Showing papers on "Superplasticity published in 1995"

Superplasticity in powder metallurgy aluminum alloys and composites

Abstract: Superplasticity in powder metallurgy aluminum alloys and composites has been reviewed through a detailed analysis. The stress-strain curves can be put into four categories: a classical well-behaved type, continuous strain hardening type, continuous strain softening type and a complex type. The origin of these different types of stress-strain curves is discussed. The microstructural features of the processed material and the role of strain have been reviewed. The role of increasing misorientation of low angle boundaries to high angle boundaries by lattice dislocation absorption is examined. Threshold stresses have been determined and analyzed. The parametric dependencies for superplastic flow in modified conventional aluminum alloys, mechanically alloyed alloys and aluminum alloy matrix composites is determined to elucidate the superplastic mechanism at high strain rates. The role of incipient melting has been analyzed. A stress exponent of 2, an activation energy equal to that for grain boundary diffusion and a grain size dependence of 2 generally describes superplastic flow in modified conventional aluminum alloys and mechanically alloyed alloys. The present results agree well with the predictions of grain boundary sliding models. This suggests that the mechanism of high strain rate superplasticity in the above-mentioned alloys is similar to conventional superplasticity. The shift of optimum superplastic strain rates to higher values is a consequence of microstructural refinement. The parametric dependencies for superplasticity in aluminum alloy matrix composites, however, is different. A true activation energy of 313 kJ mol−1 best describes the composites having SiC reinforcements. The role of shape of the reinforcement (particle or whisker) and processing history is addressed. The analysis suggests that the mechanism for superplasticity in composites is interface diffusion controlled grain boundary sliding.

Superplasticity in Earth's Lower Mantle: Evidence from Seismic Anisotropy and Rock Physics

Abstract: In contrast to the upper mantle, the lower mantle of the Earth is elastically nearly isotropic, although its dominant constituent mineral [(Mg,Fe)SiO 3 perovskite] is highly anisotropic. On the basis of high-temperature experiments on fabric development in an analog CaTiO 3 perovskite and the elastic constants of MgSiO 3 perovskite, the seismic anisotropy was calculated for the lower mantle. The results show that absence of anisotropy is strong evidence for deformation by superplasticity. In this case, no significant transient creep is expected in the lower mantle and the viscosity of the lower mantle is sensitive to grain size; hence, a reduction in grain size will result in rheological weakening.

Selected processing for non-equilibrium light alloys and products

Abstract: A new class of light or reactive elements and monophase α′-matrix magnesium- and aluminum-based alloys with superior engineering properties, for the latter being based on a homogeneous solute distribution or a corrosion-resistant and metallic shiny surface withstanding aqueous and saline environments and resulting from the control during synthesis of atomic structure over microstructure to net shape of the final product, said α′-matrix being retained upon conversion into a cast or wrought form. The manufacture of the materials relies on the control of deposition temperature and in-vacuum consolidation during vapor deposition, on maximized heat transfer or casting pressure during all-liquid processing and on controlled friction and shock power during solid state alloying using a mechanical milling technique. The alloy synthesis is followed by extrusion, rolling, forging, drawing and superplastic forming for which the conditions of mechanical working, thermal exposure and time to transfer corresponding metastable α′-matrix phases and microstructure into product form depend on thermal stability and transformation behavior at higher temperatures of said light alloy as well as on the defects inherent to a specific alloy synthesis employed. Alloying additions to the resulting α′-monophase matrix include 0.1 to 40 wt. % metalloids or light rare earth or early transition or simple or heavy rare earth metals or a combination thereof. The eventually more complex light alloys are designed to retain the low density and to improve damage tolerance of corresponding base metals and may include an artificial aging upon thermomechanical processing with or without solid solution heat and quench and annealing treatment for a controlled volume fraction and size of solid state precipitates to reinforce alloy film, layer or bulk and resulting surface qualities. Novel processes are employed to spur production and productivity for the new materials.

