Superplastic deformation of SiCp/2024 Al composite fabricated by spray atomization and codeposition

In this paper, data are presented on the microstructure and superplastic deformation mechanics of an aluminum alloy, 2024, containing 10 vol% SiC particles. The material was fabricated by spray atomization and codeposition. The properties were studied after pretreatment by isothermal hot compression and isothermal hot forward extrusion (extrusion ratio 10.0). The experimental results show that the strain-rate sensitivity index (m-value) is 0.48 and the limit elongation (the elongation at fracture) is 345 % during superplastic uniaxial tension. The optimum conditions for superplastic behavior are 753 K of deformation temperature and 1.0 × 10⁻³ s⁻¹ of initial strain rate. Superplasticity may result from the fine grain size and the well-distributed SiC particles during superplastic uniaxial tension. Moreover, the simple and convenient pretreatment used in this paper is easily applied to industrial practice.     

Structural evolution and superplastic formability of friction stir welded AA 2095 sheets

Friction stir welding was used to join superplastic AA 2095 sheets. The effect of welding rate on the grain size distribution and grain boundary misorientations in the stir zone was investigated. The superplastic behavior of the weld nugget parallel to the welding direction was also characterized at 495 °C and strain rates from 10⁻⁴s⁻¹ to 10⁻²s⁻¹. Increasing the welding rate during friction stir welding augmented the formation of a fine-equiaxed high-angle grain boundary structure within the stir zone. Increasing intensity of plastic straining during friction stir welding resulted in enhanced properties during subsequent superplastic formation. The maximum strain-to-failure was obtained for the weld made at a tool speed of 1000 rpm and a weld rate of 4.2 mm/s when tested at a superplastic forming strain rate of 10⁻³s⁻¹.     

Superplastic characteristics of fine-grained 7475 aluminum alloy

A 7475-aluminum alloy was subjected to a thermomechanical heat treatment that resulted in a final recrystallized grain size on the order of 10 μm. Tensile specimens of dimensions 10 × 4 × 2.3 mm were machined such that the tensile axis was parallel to the rolling direction. Tensile tests were carried out at high temperatures in the range of 773 to 803 K at different cross-head speeds corresponding to initial strain rates in the range of 10⁻⁴ to 10⁻² s⁻¹. Elongations of several hundred percent were observed at strain rates of <10⁻³ s⁻¹. The correlation between flow stress and strain rate suggests that the strain rate sensitivity m is close to 0.5 at the lower strain rates. The value of m decreases to ≈0.2 at high strain rates. The decrease in m suggests a transition in the rate-controlling process from superplastic deformation (m ≈ 0.5) to dislocation creep (m ≈ 0.2) with increasing strain rate. The calculated activation energies in the two deformation regions are consistent with the suggested rate-controlling processes.     

Hot deformation mechanisms in Ti-5.5Al-1Fe alloy

The mechanisms of hot deformation in the alloy Ti-5.5Al-1Fe have been studied in the temperature range 750 to 1150 °C and with the true strain rate varying from 0.001 to 100 s⁻¹ by means of isothermal compression tests. At temperatures below β transus and low strain rates, the alloy exhibited steady-state flow behavior, while, at high strain rates, either continuous flow softening or work hardening followed by flow softening was observed. In the β region, the deformation behavior is characterized by steady-state behavior at low strain rates, yield drops at intermediate strain rates, and oscillations at high strain rates. The processing maps revealed two domains. (1) In the temperature range 750 to 1050 °C and at strain rates lower than 0.01 s⁻¹, the material exhibits fine-grained superplasticity. The apparent activation energy for superplastic deformation is estimated to be about 328 kJ/mole. The optimum conditions for superplasticity are 825 °C and 0.001 s⁻¹. (2) In the β region, a domain occurs at temperatures above 1100 °C and at strain rates from 0.001 to 0.1 s⁻¹ with its peak efficiency of 47% occurring at 1150 °C and 0.01 s¹. On the basis of kinetic analysis, tensile ductility, and grain size variation, this domain is interpreted to represent dynamic recrystallization (DRX) of β phase. The apparent activation energy for DRX is estimated to be 238 kJ/mole. The grain size (d) is linearly dependent on the Zener-Hollomon parameter (Z) per the equation

Superplastic Behavior of Al-4.5Mg-0.46Mn-0.44Sc Alloy Sheet Produced by a Conventional Rolling Process

