Showing papers on "Superplasticity published in 1993"

Structure and properties of ultrafine-grained materials produced by severe plastic deformation

Abstract: Strain-heat methods of obtaining ultrafine-grained (UFG) metallic materials with grain sizes as small as 20 nm and peculiarities of their structure are considered. It is shown that intercrystalline boundaries are the main element of the structure of UFG materials and that they are typically in a non-equilibrium state. The formation of a special grain boundary phase, i.e. a thin near-boundary layer with high dynamic activity of atoms, has been found. This unusual structure leads to the manifestation of promising new elastic, strength, superplastic, damping and magnetic properties of UFG materials.

Superplasticity in advanced materials

Abstract: The ability to achieve a high tensile ductility in a polycrystalline material is of interest both from a scientific point of view and also because of potential applications in the materials forming industry. The superplasticity of conventional metallic alloys is now well-documented and understood reasonably well. However, the field of superplasticity has expanded recently beyond the traditional metallic alloys to include evidence of superplastic-like behavior in a very wide range of new and advanced materials. To date, superplasticity has been reported in mechanically alloyed metals, metal matrix composites, ceramics, ceramic matrix composites and intermetallic compounds. This review presents an overview of these new developments using the established behavior of conventional metallic alloys as a standard for comparison with the mechanical properties of these new materials. As well be demonstrated, the new materials often exhibit significant differences in their flow characteristics in comparison with the traditional superplastic metallic alloys. The successful utilization of superplastic materials in forming applications requires an understanding of the failure processes occurring in the materials in terms of both the localization of external flow and the accumulation of internal damage through the nucleation and growth of cavities. These problems are also addressed in this review.

Mechanical properties and deformation behavior of materials having ultra-fine microstructures

Abstract: Fundamental concepts structure and physical properties mechanical response superplasticity nanoindentation synthesis and processing characterization

Plastic deformation and fracture of materials

Abstract: Introductory Chapter: Microstructure and Mechanical Properties (H. Mughrabi). Flow Stress and Work Hardening (J. Sevillano). Deformation and Textures of Metals at Large Strain (E. Aernoudt, et al.). Dislocation Patterning (L. Kubin). Solid Solution Strengthening (H. Neuh?user & C. Schwink). Deformation of Intermetallic Compounds (Y. Umakoshi). Particle Strengthening (B. Reppich). High-Temperature Deformation and Creep of Crystalline Solids (W. Blum). Superplasticity in Metals, Ceramics and Intermetallics (A. Mukherjee). Inelastic Deformation and Fracture of Glassy Solids (A. Argon). Cyclic Deformation and Fatigue (S. Suresh). Fracture Mechanisms (H. Riedel). Friction and Wear (K. Kato). Index.

Formation of submicrocrystalline structure in the titanium alloy VT8 and its influence on mechanical properties

Abstract: It is shown that the formation of microstructure with grain size up to 0.06 μm may occur during the course of plastic deformation of the Ti-6Al-3.2Mo (α+β)-alloy with the initial coarse-grained lamellar structure. The formation of submicrocrystalline structure results from the development of dynamic recrystallization concurrent with the process of spheroidization. The temperature of superplastic deformation significantly decreases while strength characteristics at room temperature sharply increase in the alloy with such a microstructure.

Processing nanocrystalline ceramics for applications in superplasticity

Abstract: The production of nanocrystalline ceramics for subsequent use in superplastic forming operations requires that the ceramics be made in large quantities, with high densities, and under stringent grain growth control. To make large amounts of nanocrystalline starting powders, two popular wet chemical techniques (precipitation from salt solutions and alkoxide hydrolysis) can be used and are described in this paper. Unfortunately, pressureless sintering of these powders does not typically lead to the high densities and ultrafine grain sizes desired in the final product. Sintering data suggest that pore shrinkage occurs only when grains reach a critical size with respect to the pore size; thus, if the ceramic contains large pores, densification can require significant grain growth. Separation of large pores from grain boundaries may also occur and lead to incomplete densification, even at extremely large grain sizes. In all cases the pressureless sintering behavior of the nanocrystalline ceramics appears to adhere to well established theories used to explain the sintering of conventional, larger-grained ceramics. During both pressureless sintering and sinter-forging experiments, the grain size of a nanocrystalline ceramic is identical to the average spacing between open pores in the sample. Pressureless sintering results in the closure of these pinning pores by about 90% density and thus3leads to a substantial grain growth at densities greater than 90%. Sinter-forging, however, often allows one to maintain a stable population of small open pores (for pinning purposes) throughtout sintering, while preferentially eliminating the large pores which detract from the sample density. The deformation regime in which sinter-forging is performed has a decided effect on whether large pores or small pores are eliminated preferentially and, consequently, on whether a high density and fine grain size combination is achieved or not.