The enhancement of superplastic flow in tetragonal zirconia polycrystals with SiO2-doping

Abstract: Superplasticity in SiO{sub 2}-doped tetragonal zirconia polycrystal (2.5Y-TZP) is investigated by means of tensile testing in a temperature range 1,200--1,500 C. The grain boundary SiO{sub 2} phase reduces the flow stress and greatly enhances the tensile ductility in TZP. The stress and grain size exponents take a value close to 2 and 3, respectively, in this temperature range, but there is an abrupt change in activation energy at 1,380 C in SiO{sub 2}-doped TZP. The activation energy for superplastic flow above and below this temperature is estimated to be 182 kJ/mol and 635 kJ/mol. The enhancement of superplasticity due to SiO{sub 2}-doping is explained in terms of the accelerated plastic flow in the grain boundary SiO{sub 2} phase to accommodate the stress concentration generated by grain boundary sliding. The elongation to failure in the SiO{sub 2}-doped TZP is phenomenologically described as a function of the Zener-Hollomon parameter. At high temperatures and low strain rates, the cavities tend to align and interlink parallel to the tensile axis, and the huge elongation in excess of several hundred percent is obtained.

In situ observation of partial melting in superplastic aluminum alloy composites at high temperatures

Abstract: The possibility of partial melting and its relations to the superplasticity at high strain rates were studied with transmission electron microscopy and differential scanning calorimetry in AlCuMg (2124), AlMg (5052), and AlMgSi (6061) alloys reinforced with Si 3 N 4 particles Calorimetry measurements of all three composites showed a sharp endothermic peak at an optimum superplastic temperature At the same temperature, transmission electron microscopy showed the melting of grain boundaries and interfaces, suggesting direct correlations between partial melting and the superplasticity Solute segregation was also observed at boundaries and interfaces, and was discussed as causes for partial melting

Grain refinement and superplasticity in 5083 Al

Abstract: A preliminary investigation of thermomechanical processing of 5083 aluminum plate (Al-4.7%Mg-0.7%Mn) was undertaken to develop a fine-grain sheet for superplastic forming applications. Significant differences in grain size and the extent of superplasticity are seen in hot-rolled vs. cold-rolled sheets, with tensile elongations exceeding 600% for the cold-rolled alloy. Additionally, a separate fine-grain sheet of the same alloy, produced by Alusuisse Co., was studied in greater detail. Superplastic deformation behavior of this sheet was investigated under uniaxial tension over the temperature range of 500–565 °C. Strain rate sensitivity values greater than 0.3 were observed over a strain rate range of (3 × 10−5)–(1 × 10−2) s−1 with a maximum value of 0.65 obtained for strain rate of 5 × 10−4 s−1 at 565 °C. Constant-velocity tension tests consistently show larger strains to failure and lower strain hardening rate than the corresponding “constant-strain-rate” tests for the range investigated. A short but rapid prestraining step, prior to the normal superplastic straining, produced enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600% was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. This paper and a companion paper are used to provide the details of results obtained to date from this study.

High strength and high strain rate superplasticity in a Mg-Mg2Si composite

Abstract: The Mg-Mg2Si composite processed from rapidly solidified ribbons exhibited high strength of about 500 MPa at room temperature and superplastic behavior at high strain rates of 10−1 ~ 1 s−1 at 773 K. The high strain rate superplasticity is attributed to the very small grain size of about 1 μm.

Effect of liquid phases on the tensile elongation of superplastic aluminum alloys and composites

Abstract: One of the major drawbacks of conventional superplastic forming is that the phenomenon is only found at relatively low strain rates, typically about 10{sup {minus}4} to 10{sup {minus}3} s{sup {minus}1}. Recent studies, however, have demonstrated that superplasticity can in fact be found at considerably higher strain rates than 10{sup {minus}3} s{sup {minus}1}, i.e. at strain rates of up to 10{sup 0} to 10{sup 2} s{sup {minus}1}. This high-strain-rate superplasticity (HSRS) phenomenon was originally observed in metal matrix composites and then found in mechanically alloyed materials. The phenomenon was then studied in some detail, principally in metal matrix composites but was also pursued in the mechanically alloyed materials. More recently the effect has also been observed in metallic alloys produced by more conventional methods. Technologically, HSRS in metal matrix composites is expected to result in a viable, near-net-shape forming technique for the automobile, aerospace, and even semi-conductor industries. It was initially pointed out by Nieh et al. that the observed HSRS phenomenon may be related to the presence of some liquid phases at interfaces or grain boundaries as a result of, or at least accompanied by, the segregation of solutes to such regions. Recently, Mabuchi and Higashi, using an in situmore » TEM technique, have directly observed solute segregation at interfaces or grain boundaries and the preferential melting of these enriched grain boundaries. In the current paper, the authors present further evidence, through the analysis of existing data, to illustrate that the elongation to failure of many superplastic materials is associated with the presence, or the likelihood of the presence, of liquid phases at boundary interfaces. The materials and experimental procedures have been described previously in various papers.« less