This article describes the superplastic behavior of the Al-4.5Mg-0.46Mn-0.44Sc alloy. The investigated alloy was produced by casting and was conventionally processed to form a sheet with a thickness of 1.9 mm and an average grain size of 11 μm. The superplastic properties of the alloy were investigated using a uniaxial tensile testing with a constant cross-head speed and with a constant strain rate in the range 1 × 10⁻⁴ to 5 × 10⁻² s⁻¹ at temperatures from 390 to 550 °C. The investigations included determinations of the true-stress, true-strain characteristics, the maximum elongations to failure, the strain-rate sensitivity index m, and the microstructure of the alloy. The m-values determined with the strain-rate jump test varied from 0.35 to 0.70 in the temperature interval from 390 to 550°C and strain rates up to 2 × 10⁻² s⁻¹. The m-values decreased with increased strain during pulling. The elongations to failure were in accordance with the m-values. They increased with the temperature and were over 1000%, up to 1 × 10⁻³ s⁻¹ at 480 °C and up to 1 × 10⁻² s⁻¹ at 550 °C. A maximum elongation of 1969% was achieved at an initial strain rate of 5 × 10⁻³ s⁻¹ and 550 °C. The results show that the addition of about 0.4 wt.% of Sc to the standard Al-Mg-Mn alloy, fabricated by a conventional manufacturing route, including hot and cold rolling with subsequent recrystallization annealing, results in good superplastic ductility.     

Mechanical Characteristics of Superplastic Deformation of AZ31Magnesium Alloy

As the lightest constructional metal on earth, magnesium (and its alloys) offers a great potential for weight reduction in the transportation industry. Many automotive components have been already produced from different magnesium alloys, but they are mainly cast components. Production of magnesium outer body components is still hindered by the material’s inferior ductility at room temperature. Magnesium alloys are usually warm-formed to overcome this problem; however, it was observed that some magnesium alloys exhibits superior ductility and superplastic behavior at higher temperatures. More comprehensive investigation of magnesium’s high temperature behavior is needed for broader utilization of the metal and its alloys. In this work, the high temperature deformation aspects of the AZ31B-H24 commercial magnesium alloy are investigated through a set of uniaxial tensile tests that cover forming temperatures ranging between 23 and 500 °C, and constant true strain rates between 2 × 10⁻⁵ and 2.5 × 10⁻² s⁻¹. The study targets mainly the superplastic behavior of the alloy, by characterizing flow stress, elongation-to-fracture, and strain rate sensitivity under various conditions. In addition, the initial anisotropy is also investigated at different forming temperatures. The results of these and other mechanical and microstructural tests will be used to develop a microstructure-based constitutive model that can capture the superplastic behavior of the material. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6–9, 2005.     

Superplastic behaviour and microstructure evolution of a fine-grained TA15 titanium alloy

The fine-grained microstructure of TA15 titanium alloy was prepared through two-step forging technology combined with high and low temperatures, and a transnormal superplastic elongation of more than 2000% was obtained. The superplastic behaviour and microstructure evolution were systematically researched at different temperatures and strain rates through superplastic tensile test. The results indicate that the fine-grained TA15 alloy exhibits superplasticity at temperatures of 760–980°C and initial strain rates from 1.1 × 10⁻² to 5.5 × 10⁻⁵ s⁻¹. The optimal superplastic conditions are 940°C and 3.3 × 10⁻⁴ s⁻¹, in which the average elongation is 2526% and the maximum elongation is 2743%. During superplastic deformation, dynamic recovery and recrystallization occur obviously, and the corporate effect of strain hardening and recrystallization softening decides the superplastic ability directly.     

High-temperature deformation behavior and processing map of 7050 aluminum alloy re]20101008

The high-temperature deformation behavior and processing map of 7050 aluminum alloy were investigated by tensile tests conducted at various temperatures (340, 380, 420, and 460 °C) with various strain rates of 10⁻⁴, 10⁻³, 10⁻², and 0.1 s⁻¹. The results show that the instability region with a peak power dissipation efficiency of 100 % occurs at the low deformation temperature region of 340 °C to 380 °C and high strain rates (>10⁻³ s⁻¹). The 7050 aluminum alloy exhibited a continuous dynamic recrystallization domain with power dissipation efficiency of 35% to 60 % in the deformation temperature range of 410 °C to 460 °C and the strain rate range of 10⁻⁴–10⁻³ s⁻¹. The domain with a power dissipation efficiency of 35 % to 50 % occurring at high deformation temperatures and strain rates was interpreted to represent dynamic recovery. Dynamic recovery and continuous dynamic recrystallization provide chosen domains for excellent hot workability.     

