Showing papers on "Superplasticity published in 2002"

An examination of the constitutive equation for elevated temperature plasticity

Abstract: It was 25 years ago that the symposium on rate processes in plasticity was organized. Since then, advances in transmission electron microscopy, large-scale computation as well as molecular dynamics simulation, etc. have contributed much to our understanding of elevated temperature plasticity. The constitutive relation that links the stress–strain rate–grain size–temperature relation (Mukherjee–Bird–Dorn, MBD correlation) was presented in 1968/1969 to describe the elevated-temperature crystalline plasticity. This equation has held up well during the intervening quarter of a century. It has been applied to metals, alloys, intermetallics, ceramics, and tectonic systems, and it has worked equally well. It made the depiction of deformation mechanism maps in normalized coordinates a reality and provided a rationale for estimating life prediction by giving a quantitative estimate of the steady-state creep rate in creep damage accumulation relationship. In the case of particle-dispersed systems as well as metal matrix composites, the introduction of the concept of a threshold stress was a substantial improvement in creep studies. One of the significant applications of the MBD relation has been in superplasticity. The concept of scaling with either temperature or with strain rate, inherent in this relationship, seems to be obeyed as long as the rate-controlling mechanism is unchanged. The application of this relation to high strain-rate superplasticity and also to low-temperature superplasticity has been illustrated. Experimental data demonstrate that superplasticity of nanocrystalline metals and alloys follows the general trend of the constitutive relation but with important differences in the level of stress and strain hardening rates. It is shown that in the nanocrystalline range, molecular dynamics simulation has the potential to yield data on stress–grain size–temperature dependencies at very low grain size ranges where experimentalists cannot conduct their studies yet.

Enhancement of strength and superplasticity in a 6061 Al alloy processed by equal-channel-angular-pressing

Abstract: Pre-equal-channel-angular-pressing (ECAP) solution treatment combined with post-ECAP aging treatment has been found to be effective in enhancing the room-temperature strength of 6061 aluminum alloy. The largest increase in ultimate tensile strength (UTS) (=460 MPa) and yield stress (YS) (=425 MPa) is obtained in post-ECAP aged 6061 Al with six pressings. The strength increases by a factor of 1.4 when compared to T6 treated commercial 6061 Al. The strength of 6061 Al obtained in the present research is higher than that of ECA-pressed 6061 Al with pre-ECAP peak-aging treatment studied by other investigators. The more effective strengthening of post-ECAP low-temperature aging may be linked with the higher dislocation accumulation rate in the solutionized matrix and the presence of higher density particles in the aged matrix. Modest low-temperature (523 K) and high-temperature (813 K) superplasticity is observed in the ECAP 6061 Al, which may be a result of increased grain bundary area from grain refinement.

The use of severe plastic deformation for microstructural control

Abstract: The microstructure of a metal may be very significantly changed by subjecting the material to severe plastic deformation (SPD) through procedures such as equal-channel angular pressing (ECAP) and high-pressure torsion. These procedures lead to a substantial refinement in the grain size so that the grains are reduced to, typically, the submicrometer or even the nanometer range. This paper describes the application of SPD to pure aluminum and aluminum-based alloys, with emphasis on the factors influencing the development of homogeneous microstructures of equiaxed grains separated by high-angle grain boundaries. Materials subjected to ECAP are capable of exhibiting exceptional mechanical properties including superplastic ductilities at very rapid strain rates. Examples of this behavior are presented and results are described showing the potential for using this approach in superplastic forming applications at high strain rates.

Superplastic behaviour of ultrafine-grained Ti–6A1–4V alloys

Abstract: Superplastic behavior of the ultrafine-grained (UFG) Ti–6A1–4V alloy produced by high pressure torsion (HPT) has been studied. High elongations (more than 500%) have been observed in this alloy during tensile tests at relatively low temperatures and high strain rates. At the same time, the superplastic behavior of this alloy has several specific features such as the relatively low values of strain rate sensitivity of flow stress and significant strain hardening. Moreover, it was shown that the alloy, processed by HPT, demonstrates an outstanding room temperature strength about 1500 MPa after superplastic deformation.

Characteristics of superplasticity domain in the processing map for hot working of as-cast Mg-11.5Li-1.5Al alloy

Abstract: Processing map for hot working of as-cast Mg-11.5Li-1.5Al alloy has been developed in the temperature range 200 450degreesC and strain rate range 0.001-100 s(-1). The map exhibited a single domain with a peak efficiency of 65% occurring at 400degreesC and 0.001 s(-1). Under these conditions, the material exhibited abnormal elongation. On the basis of the elongation, the grain structure, the apparent activation energy for hot deformation (95 kJ mole (1)) and the peak efficiency of power dissipation (65% corresponding to a strain rate sensitivity of about 0.5), the domain is interpreted to represent superplasticity, At strain rates higher than about 10 s(-1), the material exhibits microstructural instability, while at temperatures of 450degreesC and it strain rate of 0.001 s (1), grain boundary cracking is observed.

