Showing papers on "Superplasticity published in 1990"
TL;DR: Newnham et al. as mentioned in this paper showed that the selection of an appropriate phase and the tailoring of the grain boundary or liquid-phase composition can sometimes drastically alter the cavitation resistance.
Abstract: Superplastic structural ceramics (Y-TZP, A1203, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or a phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain-growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second-phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain-boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain-boundary cracking during large strain deformation. Existing evidence suggests that a lower grainboundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid-phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by R. E. Newnham-contributing editor
463 citations
TL;DR: Superplasticity is defined phenomenologically as the ability of a material to exhibit exceptionally large elongations during tensile deformation, and is well established for metals and alloys.
Abstract: SUPERPLASTICITY is defined phenomenologically as the ability of a material to exhibit exceptionally large elongations during tensile deformation1. It is a property of some poly crystalline solids, and is well established for metals and alloys2. Superplasticity has also been observed in some ionic crystals, such as Y2O3-stabilized tetragonal ZrO2 polycrystals3,4, but has not been found previously for covalent crystals. Here we report superplastic elongation (by more than 150%) of a covalent crystal composite, Si3N4/SiC. The superplasticity is probably related to the presence of an inter-granular liquid phase. Combined with its hardness, this property suggests several useful applications for the novel material: for example, to form engine components—superplasticity will make it readily mouldable at high temperatures.
323 citations
TL;DR: In this paper, the grain growth kinetics in nanocrystalline materials are examined. But, the initial particle size in green body compacts is not always indicative of the final grain sizes in fully sintered ceramic bodies.
Abstract: This paper reports on scientific interest in ultra-fine grained powders for processing of ceramic components motivated by the possibilities for the enhancement of sintering rates, reduction in flaw sizes and low-temperature superplastic deformation. Previous works have developed a technique, which combines the methods established of inert gas condensation of small particles and in situ powder compaction, for synthesizing materials with grain sizes {lt}10 nm. It has been shown that this method can be adapted for the production of ceramic nanocrystalline particles. Subsequent work has demonstrated that enhanced sintering and superplastic deformation is possible in nanocrystalline ceramics (TiO{sub 2}), but not without significant grain growth. Control of grain growth, however, is necessary if the capability for synthesizing nanoscale powders is to have benefit for structural applications. It is well known that the initial particle size in green body compacts is not always indicative of the final grain sizes in fully sintered ceramic bodies. This study was initiated to examine grain growth kinetics in nanocrystalline materials. TiO{sub 2} was selected for this initial study since sintering, deformation and diffusion data are available.
174 citations
TL;DR: In this article, the microstructure and deformation characteristics of a fine-grained superelastic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have been investigated.
Abstract: The microstructure and deformation characteristics of a fine-grained superelastic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have been investigated. Both hot indentation and tensile tests were carried out at temperatures between 1273 and 1923K over the strain rate range from 2.7 × 10−5 to 2 × 10−3 s−1. It was found that the material exhibited extensive plasticity at temperatures higher than 1473K; a maximum tensile elongation of over 800% was recorded. Microstructural examination did not indicate the presence of a glassy phase at grain boundaries. Yttrium, however, was found to segregate to the grain boundaries. The microstructure of the Y-TZP was thermally unstable and appreciable grain growth was observed at emperattures higher than 1723 K; the grain growth was enhanced by external stresses, i.e. dynamic grain growth was observed. Grain growth at elevated temperatures resulted in apparent strain rate sensitivity exponents of approximately 0.33 at 1723K. This value decreased with increasing temperature. The grain size-compensated strain rate, however, was found to depend approximately on the square of the flow stress, i.e. to exhibit a true strain sensitivity value of 0.5, which suggests a grain boundary sliding mechanism. Microstructures from samples that were deformed superelastically indicated that grains remained equiaxed; this observation is consistent with a grain boundary sliding mechanism. The activation energy for superplasticity, under the conditions of constant structure, in Y-TZP was calculated to be 720 kJ/mol.
157 citations
TL;DR: The mechanical properties of Al-(Mg)-0.5Sc alloys have been investigated in this article, where room-temperature tensile and toughness properties were found to reflect a superposition of the properties of both Al-Mg and Al-3Sc. The properties of these alloys demonstrate an extremely attractive combination of strength, toughness, density, and superplastic forming fabricability.
