Showing papers on "Superplasticity published in 1994"

A unified approach to grain boundary sliding in creep and superplasticity

Abstract: Rachinger grain boundary sliding is a characteristic of high temperature deformation in both creep when the grain size is large ( d > λ ) and superplasticity when the grain size is small ( d λ ), where d and λ are the grain size and the subgrain size, respectively. An analytical procedure is used to determine the rate equation for Rachinger sliding when d > λ . Data for superplastic metals are examined to give the rate equation for Rachinger sliding when d λ . A unified model is developed for Rachinger sliding at all grain sizes, where the rate of sliding is controlled by the rate of accomodation through intragranular slip. It is demonstrated that the predictions of this model are in good agreement with experimental data under both creep and superplastic conditions.

An evaluation of the strain contributed by grain boundary sliding in superplasticity

Abstract: Grain boundary sliding is an important mode of deformation in superplasticity. Measurements of the contribution of sliding to the total strain generally give values of about 50–70% so that there is an apparent “missing strain” of about 30–50%. It is demonstrated that the experimental method used to measure the sliding contribution will lead to estimates in the range of about 45–90% even when all the deformation is by grain boundary sliding and the associated accommodation process. Since the problem of accommodation is less severe at the specimen surface, it is shown that estimates of the sliding contribution from surface marker lines will tend to lie at the lower end of this predicted range. It is concluded that grain boundary sliding accounts for essentially all the deformation under optimum superplastic conditions and there is no “missing strain” of about 30–50%.

An investigation of the role of a liquid phase in AlCuMg metal matrix composites exhibiting high strain rate superplasticity

Abstract: Tensile experiments were performed under conditions of true constant strain rate on three AlCuMg metal matrix composites containing 20 vol.% of either β-Si3N4 whiskers or α-Si3N4 particulates with diameters of 0.2 or 1.0 μm, respectively. The tests were conducted at temperatures from 758 to 833 K and at strain rates up to 102 s−1. The results show that each composite exhibits a strain rate sensitivity of >0.3 at strain rates in the vicinity of ∼10−1 −1 s−1 and with associated superplastic elongation of >200% or even up to >800% for the composite with particulates having a diameter of 1.0 μm. It is concluded that high ductilities are achieved at effective testing temperatures above the solidus temperature of the matrix alloy but the results are not consistent with a rheological model based on deformation in a semi-solid matrix. It is suggested instead that the presence of a liquid phase at the interfaces between the reinforcement and the matrix serves to relieve the stress concentrations due to sliding and thereby to limit the development of internal cavitation.

Step model of solution-precipitation creep

Abstract: Polycrystals which have intergranular thin liquid film deform by solution-precipitation creep. The strain rate is expressed by the velocity of surface step and its density. The velocity of step is proportional to the solubility and stress. It is controlled by resistance at kink, diffusion in the adsorption layer, and diffusion in the liquid film. The density of step is, (a) constant; (b) proportional to stress; or (c) increased by two dimensional nucleation. The constitutive equations were derived for diffusion-controlled and interface-reaction controlled creep. The step model was applied to creep and superplasticity of fine-grained ceramics with intergranular liquid film, and found to successfully explain the experimental results.

Superplastic deformation mechanism accommodated by the liquid phase in metal matrix composites

Abstract: The accommodation processes for superplastic metal matrix composites are required to release the stress concentration near matrix-reinforcement interfaces because of stress concentration caused by interfacial sliding. An optimum superplastic elongation was found at the temperature where local melting of interfaces was confirmed by in situ transmission electron microscopy observation. It is postulated that superplastic flow in metal matrix composites is controlled by a grain-boundary sliding mechanism accommodated with relaxing the stress concentration by an isolated liquid phase at matrix-reinforcement interfaces.

Cavitation and cavity-induced fracture during superplastic deformation

Abstract: The characteristics of fracture by cavitation in superplastic materials are reviewed. Particular attention is paid to the theoretical developmental aspects of cavity nucleation, cavity growth and cavity interlinkage. Various factors, including grain boundary sliding, impurity atoms or particles, phase proportion, deformation temperature, strain rate, strain and grain size, are discussed. Finally, methods for controlling cavitation during superplastic deformation are summarized, and problems which require further work are also presented.