Digital image correlation analysis of the deformation behavior of Pb-free solders at intermediate strain rates

Digital image correlation (DIC) is a powerful tool for quantifying local stresses and strains. The demand for environmentally benign Pb-free solders and the push toward smaller portable electronics will make it more likely for solder interconnects to en-counter mechanical shock through dropping or mishandling. Thus, quantifying the strain rate behavior of Pb-free solders from the quasi-static to the shock regime is essential for developing reliable numerical models of the mechanical shock behavior. In this paper we report on the use of DIC to measure the local strain and strain rate occurring in the neck of Sn-3.5Ag-0.7Cu specimens, at the onset of necking. Tensile tests were conducted in the range 10⁻³s⁻¹–30 s⁻¹. A parametric study was conducted to identify the optimum DIC parameters for the experimental setup. The effect of microstructure and applied strain rate on the local values of strain and strain rate is discussed     

Strain Hardening and Strain-Rate Sensitivity of an Extruded Magnesium Alloy

The strain-hardening behavior and strain-rate sensitivity of an extruded AZ31B magnesium alloy were determined at different strain rates between 10⁻² and 10⁻⁵ s⁻¹ in relation to the thickness of specimens (2.5 and 4.5 mm). Both the common approach and Lindholm’s approach were used to evaluate the strain-rate sensitivity. The yield strength (YS) and the ultimate tensile strength (UTS) increased, the ductility decreased, and the brittle fracture characteristics increased with increasing strain rate. The thinner specimens exhibited a slightly higher UTS, lower ductility, higher strain-hardening exponent, and strain-hardening rate due to smaller grain sizes. The stage III strain-hardening rate linearly decreased with increasing true stress, but increased with increasing strain rate. In comparison to the common approach, the Lindholm’s approach was observed to be more sensitive in characterizing the strain-rate sensitivity due to larger values obtained. The thinner specimens also exhibited higher strain-rate sensitivity. As the true strain increased, the strain-rate sensitivity decreased.     

Tensile behavior of Al−4%Mg−0.4%Sc−0.5% misch metal alloy at room temperature

The effects of strain rate and pre-deformation in Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm (misch metal) alloy on tensile behavior and P-L effect have been investigated. Pre-deformation of Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm alloy clearly enhances the yield strength and ultimate strength, though it decreases the fracture strain. The yield strength of pre-deformed Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm alloy is higher than that of commercially used Al−Mg based alloys. The strength of Al−4 wt.%Mg−0.5 wt.%Sc−0.5 wt.%Mm alloy was changed slightly at a strain rate between 2×10⁻⁵s⁻¹ and 2×10⁻³s⁻¹, but changed significantly when predeformation was introduced. Tensile test results of as-cast Al-4 wt.%Mg-0.4 wt.%Sc-0.5 wt.%Mm alloy show a significant oscillation of serration during deformation at room temperature, and the critical strain (ε _c ), which is the strain at the start of serration, decreases with increasing strain rate. Pre-deformation of Al−4wt%Mg−0.4wt%Sc−0.5wt%Mm also affects the serration oscillation: it decreases the critical strain at lower strain rate and increases it at higher strain rate (>2×10⁻⁴s⁻¹).     

Deformation Mechanisms of Ti-6Al-4V During Tensile Behavior at Low Strain Rate

A superplastic Ti-6Al-4V grade has been deformed at a strain rate of 5 × 10⁻⁴ s⁻¹ and at temperatures up to 1050 °C. Structural mechanisms like grain boundary sliding, dynamic recrystallization, and dynamic grain growth, occurring during deformation, have been investigated and mechanical properties such as flow stress, strain hardening, and strain at rupture have been determined. Dynamic recrystallization (DRX) brings on a decrease in the grain size. This could be of great interest because a smaller grain size allows a decrease in temperature for superplastic forming. For DRX, the driving force present in the deformed microstructure must be high enough. This means the temperature must be sufficiently low to ensure storing of enough dislocation energy but must also be high enough to provide the activation energy needed for DRX and to allow superplastic deformation. The best compromise for the temperature was found to be situated at about 800 °C; this is quite a bit lower than the 925 °C referenced in the literature as the optimum for the superplastic deformation. At this medium temperature the engineering strain that could be reached exceeds 400%, a value high enough to ensure the industrial production of complex parts by the way of the superplastic forming. Microstructural, EBSD, and mechanical investigations were used to describe the observed mechanisms, some of which are concurrent. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6-9, 2005.     

