L holds in a high temperature region, where σj is the stress at L=0. The alloys superplastically deform in this region. The experimental data obey the phenomenological and mechanical equation of state for superplasticity: e=K[(σuy-σj)/L]nexp(-U/RT), where U is the activation energy, R the gas constant and K a constant. The Hall-Petch relation σuy∝L-1/2 holds in a low temperature region. Superplastic features are hardly observed. A transition region from superplastic to normal deformation is found at intermediate temperatures. This region moves to high temperature side as the grain size increases.

Method for joining titanium alloy

Abstract: PURPOSE: To improve the structure of titanium alloy to that showing a superplasticity phenomenon and also its diffusion joining property, by performing a heat treatment on titanium alloys having a cast structure or an abnormal structure. CONSTITUTION: When a titanium alloy having a cast structure or an abnormal structure is diffusion joined, the cast structure or the abnormal structure is improved by performing a preheating treatment on the titanium alloy by repeating heatings and coolings more than two times within a temperature range from the room temperatureW600°C to 850W1,000°C. After this preheating is performed, the diffusion joining is made on the materials while they are being maintained at a constant temperature which is lower than the β-transformation point and a pressure. In this way, the structure is improved and the diffusion joining property is improved by utilizing the superplasticity. Moreover, the mechanical characteristics of the parent metal is also improved. COPYRIGHT: (C)1982,JPO&Japio

Fine Grain Aluminum Superplasticity

Abstract: : fine grained 7475 aluminum alloy sheet was evaluated for its superplastic properties and potential for superplastic forming. The sheet alloy was specially processed by the ALCOA Technical Laboratory utilizing procedures prescribed by the contractor to achieve a fine grain size. The grain size measurements on this sheet alloy after complete recrystallization revealed a short transverse dimension of 7.8 microm and longitudinal and long transverse dimension of 14 microm. Subsequent grain growth studies showed that this fine grain size developed is quite stable at temperatures as high as 516 C . High temperature testing was conducted over the temperature range of 427 C to 516 C in order to establish the strain rate sensitivity of flow stress, strain hardening characteristics, and total elongation capabilities. Superplastic forming tests were also conducted utilizing commonly used gas pressure forming methods. Small demonstration parts were formed over a range of strain rates and thinning characteristics evaluated. Metallographic evaluation of tensile test specimens and formed parts revealed the tendency of the alloy to form internal voids, or cavities, at large strains. The cavitation appears to be sensitive to the conditions of deformation, being minimized at the higher temperature and lower strain rates.

Metallurgical Characterization of Superplastic Forming

Abstract: : A systematic characterization of the superplasticity for regular- grade Ti-6Al-4V, ELI-grade Ti-6Al-4V, simulated coil-rolled Ti-6Al-4V, Ti-3Al-2. 5V, Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al-1Mo-1V, and Ti-15V-3Cr-3Sn-3Al sheets with several different microstructures and texture was performed. The effects of alloy chemistry, grain size, volume fraction of constituent phases, and anomalous microstructures on the superplastic parameters were determined by incremental strain-rate, constant stress, constant strain rate and biaxial constant stress cone-forming tests. The strain-rate time (or equivalently, strain) dependences of flow stress and strain-rate sensitivity were identified as the most important superplasticity parameters, with the continuous changes in alloy microstructures during superplastic deformation requiring proper consideration. In the alpha-beta and near-alpha titanium alloys, the flow stress decreases and necking resistance increase with decreasing grain size at 850-950 F (1562 - 1742 F) with increasing strain and time the flow stress at constant strainrate increases and the strain rate at constant applied stress decreases as a consequence of increasing grain size. Ti-6Al-4V alloys with elongated-alpha have significantly higher flow stress than equiaxed regular grade Ti-6Al-4V. 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.