High Performance Materials in Aerospace

Abstract: Design requirements for aerospace structural materials, C.J. Peel and P.J. Gregson aluminium alloys - physical metallurgy, processing and properties, P.J. Gregson titanium alloys - production, behaviour and applications, J.C. Williams nickel-based alloys - recent developments for the aero-gas turbine, M. McLean structural steels, D.P. Davies ceramic materials in aerospace, D.M. Dawson polymeric-based composite materials, N. Marks metal matrix composites, H.M. Flower superplastic forming, D. Stephen joining advanced materials by diffusion bonding, P.G. Partridge and A. Wisby adhesive bonding for aerospace applications, D. Driver rapid solidification and powder technologies for aerospace, H. Jones hot isostatic pressing, B.A. Rickinson and S. Andrews..

On the microstructural aspects of the nonhomogeneity of superplastic deformation at the level of grain groups

Abstract: The surface relief of a superplasticity deformed magensium alloy (Mg-1.5% Mn-0.3%Ce) was studied. Zones/bands of localized deformation were detected by means of vacuum etching. Between the localized deformation bands were less-deformed regions. After 20% elongation in vacuum (1.33 × 10−3 Pa), zones of intensive vacuum etching were observed as two intersecting bands of localized deformation oriented at 35–60° to the tensile axis. Spacing between the localized deformation bands is 6–8 grain diameters after a tensile elongation of 20% and 3–4 grain diameters after an elongation of 160%. The observed bands of strain localization are explained from the viewpoint of cooperative grain boundary sliding, i.e. shifting of grain groups as a whole unit along grain boundary surfaces oriented close to the maximum shear stress direction. It is suggested that the cooperative nature of GBS be taken into account when evaluating the real local strain rate and the real strain-rate sensitivity of grain boundary sliding process.

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

Abstract: The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathin (1nm) yttrium-rich amorphous film. After deformation at 1200°–1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress.

A critical assessment of flow and cavity formation in a superplastic yttria-stabilized zirconia

Abstract: Tensile tests were performed on specimens of high purity 3Y-TZP (tetragonal ZrO2 stabilized with ∼ 3 mol% of Y2O3) at temperatures from 1623 to 1803 K. Superplastic-like flow was achieved with elongations of up to >400%. The experiments show that the stress exponent, n, is ∼ 3.0–3.4 and the activation energy for flow, Q, is ∼ 602±20 kJ mol−1. Internal cavities developed in all specimens during flow, and quantitative measurements were taken of the cavity shapes and sizes over a range of experimental conditions. The results demonstrate that there is an increase in the extent of cavitation with (i) decreasing temperature at constant strain rate and (ii) increasing strain rate at constant temperature. The influence of strain rate on cavitation in 3Y-TZP is the opposite of most superplastic metals, and the difference arises because of an inhibition in the diffusion growth process in 3Y-TZP.

Superplastic behavior in a commercial 5083 aluminum alloy

Abstract: When considering the forming and post-forming properties required of a superplastic material, attractive candidates are commercial Al-Mg-Mn weldable alloys such as AA5083. There have been several investigations of hot deformation of 5083-type alloys in the literature. Only two studies evaluated commercial-purity 5083 and they achieved tensile elongations of 150% and 200%. Alloy modification has produced improved behavior in three 5083-type alloys developed specifically for SPF. Two were deemed high-purity 5083 (low Fe and Si) and achieved elongations of 450% and 630%. Engineering strains up to 700% were measured by Watanabe et al. in a 5083-based alloy with the addition of 0.6% Cu as a grain refiner. These results suggest that alloy modifications such as reduced Fe and Si contents or Cu additions may be required to improve superplastic response. Unfortunately, specific SPF-grade 5083 alloys are substantially more expensive than the commercial grade, and the addition of Cu decreases the corrosion resistance of the base material. The purpose of this work is to examine the effect of prior degrees of cold work on the SPF behavior of a standard-grade 5083 alloy. Superplastic behavior of this material at 510[degree]C is assessed and compared to published results for the SPF-grade alloys.

Superplasticity in Fe3Al-Ti alloy with large grains

Abstract: In this paper the authors report the superplasticity behavior in Fe[sub 3]Al with large grains The tensile behavior of one Fe[sub 3]Al based alloy, Fe-28Al-2Ti (in atomic percent), under different strain rates at high temperatures was examined The microstructure before and after deformation was observed by optical microscopy and transmission electron microscopy (TEM) The results revealed that the Fe[sub 3]Al alloy with a large grain size of 100[mu]m exhibited a large elongation of more than 150% from 800 C to 900 C with a strain rate range of 10[sup [minus]4] [approximately] 10[sup [minus]3]/s The maximum elongation is 3328% at 850 C under a strain rate of 1 [times] 10[sup [minus]3]/s A tensile elongation of up to 507% was obtained by slightly changing the gauge section and/or modifying the composition The reason for the large elongation is ascribed to the dynamic recovery and recrystallization in this alloy at high temperature