Grain boundary design and control for high temperature materials

Abstract: Recent experimental work has been reviewed concerning the effects of grain boundary structure on grain boundary sliding and migration, and other boundary phenomena involved in high temperature plasticity. A basic knowledge of the structure-dependent intergranular phenomena is important for a full understanding of the heterogeneity and characteristic features of high temperature deformation and fracture, because the structural effects of grain boundaries enhance the boundary-induced heterogeneity of deformation and fracture in polycrystals. The grain boundary character distribution (GBCD) has been shown to be important and useful in explaining and controlling high temperature plasticity, superplasticity and fragility in polycrystals. The potential use of grain boundary design and control for high temperature materials has been discussed on the basis of recent work concerning GBCD, texture and other microstructural factors. The recent achievement of grain boundary design and control for high temperature materials has been briefly introduced.

An investigation of the role of intragranular dislocation strain in the superplastic Pb-62% Sn eutectic alloy

Abstract: Experiments were conducted on the Pb-62% Sn eutectic alloy to determine the role of intragranular dislocation movement in the superplastic region II. By taking careful measurements within individual grains of a specimen tested at 423 K at a strain rate near the center of region II, it is demonstrated that there is evidence for the movement of some intragranular dislocations, but the strain is non-uniform and oscillatory in nature with changes from positive to negative contributions to the total strain as the deformation continues. The results show that the Sn and Pb phases behave similarly and the net contribution to the total strain is close to zero. It is concluded that grain boundary sliding and the relative translation of individual grains represents the dominant flow mechanism, and intragranular dislocation movement occurs as an accommodation process which, nevertheless, has an important influence on the flow mechanism because it represents a source of mobile extrinsic grain boundary dislocations.

Superplasticity in fine grained ceramics and ceramic composites: current understanding and future prospects

Abstract: Superplasticity in ceramics has now been reported in a wide range of materials with elongations to failure of more than 100%. Although the experimental observations of large deformation are in some ways similar to those reported in numerous metallic alloys, there are significant differences in the mechanical properties and cavitation failure characteristics of superplastic ceramics. This paper provides an overview of superplastic deformation and failure in ceramics, with specific emphasis on a 3 mol.% yttria stabilized zirconia and a zirconia-20wt.%alumina composite. It is demonstrated that there is a transition in the deformation behavior of zirconia which is dependent on the grain size and the impurity content of the material. Many of these materials fail by the nucleation, growth and interlinkage of cavities, so that the ductility is governed by the imposed stress and the grain size. Potential areas for additional research on superplastic ceramics are highlighted.

Mechanical properties, fracture mode and deformation behavior of Al70Pd20Mn10 single-quasicrystal

Abstract: The coefficient of thermal expansion (α), Young's and rigid moduli (E and G), Poisson's ratio (v), compressive yield and fracture strengths (σ y and σ f ), fracture toughness (K), Vickers hardness (H v ) and cleavage fracture mode were examined along two-, three- and five-fold symmetry axes for an Al 70 Pd 20 Mn 10 single-quasicrystal prepared by the Czochralski method. In the temperature range below 600 K, α is 1.3×10 -5 K -1 which is about 50% as high as that for conventional Al-based alloys

Influence of Amorphous Grain Boundary Phases on the Superplastic Behavior of 3‐mol%‐Yttria‐Stabilized Tetragonal Zirconia Polycrystals (3Y‐TZP)

Abstract: Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y-TZP in order to determine their influence on the superplastic behavior between 1,200 and 1,300C and on the room-temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y-TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y-TZP with 5 wt% glass, but the room-temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y-TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y[sub 2]O[sub 3] and ZrO[sub 2], but the strain rate did not scale inversely with the viscosity of the silicae phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y-TZP with and without glassy grain boundary phases.

High temperature superplastic creep and internal friction of yttria doped zirconia polycrystals

Abstract: Creep tests in compression and internal friction experiments have been performed in order to investigate the influences of microstructural parameters such as the grain size and the amount of intergranular glassy phase on the plastic deformation of yttria stabilized tetragonal zirconia polycrystals. The values of the apparent activation energies obtained from creep tests in the low stress regime and those obtained from the internal friction tests are in good agreement. The two techniques appear to be complementary approaches to the study of plastic deformation of fine grained zirconia under low stress. The mechanisms of plastic deformation are discussed in terms of grain boundary sliding models.