Constitutive Equation and Processing Map for Hot Deformation of SiC Particles Reinforced Metal Matrix Composites

Flow behavior and microstructures of Al/15% SiCp were investigated by hot compression tests using Gleeble-1500 thermomechanical simulator at temperatures ranging from 440 to 500 °C with strain rates of 0.001-1.0 s⁻¹. The high-temperature deformation behaviors of Al/15% SiCp were analyzed based on the true stress-true strain curves. The results show that the softening mechanism at low strain rate (0.001 s⁻¹) is dynamic recovery, and at high strain rates (0.01, 0.1, and 1 s⁻¹) is dynamic recrystallization (DRX). Based on these experimental data, a set of constitutive equations for Al/15% SiCp are described by the Zener-Hollomon parameter, and the coefficients of equations are found to be functions of strain. The constitutive equations reveal the dependence of flow stress on strains, strain rates, and temperatures. Furthermore, the mean error between the experimental and the calculated flow stress was computed. The result shows that the calculated results from constitutive equations are in good agreement with the experimental results. To demonstrate the potential workability of Al/15% SiCp, the processing map was established. The stable zones and the instability zones in processing map are identified and verified through micrographs. As a result, the optimum strain rates and temperatures for effective hot working of Al/15% SiCp were determined.     

Beta phase superplasticity in titanium alloys by boron modification

Addition of boron to titanium alloys produces fine TiB whiskers in situ with excellent thermal stability and good chemical compatibility with the matrix. These whiskers stabilize a fine-grain microstructure by restricting grain growth at high temperatures in the β phase field. The hot deformation behavior in the β phase field (temperature range 1050–1200 °C) of Ti-6Al-4V alloys modified with two different levels of B additions (1.6 and 2.9 wt.%) produced by powder metallurgy was investigated using hot compression tests in the strain rate range of 10⁻³ to 10⁻¹ s⁻¹ and hot tensile tests at a nominal strain rate of 6×10⁻⁴ s⁻¹. The β phase exhibits superplasticity, which occurs due to stabilization of a fine-grain microstructure by the TiB. Matrix grain boundary sliding and β/TiB interface sliding appear to contribute to the β superplasticity. The ability to achieve superplasticity at higher temperatures enable lower flow stresses, improved chemical homogeneity, and high strain rate capability due to enhanced accommodation processes. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming, sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

The positive strain-rate dependence of ductility in a 50Mo-50Re alloy

This paper is a review of the recent studies of deformation and fracture behaviors, especially the anomalous strain-rate effect on plasticity of a 50Mo-50Re alloy. The ductility of this alloy was found to increase with increase in strain rate within the range of 10⁻⁶ s⁻¹ to 1 s⁻¹ at room temperature in air. The fracture surfaces in the alloy changed from brittle to ductile in nature with increasing strain rate. A damage-toughening phenomenon was also observed in this alloy.     

Upper bound analysis of deformation and dynamic ageing behavior in elevated temperature equal channel angular pressing of Al-Mg-Si alloys

In the present study, the plastic deformation and dynamic strain ageing behavior of Al-6082 (Al-Mg-Si) alloy treated with elevated temperature equal channel angular pressing (ECAP) were investigated using upper bound analyses. Tensile tests were carried out over wide ranges of temperature and strain rate in order to evaluate the dynamic ageing conditions. ECAP processing was then experimentally performed at temperatures from room temperature up to 200 °C under various strain rates ranging between 10⁻⁴s⁻¹ and 10⁻¹s⁻¹. The upper bound analysis solutions and the experimental results are comparable. A theoretical dynamic ageing region was found to be in the temperature range of 90 °C to 260 °C, which is in agreement with the experimental observations in the temperature range of 75 °C to 175 °C.     