XRD and XPS characterization of superplastic TiO2 coatings prepared on Ti6Al4V surgical alloy by an electrochemical method

Abstract: X-ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS), in conjunction with argon ion etching, were used to characterize the microstructure and the chemical composition of alkoxy-derived TiO2 coatings prepared on Ti6Al4V surgical alloy by an electrochemical method. The as-deposited oxide coatings prepared at room temperature (up to 40 μm thick) were amorphous, but transformed into nanocrystalline anatase at 550°C. Using a micro-indentation technique, it was found that nanocrystalline anatase coatings were ductile, permitting significant plastic deformation at room temperature. The XPS data also revealed the presence of significant proportion of physisorbed (OH) and chemisorbed H2O (i.e. Ti−OH) on the oxide surface, indicating that these coatings, similar to sol-gel-prepared titania, may serve as reactive substrates for heterogeneous nucleation of apatite under physiological conditions.

Intermetallic γ-TiAl based alloy sheet materials : processing and mechanical properties

Abstract: The status of rolling of intermetallic two-phase γ-TiAl based alloys is described. As prematerials both cast and subsequently forged ingots and hot isostatic pressed pre-alloyed powder compacts were used. The results are presented and the impact of the differently processed prematerials on the sheet quality is discussed. The influence of heat-treatments on the microstructure and thus on the mechanical properties of γ-TiAl based alloy sheets is presented. Superplastic deformation behavior is observed for temperatures exceeding 900°C. The results of sheet forming by means of superplastic forming and other forming techniques are illustrated.

Microstructural examination of a superplastic yttria-stabilized zirconia: Implications for the superplasticity mechanism

Abstract: A detailed microstructural examination was conducted on specimens of high purity superplastically-deformed 3Y-TZP (tetragonal ZrO{sub 2} stabilized with {approximately}3 mol.% Y{sub 2}O{sub 3}). Several of the microstructural features were similar to conventional superplastic metallic alloys including the retention of an equiaxed grain configuration, evidence for only limited dislocation activity within the grains and the concurrent development of internal cavitation. There was no detectable amorphous phase at any of the grain boundaries or at the triple junctions but there was some segregation of yttria to the boundary regions. It is concluded that there are two types of superplastic Y-TZP materials depending upon whether there is an amorphous phase at the grain boundaries. When an amorphous phase is absent, the behavior is fairly similar to superplastic metals except that an additional mechanism operates at the lower stress levels to impede grain boundary sliding.

Characterization and analysis of low-temperature superplasticity in 8090 Al-Li alloys

Abstract: The 8090 Al-Li alloys, after a special thermomechanical process (TMP), exhibited low-temperature superplasticity (LTSP) from 350 °C to 450 °C and behaved differently from the conventional high-temperature superplasticity (HTSP). The LTSP sheets after ~700 pct elongation at 350 °C and 8 × 10−4 s−1 still possessed fine “(sub)grains” 3.7 μm in size and narrow surface Li-depletion zones 11 μm in width, resulting in a post-SP T6 strength of ~500 MPa, significantly higher than that of the 8090 alloys tested at normal superplastic temperature of 525 °C or above. Examination from the movement of surface marker lines in LTSP samples confirmed the role of grain boundary sliding (GBS), coupled with grain rotation and migration. During the initial stage (<150 pct), GBS along certain higher-angled boundaries was proceeded along a plane ±45 deg with respect to the sample surface. With increasing straining, sliding between individual grains or grain groups was observed on other planes, forming a zigzag morphology. Transmission electron microscopy (TEM) observations revealed appreciable dislocation activities, suggesting the involvement of dislocation creep. The tensile behavior and deformation mechanisms of the HTSP and LTSP sheets were investigated and analyzed over the strain rates range 10−5 to 10−2 s−1. The strain-rate sensitivity(m value) for the LTSP and HTSP materials was found to be ~0.33 and 0.50, respectively. The activation energy was extracted to be 92 kJ/mole for the LTSP sheets and to be 141 kJ/mole for the HTSP sheets. Based upon these results, the primary deformation and accommodation mechanisms for the HTSP and LTSP sheets are GBS and dislocation creep, respectively.