The role of grain boundaries in high temperature deformation

Abstract: The grain boundaries in polycrystalline materials often play a very significant role in the flow and fracture characteristics at elevated temperatures. These characteristics are reviewed by defining four distinct grain size ranges, termed macroscopic, mesoscopic, microscopic and nanoscopic respectively. The role of grain boundary sliding in the mesoscopic grain size range is examined and it is shown that sliding measurements may be analyzed to give a rate equation for the sliding process. Grain boundary sliding accounts for essentially all of the strain in metals with microscopic grain sizes in the superplastic region of flow, but in ceramic materials with microscopic grain sizes the presence of glassy phases at the boundaries is also important. Nanoscopic grain sizes provide the potential for attaining high superplastic ductilities but there are difficulties associated with achieving fully dense materials with grain sizes of the order of approximately 0.01 μm.

Processing of nano-grained materials

Abstract: Sintering and deformation were studied in nano-grained (n-)TiO 2 and n-TiAl as part of a program to develop materials for near-net shaping and superplasticity applications. An important concern for processing nano-grained materials is the control of grain growth during both densification and deformation. In this study, the effectiveness of doping n-TiO 2 with yttrium for controlling grain growth during isothermal annealing was examined. In addition, an empirical constitutive law for the densification of n-TiO 2 was determined. Comparison of sinter-forming in n-TiO 2 with larger grained oxide ceramics shows many similar features. The studies on TiAl examined the hardness as a function of grain size, indentation time and temperature. At large grain sizes, the hardness obeys the Hall-Petch relation, but below a critical grain size, approximately 30 nm, the hardness decreases with decreasing grain size. Finally, the potential for synthesizing metallic glasses with nanoscale amorphous particles is discussed.

The case for ultrahigh-carbon steels as structural materials

Abstract: Ultrahigh-carbon steels (UHCSs) are low-alloyed plain carbon steels containing 1–2.1% carbon. These steels have remarkable structural properties when processed to achieve fine ferrite grains with fine spheroidized carbides. They can be made superplastic at intermediate temperatures. Further, they can be made hard with compression toughness and strong with good tensile ductility at ambient temperatures. Contrary to conventional wisdom, UHCSs are ideal replacements for currently used high-carbon (0.5–1 % carbon) steels because they have comparable ductility but higher strength and hardness. In this article, examples of structural components formed from fine-grained spheroidized UHCSs are illustrated, and other potential structural applications are reviewed. These steels can be laminated with other metal-based materials to achieve superplasticity, high impact resistance, exceptionally high tensile ductility, and improved fatigue behavior.

Dislocation activity and differences between tensile and compressive creep of yttria doped alumina

Abstract: It is now well recognized that most important properties of materials are controlled by their internal interfaces. The contribution of each interface to the overall polycrystalline behavior depends on its geometry and on its chemistry. This paper presents a good example of the changes in the constitutive laws of high-temperature deformation of a superplastic magnesia-doped alumina by codoping with yttria. Yttrium, strongly segregated at grain boundaries, promotes strengthening of the ceramic; furthermore, this effect is much more pronounced under tensile than under compressive tests. After thorough investigations by transmission electron microscopy, the macroscopic differences were associated with a strong increase in the intragranular and intergranular dislocation activity. These features have been attributed to the impediment by yttrium of the accomodation processes of grain boundary dislocations.

The direct observation of cooperative grain‐boundary sliding and migration during superplastic deformation of lead‐tin eutectic in shear

Abstract: Direct observation of superplastic deformation in shear in a scanning electron microscope in Pb‐62% Sn eutectic alloy at different microstructural levels were performed. The sliding of grains as an entity was observed. Such cooperative grain‐boundary sliding proceeds by means of the sequential shear of grains along shear surfaces, and is accompanied by cooperative grain‐boundary migration. The pattern of shear surfaces at the level of the entire deformed volume is close to one that is predicted by slip‐lines field theory. The processes to accommodate deformation of grain‐boundary sliding include the operation of grain‐boundary migration, intragranular deformation, and grain rotation. A critical analysis of the proposed geometrical models for superplastic flow is given. The model of sequential shear of grains as a result of the movement of cellular dislocations is extended to the case of the two‐phase material, having structure that is typical for superplastic eutectics.

