The nucleation and growth of cavities in a superplastic quasi-single phase copper alloy

Experiments were conducted on a superplastic quasi-single phase copper alloy, Coronze CDA 638, to provide information on the nucleation and growth of internal cavities during deformation. It is shown that, at a temperature of 823 K, the cavities are generally associated with the presence of large Co-rich particles on the grain boundaries, with nucleation occurring at the particle/grain boundary interfaces. The cavities tend to form in stringers and these stringers are always oriented along the rolling direction regardless of the direction of the tensile axis. Thus, the cavity stringers are clearly associated with the Co-rich particles which also lie in stringers parallel to the rolling direction. A detailed series of tests at an initial strain rate of 1.3 × 10⁻⁵s⁻¹ shows that the cavities grow by a diffusion-controlled mechanism for cavity radii <15 μm and by the power-law mechanism for radii >20μm. It is demonstrated that this conclusion is consistent both with direct measurements of the cavity growth rates and with estimates of the increases in cavity size due to superplastic diffusion growth. The results therefore emphasize the importance of diffusion-controlled cavity growth at low strain rates.     

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

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.     

Large strain behaviour of a superplastic copper alloy—II. Cavitation and fracture

A quantitative study of cavitation damage and fracture of a superplastic copper alloy, Coronze 638, has been made. Cavities are found to nucleate at large particles present in the form of stringers. The size and shape of cavities, as well as the level of damage up to fracture are essentially independent of strain rate over regions I and II of the σ−ϵ curve, as are the true strain to fracture and the development of t instabilities. As the strain rate increases into region III, the level of damage to failure decreases, while the true failure strain increases and necks become sharper. Extensive cavity coalescence is observed up to strains of about 1.5, producing a number of large (> 100 μm) cavities which exhibit a high stability, and little tendency to coalescence. This allows the sample to sustain a very high level of cavitation without failure. The mechanism of cavity growth for small isolated cavities (< 10 μm) is thought to be diffusive growth constrained by matrix creep at low strain rates, with a transition to plasticity controlled growth at large strain rates. For larger cavities growth appears to be entirely creep controlled. Final fracture occurs by the material exhaustion in the ligaments between voids once the reduction in the cross section exceeds about 30%. No large instability either in flow or damage seems to be involved in this process.     

镁合金的超塑性与损伤定量分析

研究了轧制AZ31B镁合金板材的超塑性与空洞损伤，对拉伸试样在超塑性变形各阶段轴剖面的空洞进行了观察，通过对空洞演化的分析建立了空洞体积分数与变形程度的定量关系。研究结果表明：AZ31B镁合金板材在一定的变形条件下具有良好的超塑性；变形伴随着空洞的形核、长大，继而发生空洞的连接，导致材料断裂；空洞体积分数随着变形程度的增加呈指数规律变化。     

Cavitation at grain and phase boundaries during superplastic flow of an aluminum bronze

Cavities have been observed to form at grain and phase boundaries under certain strain rate conditions during superplastic tensile deformation of a Cu-9.5 pct Al-4 pct Fe aluminum-bronze. The cavities form preferentially at α-β interfaces or triple junctions involving both phases. The process of cavitation is associated with grain boundary sliding and cavity nucleation probably occurs at points of stress concentration in the sliding interfaces. The ductility is not markedly impaired by the cavities because the high strain-rate sensitivity of the material inhibits the interlinkage of cavities at high strains. A range of strains and strain rates for superplastic forming processes has been determined at which the volume fraction of cavities present was tolerable.     

Cavitation and cavity growth during superplastic flow in microduplex Cu-Zn-Ni alloys

A study has been made of cavity growth during superplastic tensile deformation of two microduplex α/β nickel-silvers, one a Cu-Zn-Ni alloy and the other a Cu-Zn-Ni-Mn alloy. For cavities with radii of >0.5 /gmm, measured growth rates were found to be in good agreement with values calculated on the assumption that cavity growth was controlled by viscous flow of the matrix. For smaller cavity sizes a diffusional growth mechanism could predominate. Metallography revealed that cavity morphology changed with strain in a manner consistent with diffusion-controlled growth at small sizes, and matrix deformation controlled growth at intermediate and large cavity sizes. Density studies showed that the overall level of cavitation was independent of both strain rate and temperature, and was influenced only by strain.     