Effect of temperature on the mechanical properties of mechanically-alloyed materials at high strain rates

Abstract: The superplastic properties of several mechanically-alloyed materials. including IN9021, IN9052, IN905XL and 15 vol.% SiCp/IN9021, have been characterized. The tensile elongation to failure, strain rate sensitivity and activation energy are found to be dependent upon the testing temperature. Specifically, marked changes in these properties are closely related to incipient melting points in the materials. These observations confirm the suggestion that the presence of a small amount of liquid phase at interfaces and grain boundaries not only enhances the strain rate for superplasticity, but also has a strong influence on the deformation mechanisms. When the amount of liquid phase increases, as a result of increasing temperature, the expected observation is noted, i.e. the presence of the liquid phase degrades the material properties. A model is proposed to explain the experimental observations.

High temperature deformation behavior of physical vapor deposited Ti-6Al-4V

Abstract: A detailed study has been conducted of the high temperature creep and microstructural evolution accompanying the creep deformation of an initially nanocrystalline Ti-6Al-4V alloy. For test temperatures of 600 and 680°C the alloy transformed from an (α + α′) to a single phase α during creep testing and exhibited exceptionally low creep resistance. During testing between 760 and 900°C, the alloy transformed to a conventional (α + β) microstructure and exhibited up to ten times the creep rates of conventional grain size (superplastic) Ti-6Al-4V. Creep models based on grain boundary sliding, dislocation and diffusional creep were combined with relationships for phase evolution and grain growth to predict stress—strain rate relationships at each test temperature. The analysis indicates that in the low temperature region dislocation accommodated GBS, in conjunction with diffusional flow, are responsible for creep whilst in the high temperature region diffusion accommodated GBS is the dominant mechanism.

Optimizing the properties of TiAl sheet material for application in heat protection shields or propulsion systems

Abstract: The present work represents a survey on the production of TiAl sheet material on an industrial scale and on the properties of these sheets. Using different final heat treatments, near-gamma and duplex microstructures have been produced. The mechanical properties as a function of the microstructure have been examined at room temperature (RT) and 700 °C. Additionally, the parameter field of temperature and strain rate for which superplastic forming of the sheet material is possible has been established. Mechanical testing at RT revealed that the strength of Ti-48Al-2Cr sheet material is mainly governed by grain size if the microstructure is homogeneous. The volume fraction and distribution of the second phase (α2-Ti3Al) appears to be of minor influence. Room temperature ductilities between 0.25 and 0.051 have been measured. At 700 °C the duplex microstructures show slightly improved strength compared to near-gamma microstructures. Depending on the deformation rate, fracture strains between 0.11 and 0.80 have been detected. Superplastic behaviour of Ti-48Al-2Cr (at.%) is governed by dynamic recrystallization or dynamic grain growth, depending on the initial grain size and the deformation parameters temperature and strain rate.

Combined heating cycles to improve efficiency in inductive heating operations

Abstract: Our induction heating workcell permits rapid and controlled heating and cooling of a workpiece within a wide temperature range. The induction workcell allows us to combine manufacturing operations, like SPF, brazing, and annealing, into a single heating cycle to save time, energy, capital, touch labor, and factory space. Superplastic forming (SPF) and brazing occur at temperatures that differ by about 150°-200° F. (85°-105° C.) or more as do SPF and β-annealing of titanium. We can combine these operations into an economical single cycle in which both operations are done during a single heating of the press Multisheet SPF parts with braze joints at selected locations are made by (a) heating a multisheet pack to its superplastic forming range below the melting point of the braze alloy, (b) superplastically forming the pack at the SPF temperature to form the sheets and to define braze joints having unmelted braze alloy, (c) increasing the temperature to the braze temperature of the braze alloy, and (d) cooling the pack below the superplastic range to freeze the braze alloy in the braze joint.