Positive exponent superplasticity in advanced aluminum alloys with nano or near-nano scale grained structures

Abstract: Very high strain rate superplasticity was observed in near-nano scale aluminum alloys. These alloy systems include the group of mechanically alloyed IN9021, IN90211, IN9052, IN905XL alloys and SiCp/IN9021 composite, an A-14wt.%Ni-14wt.%Mm (Mm is mischmetal) crystalline alloy consolidated from its amorphous powders and an Al-14wt.%Ni-7wt.%Mm-1wt.%Zr alloy consolidated from nanocrystalline powders, and an Al-7wt.%Cr-1wt.%Fe alloy produced by physical vapor deposition. In each of the above advanced alloy systems, large elongations of more than 500% were obtained at extremely high strain rates of more than 1 s−1 (called positive exponent superplasticity). In the mechanical alloyed IN9021 aluminum alloy with a fine grained structure 500 nm in size, a maximum elongation of 1250% was recorded at an extremely high strain rate of 50 s−1 at 823 K. In addition, a maximum elongation of 610% was obtained at a very high strain rate of 5 s−1 of 823 K in the mechanically alloyed 15vol.%SiCp/IN9021 aluminum composite. A maximum elongation of 650% was observed at a high strain rate of 1 s−1 in both AlNiMm and AlNiMmZr alloys consolidated from amorphous or nanocrystalline powders. Also a maximum elongation of 505% was obtained at a high strain rate of 1 s−1 in a VQ AlCrFe alloy.

Superplastic behavior in a two-phase TiAl alloy

Abstract: Gamma alloys based on TiAl exhibit high strength to density ratio, better oxidation resistance than Ti[sub 3]Al alloys, and good creep properties at elevated temperatures. Recent studies have shown that the room temperature ductility of gamma titanium aluminide alloys can be improved up to 3--4% by alloy modification and thermomechanical processing. These technological achievements further broaden the range of structural application of these alloys. Significant advances have also been made recently to improve the high temperature ductility in ordered intermetallics by optimizing the microstructure suitable for structural superplasticity through careful thermomechanical processing. The superplastic behavior of titanium aluminides based on Ti[sub 3]Al and other ordered intermetallics have been demonstrated but only few studies have been done to show superplasticity in [gamma][minus]TiAl. Full characterization of superplasticity in TiAl based alloy is still lacking. In this investigation, the superplastic deformation of a [gamma][minus]TiAl with duplex microstructure, equiaxed [gamma] and lamellar [alpha][sub 2] + [gamma], was studied with respect to the effect of testing temperature and strain rate. Microstructural evolution during deformation was examined and correlated to the mechanical properties.

The absence of relative grain translation during superplastic deformation of an Al-Li-Mg-Cu-Zr alloy

Abstract: The deformation of AA8090 Al-Li-Mg-Cu-Zr alloy at elevated temperature and slow strain rates has been investigated in uniaxial tension. Under suitable conditions, this material exhibited a high strain-rate sensitivity of the flow stress and was superplastic. This superplastic behavior was obtained in material with an initially elongated grain structure combined with a distribution of similarly oriented grains and low-angle grain boundaries that was not conducive to boundary sliding. Observations of the development of microstructure and of the crystallographic preferred orientation indicated that no significant rigid body translation and little rotation of grain interiors occurred up to strains of about 0.4 and that the probability of relative translation of grain interiors up to strains of at least 1 was low. The changes of structure observed could be accounted for by a combination of grain growth and grain rotation. The consequence of these observations on the grain switching and grain boundary sliding mechanisms generally assumed to operate during superplastic deformation is discussed, with the conclusion that those mechanisms may not be wholly appropriate for explaining high rate sensitivity in this material over the range of strain rates investigated.

Superplasticity and deformation induced grain growth

Abstract: Superplastic deformation, especially in quasi-stable fine equiaxied grain structures, is accompanied by grain growth whose rate exceeds that which can occur without deformation. The deformation induced component of the grain growth stabilizes the deformation itself through an increase in the flow stress. An empirical expression for the grain growth is demonstrated which describes the behavior of microduplex and second-phase dispersed alloys. Finally, a new deformation model of superplasticity is proposed to explain the grain growth.

The influence of the rolling direction on the mechanical behavior and cavity formation during superplastic deformation of 7075 A1 alloy

Abstract: The superplastic 7075A1 alloy was tested over a range of strain rates 10−2−10−4s−1 at a temperature range 430–510°C using specimens machined with the rolling direction parallel and perpendicular to the tensile axis. It is shown that the mechanical properties of the alloy, including the elongations to failure, are essentially identical. Microstructural observations show that the cavities tend to form in stringers and these stringers are always oriented along the tensile axis regardless of the rolling direction. The cavities are not nucleated primarily at large Fe-rich or Si-rich particles, nor do they grow from pre-existing microvoids which may be introduced during thermomechanical processing. The cavities are nucleated preferentially at small particles or some irregularities in the grain boundary during superplastic deformation.