Grain boundary behavior in superplastic Mg-doped alumina with yttria codoping

Abstract: Microstructural Transmission Electron Microscopy investigations are performed on fine grained magnesium and yttrium codoped aluminas deformed by compression and tension. Striking differences with aluminas doped with only magnesium occur in GB characteristics and intragranular phenomena: the number of “special” near-coincidence GBs is small and GB planes present a tendency to align parallel to the basal plane, a characteristic of non Mg-doped aluminas. The microstructure after deformation presents evidence of plastic deformation: Isolated dislocations occur in “general” GBs and adjacent grains present basal slip and basal microtwins. The lattice defects are much more numerous after tension than after compression. These observations account for a strengthening role of yttrium in the dislocation accomodation processes in the grain boundaries, resulting in high local stress concentrations, and are in agreement with the main modifications of the macroscopic behavior when yttrium is added.

Thermal Stability and Superplastic Characteristics in Si 3 N 4 /Al–Mg–Si Composites

Abstract: Different scanning calorimeter (DSC) investigations and tension tests at 773-848 K have been carried out in four different superplastic Si 3 N 4 /Al-Mg-Si composites. The DSC investigations revealed that the melting point of each composite was lower than the melting point of its matrix alloy. This is probably attributed to local melting due to segregation of solute atoms near the matrix/reinforcement interfaces. The optimum superplastic temperature for each composite, where a maximum elongation was obtained, was close to its melting point

Low temperature superplastic flow of yttria stabilized tetragonal zirconia polycrystals

Abstract: Very fine-grained (0·2 um), dense Y-TZP ceramics have been produced by free sintering at 1150°C. The superplastic deformation of these materials is studied in compression at low temperatures (1100-1300°C). A significant enhancement in initial strain rates was observed compared to a coarser-grained (0·4 ?m) commercially available material (Tosoh, Japan). Doping with small amounts of Fe2O3 led to a further enhancement of strain rate. Deformation occurred via interface reaction controlled grain boundary sliding. Prior to deformation a continuous glassy silicate film was observed at the grain boundaries. The applicability of interface reaction controlled solution-precipitation creep models is discussed. The absence of a steady-state during deformation at low stresses is attributed to dewetting of the silicate film.

Superplastic aluminum alloy and process for producing same

Abstract: The present invention relates to a process for producing a superplastic aluminum alloy capable of being used for plastic working such as extrusion, forging and rolling. An object of the present invention is to provide an ingot-made high speed superplastic aluminum alloy in which superplasticity is developed at a strain rate higher than that of conventional static recrystallization type superplastic aluminum alloys, and a process for producing the same. The superplastic aluminum alloy of the invention has structure which is obtained by adding to a basic alloy containing from at least 4.0 to 15% by weight of Mg and from 0.1 to 1.0% by weight of one or more elements selected from the group consisting of Mm, Zr, V, W, Ti, Ni, Nb, Ca, Co, Mo and Ta, and further selective elements of Sc, Cu, Li, Sn, In and Cd, which contains from 0.1 to 4.0% by volume fraction of spheroidal precipitates of intermetallic compounds having a particle size from 10 to 200 nm, and which has a mean grain size from 0.1 to 10 µm.

High strain rate superplasticity of AlN particulate reinforced aluminium alloy composites

Abstract: Ceramic whisker or particulate reinforced aluminium alloy composites have a great potential for automobile engineering components, aerospace structures, semi-conductor packaging and so on, because of the composites ability to exhibit a high specific elastic modulus and specific tensile strength, excellent wear resistance and heat resistance, low thermal expansion and good dimensional stability A serious problem involving practical application of ceramic whisker or particulate reinforced aluminium alloy composites is due to the low tensile ductility, fracture toughness at room temperature and, also, their hardness qualities that make it difficult to deform by conventional forming processing and machining by ordinary tools It has been found, however, that aluminium alloy composites reinforced by SiC or Si[sub 3]N[sub 4] whiskers or particulates produce superplasticity at a high strain rate of about 01s[sup [minus]1] Superplastic deformation mechanisms of the ceramic whisker or particulate reinforced aluminium alloy composites are fine grain boundary sliding, interfacial sliding at a liquid phase and dynamic recrystallization An AlN particulate reinforced aluminium alloy composite exhibits a high elastic modulus and a high thermal conductivity, and their thermal expansion is similar to silicon in that the AlN particulate reinforced aluminum alloy composite is expected to apply to semi-conductor packaging in the aerospacemore » structure In addition, if the composite could produce superplasticity at high strain rates, the market of aerospace application for superplastic composites could be expanded The purpose of this study is to make clear if an AlN particulate reinforced aluminium alloy composite can produce superplasticity at high strain rate and the superplastic characteristics« less