An analysis of cavity nucleation in superplasticity

Superplastic alloys possess either a quasi-single phase or a microduplex microstructure: in quasi-single phase alloys, cavities are observed to nucleate predominantly at coarse grain boundary particles whereas in microduplex alloys, cavities tend to form at interphase boundaries and at triple point junctions. A general analysis is presented for cavity nucleation, in both microstructures, under the stress concentrations caused by bursts of grain boundary sliding during superplastic deformation. In quasi-single phase alloys, calculations indicate the cavities nucleate at coarse particles located at grain boundaries because local interphase diffusion creep cannot accommodate the stress concentrations sufficiently rapidly. The analysis demonstrates that it is possible for cavities to nucleate at grain boundary ledges under some limited experimental conditions. It is demonstrated also that the present analysis is in agreement with the available experimental data on a quasi-single phase Cu-based superplastic alloy and a microduplex superplastic Zn-22% Al eutectoid alloy. Calculations show that small pre-existing cavities, if present, are likely to be sintered rapidly prior to superplastic déformation at elevated temperatures.     

Strain-rate effects on high-temperature fracture behavior of a 2124AI-SiCw composite

The high-temperature fracture behavior of a 2124Al-SiC_w composite compared with a 2124A1 alloy was investigated in this study. Axisymmetric tensile tests were carried out over a temperature range from 25 °C to 550 °C and at strain rates from 5 × 10^-5 s^-1 to 0.3 s^-1. Detailed fractographical observations and cross-sectional microstructure analyses were also made to identify local micromechanical processes of cavity initiation at high temperature. One of the important results is that the cavity initiation sites of the composite are strongly influenced by the strain rate at high temperatures: cavities initiate at whisker ends at low strain rates and at whisker sides at high strain rates. Furthermore, the favored direction of cavity growth is also dependent upon the strain rate, being approximately 45 deg at low strain rates and perpendicular to the tensile axis at high strain rates. Such different local fracture processes at different strain rates are interpreted in terms of the role of the SiC whiskers on the load carrier in the composite at high temperatures. Formerly Research Assistant with the Department of Materials Science and Engineering, Pohang Institute of Science and Technology     

Evaluating the Superplastic Flow of a Magnesium AZ31 Alloy Processed by Equal-Channel Angular Pressing

Experiments show that the magnesium AZ31 (Mg-3 pct Al-1 pct Zn) alloy exhibits excellent superplastic properties at 623 K (350 °C) after processing by equal-channel angular pressing using a die with a channel angle of 135 deg and a range of decreasing processing temperatures from 473 K to 413 K (200 °C to 140 °C). A maximum elongation to failure of ~1200 pct was achieved in this alloy at a tensile strain rate of 1.0 × 10^?4 s^?1. Microstructural inspection showed evidence for cavity formation and grain growth during tensile testing with the grain growth leading to significant strain hardening. An examination of the experimental data shows that grain boundary sliding is dominant during superplastic flow. Furthermore, a comprehensive review of the present results and extensive published data for the AZ31 alloy shows the exponent of the inverse grain size is given by p ≈ 2 which is consistent with grain boundary sliding as the rate-controlling flow mechanism.     

An analysis of cavity growth during superplasticity

Cavity growth at high temperatures may be controlled by vacancy diffusion, giving cavities which are approximately spherical and randomly distributed, or by power-law creep, giving cavities which are elongated and aligned in the direction of the tensile stress. In general, diffusion growth is favored at low total strains, and there is a transition to power-law growth at a critical cavity radius,r _c. The value ofr _c increases with decreasing strain-rate, so that there is also a transition from predominanly power-law growth at high stress levels to predominantly diffusion growth at low stress levels. Both types of cavities have been observed in superplastic materials, but the diffusion growth rate may be enhanced if the cavity intersects a number of grain boundaries. The analysis is in good agreement with experimental results reported for three diffent superplastic materials. DAVID A. MILLER, formerly Research Associate, Department of Materials Science, University of Southern California.     