Abstract: The mechanical properties of Al-(Mg)-0.5Sc alloys have been investigated. Room-temperature tensile and toughness properties were found to reflect a superposition of the properties of Al-Mg and Al-0.5Sc alloys and are quite competitive with high-performance Al alloys. A combination of substructure refinement by Mg and stabilization by Al3Sc precipitates produces exceptional superplasticity as exemplified by superplastic forming (SPF) elongations in excess of 1000 pct at a strain rate of 0.01 s-1. Overall, these alloys demonstrate an extremely attractive combination of strength, toughness, density, and SPF fabricability.
144 citations
TL;DR: In this paper, the relationship between titanium alloy microstructures and mechanical properties is discussed, and developments in high temperature processing, including superplastic forming and isothermalforging, and the use of hydrogen as a temporary alloying addition are considered.
Abstract: The relationships between titanium alloy microstructures and mechanical properties are reviewed. The influence of hot working on microstructure development is discussed, and developments in high temperature processing, including superplastic forming and isothermalforging, and the use of hydrogen as a temporary alloying addition are considered. Recent work aimed at modelling the high temperature deformation and microstructure development is also examined.MST/1265
136 citations
TL;DR: A review of the major characteristics of superplasticity in ceramics can be found in this paper, where it is shown that there are both similarities to and differences from metals.
Abstract: It is now recognized that superplasticity is a potential deformation process in ceramics. This review summarizes the major characteristics of superplasticity and examines the reports of both transformation and structural superplasticity in ceramic and other non-metallic materials. It is shown that there are both similarities to and differences from metals. Similarities include the variation of strain rate with stress and grain size, but an important difference is the necessity to consider the role of intergranular glassy phases in ceramics. Superplasticity is also important in intermetallic compounds, and in geological materials where there is evidence for superplastic deformation both in laboratory experiments and in natural deformation.
128 citations
TL;DR: In this paper, the deformation inhomogeneities induced by the ceramic particles are examined, and the role of the particles in the nucleation and growth of recrystallised grains during subsequent heat treatment is considered.
Abstract: The development of microstructure during the deformation and annealing of discontinuously reinforced metal-matrix composites is discussed with particular reference to composites having a pure aluminium matrix. The deformation inhomogeneities induced by the ceramic particles are examined, and the role of the particles in the nucleation and growth of recrystallised grains during subsequent heat treatment is considered. The effect of the ceramic particles on the microstructures developed during multipass hot rolling is shown to be complex, depending on particle size and volume fraction. Superplasticity is shown to occur by several mechanisms in particulate composites. The effect of deformation processing on the ceramic is discussed, with emphasis on the fracture and realignment of ceramic particles, platelets, and whiskers.MST/1299
124 citations
TL;DR: In this article, a Si 3 N 4 whisker-reinforced 2124 Al composite fabricated by a simple powder metallurgical method without TMP could produce superplastic behavior.
Abstract: The aim of this study is to establish whether Si 3 N 4 whisker-reinforced 2124 Al composite fabricated by a simple powder metallurgical method without TMP could produce superplastic behaviour. The relationship between mechanical properties and strain rate in this composite is then investigated by performing tensile tests at a temperature of 525°C
119 citations
TL;DR: In this article, a small addition of CuO to 2-mol-Yz03-stabilized tetragonal zirconia polycrystals significantly enhances superplasticity by forming an amor hous grain-boundary phase containing primarily Cu', Y", Zr", and 0'-.
Abstract: .. . A small addition of CuO to 2-mol%-Yz03-stabilized tetragonal zirconia polycrystals significantly enhances superplasticity by forming an amor hous grain-boundary phase containing primarily Cu', Y", Zr", and 0'-. This phase apparently melts at around 1130°C, but it already provides a fast diffusion path even below the melting temperature. There are abrupt changes in stress exponent, activation energy, and grain size exponent across the melting temperature. Superplasticity is diffusion-controlled below the melting temperature and is interfaced-controlled above that. [Key words: yttria-stabilized tetragonal zirconia polycrystals, plasticity, liquid phase, creep, grain boundaries.]
116 citations
01 Jun 1990-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, a two-phase Mg-9Li alloy was made fine grained (d≈6-35 μm) by extensive cold rolling into foils and press bonding the foils at low homologous temperatures.
Abstract: A two-phase Mg-9Li alloy (where the composition is in weight per cent) was made fine grained (d≈6–35 μm) by extensive cold rolling into foils and press bonding the foils at low homologous temperatures. The material exhibited superplastic properties in the temperature range 423–523 K ((0.49–0.61)T/Tm) with elongations to failure as high as 460%. The creep rate was determined to be inversely proportional to approximately the square of grain size and to the second power of stress. The activation energy for superplastic flow was equal to that for lattice diffusion in the major b.c.c. β phase. The Mg-9Li laminate alloy was found to be similar in its flow characteristics to MA-21, a multicomponent Mg-8.1Li fine-grained commercial Soviet alloy.