The role of deformation localization in superplastic flow

Abstract: The model Zn22%Al alloy is used to show superplastic deformation takes place when localized deformation bands separating the material volume into large grain conglomerates are formed and developed The formation of these bands is due to the cooperative grain boundary sliding which can be stimulated by intragranular sliding The number and shape of the bands, as well as their contribution to the total elongation (the technique of surface markers was used) depend on strain rate The movement of grain in the specimen volume takes place by the mutual displacement and rotation of grain conglomerates The data presented can serve as a basis for the development of general physical theory of superplasticity based on macroscopic and mezoscopic deformation

High temperature mechanical properties of vanadium alloyed γ base titanium-aluminides

Abstract: High temperature mechanical properties of vanadium alloyed [gamma] base Ti aluminides have been related to the microstructure. The microstructures were controlled by chemistry modification -- vanadium alloying between 5.3 and 12.7 mol.% for phase distribution control -- and isothermal forging. The properties were evaluated using tensile tests at strain rates between 3 [times] 10[sup [minus]4]s[sup [minus]1] and 0.1s[sup [minus]1] and at temperatures between 1070 and 1420 K. The [gamma] plus [beta] phase intermetallic exhibit excellent hot workability with an elongation more than 600% and an m-value of 0.8. The mechanism of the superplastic deformation was critically discussed.

Ultra-fine Equiaxed Grain Refinement and Improvement of Mechanical Properties of α+β Type Titanium Alloys by Hydrogenation, Hot Working, Heat Treatment and Dehydrogenation

Abstract: Titanium alloys absorb large amounts of hydrogen and it is desorbed in vacuum at elevated temperatures. This paper describes equiaxed grain refinement and the improvement of mechanical properties of titanium alloys utilizing this phenomenon. Particular attention has been paid to the control of three different types of dislocation structures introduced by the following treatments. (1) Precipitation of hydrides: As hydrogenated titanium alloys undergo aging, high-density dislocations are introduced in the interior of hydrides as well as in the surrounding region. (2) Formation of cell walls by hot working: The β transus temperature is lowered by hydrogenation so that the hotworking temperature can be lowered enough to retain the dislocation cell structure formed during the deformation

A comparative study of cavitation characteristics in composite and 7475 aluminum alloy

Abstract: Recently, it has been demonstrated that many aluminum matrix composites with discontinuous SiC or Si[sub 3]N[sub 4] show superplastic behavior. In particular, it should be noted that some aluminum matrix composites show superplasticity at high strain rates (> 10[sup [minus]2] s[sup [minus]1]). It is well established that cavitation occurs during superplastic flow in a wide number of metallic materials. However, the cavitation behavior in superplastic metal matrix composites is not well understood. It is important to investigate characteristics of cavitation in metal matrix composites for superplastic forming applications. In this investigation, cavitation in a 20 vol% Si[sub 3]N[sub 4p]/Al-Mg-Si composite, which exhibits superplasticity at high strain rates, has been investigated and the data of the composite have been compared with those of the superplastic 7475 alloy which is a typical superplastic aluminum alloy.

Mechanical properties and deformation behaviour of large Al70Pd20Mn10 single quasi-crystals

Abstract: An icosahedral single quasi-crystal of Al 70 Pd 20 Mn 10 was found to be deformed in the temperature range above 1000 K, accompanying the generation of a number of deformation markings. The deformation appears to occur inhomogeneously along the deformation bands which lie on the fivefold plane and the region in the deformation bands is identified to have deviated alloy compositions and approximant crystalline structures. As the test temperature increases, the density of the deformation markings increases accompanying the decrease in their spacing. Superplasticity is also obtained at temperatures above 0.98 T m , presumably because of the sliding at the interface between icosahedral and approximant crystalline phases. Furthermore, a number of dislocations are observed in the icosahedral phase deformed at temperatures above 1000 K and the dislocation density is much higher near the interface. The Burgers vector was analysed to have the twofold direction for the dislocations on the fivefold plane. It is therefore concluded that the icosahedral single quasi-crystal can be deformed via dislocations along the interface between icosahedral and approximant crystalline phases at temperatures above 1000 K.