Constitutive behavior of superplastic materials

Abstract: Superplasticity is an intriguing inelastic process in solid materials with deformation upto several thousand percent. Forming sheet and bulk materials using superplastic forming has become an established manufacturing method in aerospace and lately in other industries. Developing the right constitutive behavior is important not only for modeling the process for manufacturing by engineering mechanicians but for choosing the right composition and processing for material scientists. Such an ideal constitutive equation has been eluding the analysts so far. This paper examines some of the fundamental misgivings about the origin of inelastic process in superplasticity compared to other well known deformation processes. Also an attempt is made to understand the basic characteristics of superplastic inelastic deformation at macroscopic, mesoscopic and atomic levels.

Continuous Dynamic Recrystallization in a Superplastic 7075 Aluminum Alloy

Abstract: New grain evolution taking place during superplasticity was studied by means of tensile tests as well as metallographic observation for a unrecrystallized coarse-grained 7075 aluminum alloy. Grain boundary sliding (GBS) frequently takes place even on the layered high angle boundaries (HABs) parallel to the tensile axis and brings about rotation of subgrains near the HABs and subsequently in grain interiors. The misorientations of (sub)grain boundaries evolved in the pancaked grains increase accompanied by a randomization of the initial texture, followed by development of new grains with HABs. This indicates that unrecrystallized and pancaked grain structure developed by cold rolling is an important prerequisite not only for the appearance of superplasticity, but also for the dynamic evolution of new fine grains. It is concluded that the mechanism of new grain evolution can be a deformation-induced continuous reaction, that is continuous dynamic recrystallization (CDRX). A model for CDRX is discussed in detail comparing with previous several models.

Deformation mechanisms during low-and high-temperature superplasticity in 5083 Al-Mg alloy

Abstract: The controlling deformation mechanisms and grain boundary sliding behavior during low-, medium-, and high-temperature superplasticity (LTSP, MTSP, and HTSP) in fine-grained 5083 Al-Mg base alloys are systematically examined as a function of strain. Grain boundary sliding was observed to proceed at temperatures as low as 200 °C. With increasing LTSP straining from the initial (e 1.0), the strain rate sensitivity m, plastic anisotropy factor R, high-angle grain boundary fraction, grain size exponent p, and grain boundary sliding contribution all increased. During the initial LTSP stage, there was little grain size dependence and the primary deformation mechanisms were solute drag creep plus minor power-law creep. At later stages, grain size dependence increased and grain boundary sliding gradually controlled the deformation. During MTSP and HTSP, solute drag creep and grain boundary sliding were the dominant deformation mechanisms.

Deformation behavior of differently processed γ-titanium aluminides

Abstract: A short review on forging and rolling of TiAl-ingot material as well as HIPed prealloyed powder is presented. The ingots show a lamellar microstructure with a strong cast texture and alignment of the lamellae, resulting in strong mechanical anisotropies. Hot-forging and heat-treatments are required to obtain isotropic mechanical properties due to a more random texture. The influence of strain rate and temperature on the development of microstructure during forging was studied. The role of twinning and dynamic recrystallization (DRX) will be discussed. After rolling a fine-grained microstructure is observed with a strong modified cube texture, which give rise to improved strength and creep resistance in the transverse direction (TD) of the sheets. Above 1000 °C the sheets can be deformed superplastically. DRX gives microstructures which obey a Zener–Hollomon power law.

Superplastic forming of a spherical shell out a welded envelope

Abstract: Superplastic free forming of edge welded titanium envelopes is investigated theoretically and experimentally. The envelope is transformed into a spherical shell without clamping of its edges so that the final diameter of the spherical shell, D s , produced is less than the initial diameter of the envelope, D 0 . Mathematical modelling of the process under consideration is based on the principal equations of the membrane theory of shells. Experimental justification of the model suggested is carried out using the welded envelopes of various dimensions made from different commercial titanium alloys. Comparison of the theoretical predictions with corresponding experimental data is carried out obtaining good agreement. It is found that the upper limit of the value of D 0 / D s is equal to 1.25, which corresponds to the case of superplastic forming of an isotropic envelope when the influence of the welded joint is negligible. It is shown that superplastic free forming makes it possible to produce spherical shells having more uniform thickness distribution as compared with conventional procedure where the envelope is clamped rigidly along its periphery. Technological operations providing the above-mentioned advantages of superplastic free forming include the suitable choice of D 0 , the correct arrangement of the sheets and an appropriate method of welding. The model developed enables one to calculate the pressure–time cycle, thickness distribution and initial geometry of the envelope.