Transformation process for production of ultrahigh carbon steels and new alloys

Abstract: Ultrahigh carbon steels with superplastic properties are produced by heating a steel containing ferrite and carbide phases to a soaking temperature approximately 50° C. above the A1 transformation temperature, soaking the steel above the A1 temperature for a sufficient time that the major portion of the carbides dissolve into the austenite matrix, and then cooling the steel in a controlled manner within predetermined limits of cooling rate or transformation temperature, to obtain a steel having substantially spheroidal carbides. New alloy compositions contain aluminum and solute additions which promote the formation of a fine grain size and improve the resistance of the carbides to coarsening at the forming temperature.

Effects of Zr on the high strain rate superplasticity of 2124 Al

Abstract: Many years ago, Nieh and Wadsworth, reported an observation that a 0.6wt%Zr-modified 2124 Al alloy, which has a fine grain size of about 1[mu]m, behaved superplastically at 475 C at high strain rates ([approximately] 10[sup [minus]1]s[sup [minus]1]). The present paper is an extension of the above study, demonstrating the effects of Zr additions to an Al alloy on superplastic strain rates. The deformation properties of a 2124 Al alloy containing 0.6wt%Zr have been characterized. As a result of the Zr addition, the alloy has a relatively fine grain size ([approximately] 1[mu]m). At relatively low strain rates (< 10[sup [minus]2]s[sup [minus]1]), the fine-grained 2124-0.6Zr alloy behaves like conventional coarse-grained alloys, i.e., it deforms by a dislocation climb mechanism at elevated temperatures (approximately 425-500 C). At high strain rates, however, the 2124-0.6Zr alloy exhibits superplasticity, similar to that observed in SiC whisker reinforced 2124 Al composites. The maximum tensile elongation is about 500%, recorded at 475 C, and at a strain rate of 3.3 [times] 10[sup [minus]1]s[sup [minus]1]. The high strain rate phenomenon is consistent with the general trend observed in aluminum-based alloys, namely, an increased strain rate for optimal superplastic flow with a decrease in grain size.

Superplastic Gas‐Pressure Deformiation of YTZ Sheet

Abstract: The superplastic deformation behavior of fine-grained, yttria-stabilized tetragonal zirconia under conditions of biaxial gas-pressure deformation is described. Sheet specimens were deformed into hemispherical caps at temperatures ranging from 1,450 to 1,600C and at imposed gas pressures ranging from 345 to 2,760 kPa. For the forming conditions examined, hemispherical caps were formed at times ranging from 2 to 476 min. A gas-pressure-forming apparatus used to conduct the experiments is described and a mechanical analysis of the deformation process is presented. Also discussed is the role of grain growth and the relationship between data obtained in uniaxial testing and the behavior observed during the biaxial deformation experiments of this study. A comparison is made between the observed thickness distribution in the hemispherical caps with predictions from an analytical model.

Forming of diffusion bonded joints in superplastically formed metal structures

Abstract: A method of forming a diffusion bonded joint during superplastic forming of a hollow metal structure (43) from sheet material (15, 16, 17, 18) using a two-piece die (1) having opposed clenching faces (9, 10) disposed along a split line (2) thereof, the structure 43 being formed with a joint at the split line, the method including placing the sheets (15, 16, 17, 18) between the faces (9, 10) clenching the sheets with a pressure sufficient to locate same during superplastic forming while being insufficient to deform the metal and prevent superplastic flow in the sheets between the clenching faces (9, 10), heating the sheets to a superplastic forming temperature and applying a gas pressure between the sheets to commence superplastic forming, when superplastic metal flow between the clenching faces substantially ceases upset forging metal from between the faces towards the interior of the die to increase wall thickness at the joint upon trimming off a flange formed by that part of the sheets trapped between the clenching faces. A die to carry out the above method, a die and sheet assembly and a structure manufactured according to the method are also provided.

Advances in superplasticity and superplastic forming

Abstract: A symposium on Advances in Superplasticity and Superplastic forming was held in TMS Chicago Fall meeting during November 2--5, 1992. There were three sessions with 15 papers presented, with renewed interest in superplasticity. The interest ranged from the understanding of the fundamental deformation mechanisms to the application of this phenomenon to new materials including metal matrix composites and ceramics. Individual papers have been processed separately for inclusion in the appropriate data bases.