TL;DR: The material exhibited superplastic elongation (>150%) in a tension test at temperatures from 1000° to 1100°C and at strain rates from 7.2×10−5 to 3.6 × 10−4 s−1 as mentioned in this paper.
Abstract: Dense and translucent hydroxyapatite polycrystals (Ca10(PO4)6(OH)2 with a grain size of 0.64 μMm) were obtained by hot isostatic pressing at 203 MPa and 1000°C for 2 h in argon. The material exhibited superplastic elongation (>150%) in a tension test at temperatures from 1000° to 1100°C and at strain rates from 7.2×10−5 to 3.6 × 10−4 s−1. Extensive strain hardening was observed. The stress exponent of the yield stress was larger than 3.
TL;DR: In this article, the effects of an electric current on dislocation mobility and mechanical properties at low homologous temperatures (T < 0.5Tm) reveal a polarity effect and yield an electron wind force in some agreement with theory.
Abstract: Investigations of the effects of an electric current on dislocation mobility and mechanical properties at low homologous temperatures (T < 0.5Tm) reveal a polarity effect and yield an electron wind force in some agreement with theory. An external directcurrent electric field has been reported to influence the creep rate of unalloyed metals at high homologous temperatures. During superplastic deformation of the 7475 Al alloy, such a field has been found to decrease the flow stress, reduce strain hardening, increase strain-rate hardening, reduce grain boundary cavitation and reduce grain growth. The effects of the field were polarity dependent and extended to the center of 1–2 mm thick specimens. No significant effect of the field on the flow stress occurred at low homologous temperatures. This suggests that the field influences atomic mobility through vacancy generation and/or migration. The occurrence of an uneven electron density at the interfaces between phases and at grain boundaries has been proposed as a factor, but this idea needs further consideration.
TL;DR: In this paper, a continuum theory for non-Newtonian flow of a two-phase composite containing rigid inclusions is presented, which predicts flow suppression by a factor of (1 - V)q, where V is the volume fraction of the rigid inclusion and q depends on the stress exponent and the inclusion shape.
Abstract: A continuum theory for non-Newtonian flow of a two-phase composite containing rigid inclusions is presented. It predicts flow suppression by a factor of (1 - V)q, where V is the volume fraction of the rigid inclusion and q depends on the stress exponent and the inclusion shape. Stress concentrations in the rigid inclusion have also been evaluated. As the stress exponent increases, flow suppression is more pronounced even though stress concentration is less severe. To test this theory, superplastic flow of zirconia/mullite composites, in which zirconia is a soft, non-Newtonian superplastic matrix and mullite is a rigid phase of various size, shape, and amount, is studied. The continuum theory is found to describe the two-phase superplastic flow reasonably well.
01 Jan 1990
TL;DR: The proceedings of the 1990 Symposium on Superplasticity in metals, ceramics, and intermetallics as discussed by the authors have been published as part of the materials Research Society 1990 Spring Meeting.
Abstract: This volume represent the proceedings of the Symposium on Superplasticity in metals, Ceramics, and intermetallics, part of the materials Research Society 1990 Spring Meeting. Topics covered include: fundamentals and theory, superplastic microstructure synthesis and uses, superplastic forming and diffusion bonding, intermetallic superplasticity, metallurgical superplasticity, ceramic superplasticity, and superplastic alloys.
Patent•
TDK1
TL;DR: In this article, a living hard tissue replacement is prepared by superplastic forming a ceramic material which contains CaO, SiO 2, and MgO, is substantially free of calcium phosphate, but capable of forming a calcium phosphate base compound upon contact with phosphorus-containing water.
Abstract: A living hard tissue replacement is prepared by superplastic forming a ceramic material which contains CaO, SiO 2 , and MgO, is substantially free of calcium phosphate, but capable of forming a calcium phosphate base compound upon contact with phosphorus-containing water. A replacement is also prepared by superplastic forming a sintered composite body comprising a calcium phosphate base ceramic matrix and an inorganic filler dispersed therein. Both the replacements are suitable as artificial dental roots and crowns.
TL;DR: In this article, the role of the grain-boundary glass phase on high temperature deformation behavior of tetragonal zirconia polycrystal (TZP) was investigated by compression test in the temperature range of 1000° to 1500°C.