Low-temperature superplastic behavior of a submicrometer-grained 5083 Al alloy fabricated by severe plastic deformation

Abstract: A submicrometer-grained structure was introduced in a commercial 5083 Al alloy by imposing an effective strain of ∼8 through equal channel angular pressing. In order to examine the low-temperature superplastic behavior, the as-equal channel angular pressed (as-ECAP) samples were tensile tested in the strain rate range of 10−5 to 10−2 s−1 at temperatures of 498 to 548 K corresponding to 0.58 to 0.65 Tm, where Tm is the incipient melting point. The mechanical data of the alloy at 498 and 548 K exhibited a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain rate sensitivity was 0.1 to 0.2 in the low- and high-strain rate regions and 0.4 in the intermediate-strain rate region, indicating the potential for superplasticity. At 523 K, instead of the sigmoidal behavior, a strain rate sensitivity of 0.4 was maintained to low strain rates. A maximum elongation of 315 pct was obtained at 548 K and 5×10−4 s−1. The activation energy for deformation in the intermediate-strain rate region was estimated as 63 kJ/mol. Low-temperature superplasticity of the ultrafine grained 5083 Al alloy was attributed to grain boundary sliding that is rate-controlled by grain boundary diffusion, with a low activation energy associated with nonequilibrium grain boundaries. Cavity stringers parallel to the tensile axis were developed during deformation, and the failure occurred in a quasi-brittle manner with moderately diffusive necking.

High Strain Rate and/or Low Temperature Superplasticity in AZ31 Mg Alloys Processed by Simple High-Ratio Extrusion Methods

Abstract: There have been numerous efforts in processing metallic alloys into fine-grained materials, so as to exhibit high strain rate superplasticity (HSRSP) and/or low temperature superplasticity (LTSP). The current study applied the most simple and feasible one-step extrusion method on the commercial AZ31 magnesium ingot to result in HSRL 200°C elongations of 600% at 1×10−4 s−1 and 425% at 1×10−3 s−1; and 300°C elongations of 900% at 1×10−4 s−1, 520% at 8×10−3 s−1, 300% at 2×10−2 s−1, and 210% at 1×10−1 s−1. This suggests that the current AZ31 Mg alloy has possessed HSRSP at relatively low temperatures of 280–300°C, as well as LTSP at 200°C. The low flow stress of 15–30 MPa and the true strain rate sensitivity of 0.3–0.4 both suggest that grain boundary sliding and solute drag creep have operated under these loading conditions. The current results imply that the simple high-ratio extrusion method might be a feasible processing mean for industry applications.

Diffusion creep in oxide ceramics

Abstract: Although the models for diffusion creep were developed over 50 years ago, recently there has been considerable debate in the literature regarding the experimental validity of the models. This report summarizes recent theoretical developments in diffusion creep, with specific emphasis on ceramics. Concomitant microstructural changes during high temperature deformation, such as grain growth and cavitation, frequently interfere with the identification of the rate controlling process. Recent careful experiments coupled with appropriate microstructural characterization will be reviewed in structural oxide ceramics such as alumina and tetragonal zirconia; it will be demonstrated that these oxides deform by diffusion creep. Detailed analysis of the data in tetragonal zirconia will also be utilized to demonstrate the significance of diffusion creep in superplastic ceramics.

Superplasticity and cavitation of recycled AZ31 magnesium alloy fabricated by solid recycling process

Abstract: Superplastic behaviour of Mg-alloy AZ31 was investigated to clarify the possibility of its use for superplastic forming (SPF) and to accurately evaluate material characteristics under a biaxial stress by utilizing a multi-dome test. The material characteristics were evaluated under three different superplastic temperatures , 643, 673, and 703 K in order to determine the most suitable superplastic temperature. Finite Element Method (FEM) simulation of rectangular pan forming was carried out to predict the formability of the material into a complex shape. The superplastic material properties are used for the simulation of a rectangular pan. Finally, the simulation results are compared with the experimental results to determine the accuracy of the superplastic material characteristics. The experimental results revealed that the m values are greater than 0.3 under the three superplastic temperatures, which is indicative of superplasticity. The optimum superplastic temperature is 673 K, at which a maximum m value and no grain growth were observed. The results of the FEM simulation revealed that certain localized thinning occurred at the die entrance of the deformed rectangular pan due to the insufficient ductility of the material. The simulation results also showed that the optimum superplastic temperature of AZ31 is 673 K.