Transformational superelasticity in sputtered titanium-nickel thin films

Abstract: Transformational superelasticity, an effect of increasing technical importance, is displayed by alloy systems which are perhaps better known for their shape-memory characteristics. Superelastic materials are capable of large, recoverable anelastic strains associated with the formation and reversion of a stress-induced martensite phase from an ordered intermetallic parent lattice. In the present paper, the authors report the achievement of classic transformational superelasticity in sputtered thin films of titanium-nickel. In this case, the material has been processed so as to possess very fine grain size, which has conferred high austenite strength without post-deposition thermomechanical treatment. The films displayed austenite strengths of 0.8 GPa, fully recovered anelastic strains exceeding 4%, and stored strain energies near 22 MJ/m{sup 3}, clearly demonstrating the feasibility of producing thin film superelastic materials.

Partial melting and segregation behavior in a superplastic Si3N4/Al–Mg alloy composite

Abstract: AJ-Mg alloy (5052) composite reinforced with Si 3 N 4 particulates was investigated by transmission electron microscopy and electron energy loss spectroscopy. Partial melting was observed at matrix/reinforcement interfaces and matrix grain boundaries at a temperature near an optimum superplastic temperature. Segregation of solute elements (Mg and Si) was observed at the interfaces and grain boundaries. Both partial melting and solute segregation were found to depend on grain boundaries. The obtained results were explained by a decrease of the solidus temperature due to segregation whose extent depends on the type of the grain boundary structure.

Study of superplastic deformation in an FeAl based alloy with large grains

Abstract: In this paper some results of studies on a superplastically deformed FeAl based alloy are reported. The tensile behavior of the FeAl based alloy Fe-36.5Al-2Ti (in atomic percent) under different strain rates at high temperatures was examined by optical microscopy. The results revealed that the FeAl based alloy with the grain size of 350 {micro}m exhibited a large elongation of more than 140% at 900 C and 1,000 C under a strain rate range of 1.39{times}10{sup {minus}4}/s{approximately}2.78{times}10{sup {minus}2}/s. The maximum elongation is 208% at 1,000 C under a strain rate of 1.39{times}10{sup {minus}2}/s. The reason for the large elongation is ascribed to the dynamic recovery and recrystallization in this alloy during deformation at high temperatures.

Low-pressure diffusion bonding of SAE 316 stainless steel by inserting a superplastic interlayer

Abstract: Diffusion bonding is a solid-state joining technique in which two similar or dissimilar materials are brought together under pressure at a temperature below the melting point of the materials. For a material with lower flow stress, the applied pressure needed to provide a intimate contact surface will also be low. Another advantage in this case is that even if the workpieces possess a rougher surface it can be effectively bonded. A superplastic alloy is a typical example of such a material with lower flow stress. Furthermore, a superplastic alloy possesses very fine grains and thus more grain boundary diffusion paths will be present, which provides another beneficial effect for diffusion bonding. However, most commercial technical alloys do not have superplastic characteristics. In order to use the above advantages of lower flow stress and more diffusion paths only existing for superplastic materials, an innovative process has been proposed. By inserting a superplastic interlayer with diffusion bonding compatibility in between the workpieces to be bonded, a better bond may be obtained. In the present study, a SAE 316 stainless steel was diffusion bonded by this method. A SuperDux 65 stainless steel plate was employed as its superplastic interlayer.

Joining of Y-TZP parts

Abstract: Based on the grain boundary sliding (GBS) mechanism, the authors used the structural superplasticity of a 3 mol% Y{sub 2}O{sub 3}-tetragonal ZrO{sub 2} polycrystals (Y-TZP) as a new technique to join two pieces of this material. The fully dense 3 mol% Y-TZP material was obtained from 3Y-TZP powders (Tosoh Co. Japan) which were cold pressed, sintered at 1,430 C for 2 hours and then isostatic hot pressed at 1,400 C for 2 hours. The average grain size of the starting (as-HIPed) material was 0.3 {micro}m. This material was annealed at 1,600 C, for different amounts of time in order to get grain sizes of 0.5, 0.8 and 1.6 {micro}m. From these four type of materials, samples of dimensions 5x3x3 mm were cut and polished using diamond paste of decreasing sizes down to 1 {micro}m. Compression tests at constant cross head speed were carried out in air, and at temperatures between 1,350 and 1,450 C. Microstructural observations and macroscopic parameters as K{sub c} and H{sub v} in Y-TZP show clearly that grain boundary sliding can be used to perform the joining of ceramics through a very small deformation at lower temperatures and for shorter period of time than for direct diffusionalmore » bonding.« less