Abstract: Superplastic deformation of tetragonal zirconia polycrystal (TZP) was investigated by compression test in the temperature range of 1000° to 1500°C. Special attention was paid to the role of the grain-boundary glass phase on hightemperature deformation behavior. A small addition of glass phase markedly improved the high-temperature deformability of TZP. Lithium silicate glass was much superior to aluminosilicate or lithium aluminum silicate glasses for lowering the high-temperature flow stress. The deformation mechanism was discussed on the basis of mechanical testing data and microstructural examinations.
TL;DR: In this paper, the occurrence of grain boundary sliding during creep in fine grained alumina was examined by inscribing marker lines on the tensile surfaces of specimens, prior to testing in four-point bending mode.
Abstract: The occurrence of grain-boundary sliding during creep in fine grained alumina was examined by inscribing marker lines on the tensile surfaces of specimens, prior to testing in four-point bending mode. There was considerable microstructural evidence for the occurrence of grainboundary sliding and grain rotation during creep deformation. Experimental measurements of the offsets in the marker lines at grain boundaries reveal that the grain-boundary sliding contribution to the total strain during creep deformation is 70 ± 6.2%. The extensive grain boundary sliding observed, together with the other mechanical properties, suggests that polycrystalline alumina exhibits superplastic characteristics. Several possible rate controlling mechanisms are examined critically in light of the present results and it is concluded that creep occurs either by an independent grain-boundary sliding mechanism or by an interface controlled diffusion mechanism.
TL;DR: In this paper, texture development during the thermomechanical processing of high strength aluminium alloys is reviewed and the implications of texture development are considered by examining the effects that texture can have on tensile property anisotropy and fatigue and fracture behaviour.
Abstract: Texture development during the thermomechanical processing of high strength aluminium alloys is reviewed. The alloys dealt with include both conventional heat treatable alloys, and unconventional materials such as rapidly quenched alloys and metal-matrix composites. The processing routes considered include hot and cold rolling, extrusion, forging, recrystallisation, and superplastic deformation. The information is presented as (111) pole figures and orientation distribution functions, in order to illustrate the much greater degree of detailed information that can be extracted from the latter method of analysis. The implications of texture development are considered by examining the effects that texture can have on tensile property anisotropy and fatigue and fracture behaviour.MST/1292
TL;DR: In this article, the effect of thermal cycle amplitude and MMC microstructure on the enhanced creep rate of particle reinforced metal matrix composites has been investigated over a range of reinforcement sizes and volume fractions.
Abstract: Superplasticity during temperature cycling of particle reinforced metal matrix composites has been studied over a range of reinforcement sizes and volume fractions. Above a critical volume and thermal cycle amplitude, the mean strain per cycle is proportional to stress and approximately proportional to cycle amplitude. For a given thermal cycle the constant of proportionality with respect to stress increases with reinforcement fraction to a maximum at around 30%; it then decreases with further increase in reinforcement. Transmission electron microscopy revealed no characteristics dislocation substructure; even after 90% strain the material was indistinguishable from its undeformed state. The experimental results confirm an internal plastic flow model for the phenomenon rather than an enhanced creep. A model of the process derived from the Levy-Von Mises equations predicts both the effect of thermal cycle amplitude the MMC microstructure on the enhanced creep rate.
01 Sep 1990-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the tensile behavior of mechanically alloyed (dispersion strengthened) IN90211 was investigated at strain rates between 0.0001 and 340 s−1 at temperatures between 425 and 475 °C.
Abstract: The tensile behavior of mechanically alloyed (dispersion strengthened) IN90211 was investigated at strain rates between 0.0001 and 340 s−1 at temperatures between 425 and 475 °C. At strain rates above 0.1 s−1, superplastic elongations were obtained (maximum elongation of 525% at 475 °C and 2.5 s−1). Superplastic elongations were found to result from grain boundary sliding. The data were analyzed assuming that a threshold stress resists dislocation motion. The threshold stresses were obtained assuming n = 2 (grain boundary sliding) or n = 3 (solute drag) for the stress exponent. Both assumptions provided equally credible values for a temperature-dependent threshold stress between 1% and 20% of the Orowan looping stress. The n = 3 threshold stresses agreed with load relaxation data, but the n = 2 values corresponded to the lower limit of the superplastic deformation regime, as indicated by creep tests at lower strain rates. Based upon the threshold stress theories in the literature, the threshold stresses are suggested to arise from a combination of local and general climb of lattice dislocations over particles. Consideration of the activation energies, details of flow behavior, and stress relaxation experiments provided strong evidence for the n = 3 solute drag mechanism to be the rate-limiting process in the superplastic deformation regime.