Development of Processing Map for 7075 Al/20% SiCp Composite

The hot deformation behavior and microstructure evolution of 7075 Al/20% SiC_p composite have been studied using the processing map. Compression tests were carried out in the temperature range of 300-500 °C and at the strain rate range of 0.001-1.0 s⁻¹. The stable and unstable regions in the map were verified with the microstructural observations of the deformed compression specimens. The “stable” regions, i.e., dynamic recrystallization and “unstable” regions such as debonding of SiC particles, matrix crack, and adiabatic shear band formation were identified from the processing map and compared with the reported microstructural observations of the deformed compression specimens. The optimum hot working conditions for this composite were identified.     

Effect of stress state on the high temperature workability of AZ31 Mg alloy

The influence of stress state on the high temperature workability of rolled AZ31 Mg alloy was investigated on the basis of a processing map. To construct the processing map, high temperature compression tests were carried out on samples oriented parallel to the rolling direction at various temperatures (25 °C∼450 °C) and strain rates (10⁻³ s⁻¹∼5s⁻¹), and then the results were compared with those of a torsion test. The overall efficiency profiles of both the compression and torsion processing maps were similar to each other, but the index of dissipation efficiency in the torsion was somewhat lower than that in the compression. The microstructure of the compressed specimens revealed much finer grained structure than that of the torsion specimens. Such microstructural differences were attributed to the different tendencies of twin formation and texture evolution depending on the stress state.     

Superplastic deformation behaviour of friction stir processed 7075Al alloy

Commercial 7075Al rolled plates were subjected to friction stir processing (FSP) with different processing parameters, resulting in two fine-grained 7075Al alloys with a grain size of 3.8 and 7.5 μm. Heat treatment at 490 °C for 1 h showed that the fine grain microstructures were stable at high temperatures. Superplastic investigations in the temperature range of 420–530 °C and strain rate range of 1×10⁻³–1×10⁻¹ s⁻¹ demonstrated that a decrease in grain size resulted in significantly enhanced superplasticity and a shift to higher optimum strain rate and lower optimum deformation temperature. For the 3.8 μm 7075Al alloy, superplastic elongations of >1250% were obtained at 480 °C in the strain rate range of 3×10⁻³–3×10⁻² s⁻¹, whereas the 7.5 μm 7075Al alloy exhibited a maximum ductility of 1042% at 500 °C and 3×10⁻³ s⁻¹. The analyses of the superplastic data for the two alloys revealed a stress exponent of 2, an inverse grain size dependence of 2, and an activation energy close to that for grain boundary self-diffusion. This indicates that grain boundary sliding is the main deformation mechanism for the FSP 7075Al. This was verified by SEM examinations on the surfaces of deformed specimens.     

A study of the strain rate microstructural response and wear of metals

Titanium (Ti) and copper (Cu) pins were slid against alumina in a pin-on-disk machine at a load of 50 N and sliding speeds varying from 0.1 to 4 ms⁻¹. The evolution of the microstructure in the subsurface of the material and the wear rate was co-related to the strain rate microstructural response of the material in uniaxial compression, at different strain rates (0.1–100 s⁻¹) and temperatures (298–673 K). The strain rates and temperatures in the plastically deforming zone near the surface of the pins were determined using noniterative methods. The strain rates were found to be in the region of 100 s⁻¹ near the surface and decreases as one moves into the sub-surface of the pin. The temperatures increased as the speed increased. These estimated strain rates and temperatures were superimposed on the strain rate microstructural response maps of these materials. The uniaxial compression test results of Ti showed adiabatic shear banding as a microstructural mechanism that evolves at high strain rates (≥10 s⁻¹) and lower temperatures (<575 K). Adiabatic shear bands are sites of easy crack nucleation and propagation. When Ti is slid at low speeds the near surface region of the pins deform in the adiabatic shear banding regions in the strain rate microstructural response map. At such speeds the wear rate is found to be high and reduces as the sliding speed is increased, when the material undergoes a more homogeneous deformation. The microstructural response of Cu under uniaxial compression showed that the material undergoes flow banding at intermediate strain rates (1 s⁻¹) and temperatures of up to 473 K. The subsurface microstructure of the pins slid at low speeds showed subsurface cracking and sheet like debris formation. This happen at lower speeds because the flow banding and crack nucleation is expected in the subsurface where the strain rates and temperatures are lower. The present test results show a clear relation to exist between the strain rate response of the material in uniaxial compression and its subsurface microstructural evolution and wear rate.