On the optimization of superplastic blow-forming processes

Abstract: The superplastic blow-forming process of thin sheets is analyzed, and an optimal stable deformation path that reduces production time is obtained. The analysis is based on an analytical model for the superplastic forming (SPF) of a long rectangular box made of Ti-6Al-4V alloy at 900 °C use of a microstructure-based constitutive equation for the strain rate and grain growth, a stability criterion, and a variable strain rate control. It is shown that by imposing a variable strain rate control scheme derived from the stability analysis, an optimal forming time can be developed while maintaining a stable deformation path. Some other control schemes also show effectiveness in either reducing the localized thinning in the formed sheet or reducing the required forming time. Effects of friction and initial grain sizes on the forming pressure profile and the thickness distribution of the formed sheet are also investigated.

Silicon nitride ceramic and process for forming the same

Abstract: There are provided a process for forming a silicon nitride sintered body, encompassing a sialon sintered body, by making much of the superplasticity of the sintered body intact as a simple material without formation thereof into a composite material, and a formed sintered body produced by the foregoing process. A silicon nitride sintered body (encompassing a sialon sintered body) having a relative density of at least 95% and a linear density of 120 to 250 in terms of the number of grains per 50 μm in length in a two-dimensional cross section of the sintered body is formed through plastic deformation thereof at a strain rate of at most 10-1 /sec under a tensile or compressive pressure at a temperature of 1,300 to 1,700° C. The formed sintered body has a degree of orientation of 5 to 80% as examined according to a method specified by Saltykov, a linear density of 80 to 200, and excellent mechanical properties especially at ordinary temperatures.

Superplasticity and superplastic forming, 1995

Abstract: A conference on Superplasticity and Superplastic Forming was held as part of a TMS Annual Meeting in Las Vegas, Nevada, USA, February 13--15, 1995. This was one of a continuing series of conferences on this topic sponsored by the TMS Shaping and Forming Committee. The subject of Superplasticity and Superplastic Forming (SPF) has been experiencing steady growth for the past 15 years. Starting with applications of superplastic titanium and aluminum alloys for aerospace and architectural uses, the technology has been moving steadily into other specialized materials and applications. More widespread applications are anticipated because of lower manufacturing costs offered by superplastic forming when compared to built-up machined assemblies and structures. Introduction of SPF into new arenas, however, does require development of more efficient design strategies suitable for those applications. On the basis of current interest, it is expected that automobile markets and high temperature materials including ceramics will show growth in the future. Concurrently, efforts to understand the mechanisms of superplastic flow have steadily continued in recent years with emphasis on high rate superplasticity. A considerable amount of new research addressing these issues is reported in these proceedings, bringing forth important considerations for materials testing, analysis and new phenomenology. Thirtymore » papers have been processed separately for inclusion of the data base.« less

Superplasticity in a powder metallurgy magnesium composite

Abstract: Metal-matrix composites (MMC) have great potential to be used in high-performance aerospace and automobile applications. It is important, therefore, to develop secondary processing for MMCs which can effectively produce complex engineering components directly from wrought products. Many studies now have been performed that demonstrate superplasticity can be developed in MMCs. Superplasticity has been reported in some Mg alloys, including ZK60, AZ31, AZ61, and Mg-Li alloys. But, none of these alloys showed superplasticity at high strain rates. Although HSRS has been extensively demonstrated in Al-base MMCs, both in PM and IM products, neither conventional superplasticity nor HSRS has yet been shown in any Mg-base composite. The purpose of this paper is to present, for the first time, the observation of HSRS in a 17 vol% SiC particulate-reinforced ZK60A magnesium composite (ZK60/SiC/17p).