TL;DR: In this paper, a finite element based solution to the problem of superplastic forming is presented, and examples for typical components including diffusion bonding effects are discussed and compared to typical components.
Abstract: Superplastic forming is a manufacturing process whereby titanium or aluminium sheet is blow formed into a die to produce very light and strong aerospace components. Of crucial importance is the prediction of the final thickness distribution and the pressure cycle necessary to maintain superplasticity. This paper discusses a finite element based solution to these problems and presents examples for typical components including diffusion bonding effects.
TL;DR: In this article, a Si 3 N 4 whisker reinforced 6061 aluminium composite has been successfully fabricated, which has the features: simple fabrication process by elimination of TMP, superplastic deforming rate up to 10 -1 sec -1 strain rate and super-plastic temperature below the solidus line of matrix.
Abstract: We have successfully fabricated superplastic Si 3 N 4 whisker reinforced 6061 aluminium composite which has the features: simple fabrication process by elimination of TMP, superplastic deforming rate up to 10 -1 sec -1 strain rate and superplastic deforming temperature below the solidus line of matrix. The aim of this study is to investigate superplastic behaviour of this composite
TL;DR: In this paper, a foil metallurgy processing technique has been developed to prepare fine-grained laminates based on a two-phase Mg-9Li alloy and finegrained particulate composites based on hard B4C powders embedded in the two phase Mg 9Li alloy.
Abstract: A foil metallurgy processing technique has been developed to prepare fine-grained laminates based on a two-phase Mg-9Li alloy and fine-grained particulate composites based on hard B4C powders embedded in the two-phase Mg-9Li alloy. The processing steps involve principally cold-rolling and low-temperature recovery processing for preparation of foils, and low-temperature press-bonding for preparation of laminates and composites. In this manner, contamination of the highly reactive alloy is minimized. Good tensile strength and ductility were achieved at room temperature with specific stiffness values of about 3.1 × 106 m3. Both the fine-grained laminates and the particulate composite are superplastic at 200 ° C, exhibiting a strain-rate-sensitivity exponent,m, of 0.5.
TL;DR: In this paper, a detailed series of tests at an initial strain rate of 1.3 × 10−5s−1 shows that the cavities grow by a diffusion-controlled mechanism for cavity radii 20μm.
Abstract: Experiments were conducted on a superplastic quasi-single phase copper alloy, Coronze CDA 638, to provide information on the nucleation and growth of internal cavities during deformation. It is shown that, at a temperature of 823 K, the cavities are generally associated with the presence of large Co-rich particles on the grain boundaries, with nucleation occurring at the particle/grain boundary interfaces. The cavities tend to form in stringers and these stringers are always oriented along the rolling direction regardless of the direction of the tensile axis. Thus, the cavity stringers are clearly associated with the Co-rich particles which also lie in stringers parallel to the rolling direction. A detailed series of tests at an initial strain rate of 1.3 × 10−5s−1 shows that the cavities grow by a diffusion-controlled mechanism for cavity radii 20μm. It is demonstrated that this conclusion is consistent both with direct measurements of the cavity growth rates and with estimates of the increases in cavity size due to superplastic diffusion growth. The results therefore emphasize the importance of diffusion-controlled cavity growth at low strain rates.
TL;DR: In this paper, as-solidified eutectic Pb-Sn solder joints exhibit superplastic behavior in shear creep loading, and the total shear deformation of joints in stress-rupture tests performed at 65° C are found to exceed 150%.
Abstract: This paper presents experimental evidence that as-solidified eutectic Pb-Sn solder joints can exhibit superplastic behavior in shear creep loading. Stepped load creep tests of as-solidified joints show a change in the stress exponent from a high value typical of con-ventional creep at high stress and strain rate to a superplastic value near 2 at lower stress and strain rates. In addition, the change in stress exponent is accompanied by a change in the activation energy for creep from a value near that for bulk self-diffusion (20 kcal/mol) to a value near that for grain boundary diffusion (12 kcal/mol). The total shear deformation of joints in stress-rupture tests performed at 65° C are found to ex-ceed 150%. The concomitant observation that quenched solder joints creep faster than air-cooled ones is attributed to a grain, or phase, size dependence of the strain rate. The source of superplastic behavior is a fine, equiaxed microstructure. It is not yet clear whether the superplastic microstructure is present in the as-solidified joint, or develops during the early stages of plastic deformation.