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

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 T _m, where T _m 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.     

An analysis of the effect of cavity nucleation rate and cavity coalescence on the tensile behavior of superplastic materials

A model utilizing a simple force-equilibrium approach was developed to establish the effect of the cavity nucleation rate and cavity coalescence on the uniaxial tensile behavior of superplastic metals. All cavities were assumed to be spherical and uniformly distributed within the material, irrespective of the degree of deformation. Material input parameters for the model comprised the cavity nucleation rate (N), the strain-rate sensitivity of the flow stress (m), and the growth parameter for individual cavities (η), which was taken to be a function of m. The effect of cavity coalescence on average void size and volume fraction was treated using an empirical relation, which correlates an average void growth rate to the growth rate of individual, noninteracting cavities. Model predictions indicated that the macroscopic quantities often used to describe cavitation behavior, i.e., “initial cavity volume fraction” (C _{v 0}) and “apparent cavity growth rate” (η _APP) describe the combined influence of cavity nucleation, growth, and coalescence. With regard to the overall tensile behavior, simulation results revealed that increasing cavity nucleation rates reduce ductility in a manner analogous to the effect of decreases in the strain-rate sensitivity. In addition, the failure mode was established with regard to the relative magnitudes of the cavity nucleation rate and the strain-rate sensitivity. Model predictions of tensile elongation and cavity-size distributions were validated by comparison to measurements found in the literature for cavitating superplastic materials.     

New aspects on the superplasticity of fine-grained 7475 aluminum alloys

Thermomechanical processes were developed which give fine grain sizes of 6 and 8 μm in the 7475 Al alloy. Superplastic properties of this material were evaluated in the temperature range of 400 °C to 545 °C over the strain-rate range of 2.8 x 10^-4 to 2.8 X 10^-2 s^-1. The maximum ductility exhibited by the alloy was approximately 2000 pct, and optimum superplasticity was achieved at a strain rate of 2.8 X 10^-3 s^-1 which is higher by an order of magnitude than other 7475 Al alloys. This result is attributed to the presence of fine dispersoids which maintain the fine grain size at high homologous temperatures. The flow stress and strain-rate sensitivity strongly depend on the grain size. The superplastic 7475 Al alloy has strain-rate sensitivities of 0.67 (6 μm) and 0.5 (13 μm) and an activation energy which is similar to the one for grain boundary diffusion of aluminum. Microstructural investigation after superplastic tests revealed zones free of dispersoid particles at grain boundaries primarily normal to the tensile direction. These dispersoidfree zones (DFZs) appear even after 100 pct elongation and are occasionally as large as 5 μm across. This result demonstrates the importance of diffusional flow in superplastic deformation of the fine-grained 7475 Al alloy especially at low elongations.     

A model study of cavity growth in superplasticity using single premachined holes

Experiments were conducted on a superplastic copper alloy to investigate the growth of single holes machined in the gage length prior to testing. Specimens were deformed in tension in the three regions of flow associated with superplastic materials. Within each flow region, three distinct stages of hole growth were identified. Initially, in stage 1, the hole simultaneously increases in length along the tensile axis but decreases in the dimension measured perpendicular to the tensile axis (“transverse contraction”). Subsequently, in stage 2, the hole grows both along and perpendicular to the tensile axis (“transverse growth”). Finally, in stage 3, a crack nucleates on either side of the hole and propagates to cause failure (“crack propagation”). It is shown that the transitions between the different stages of growth is dependent upon the initiation and development of macroscopic necking adjacent to the hole.     

Experimental studies on the superplastic forming of square shaped components from sheets of Ti-6Al-4V alloy

Superplasticity is the ability of a polycrystalline material to exhibit, in a relatively isotropic manner, large elongations when deformed in tension. This property is exploited during superplastic forming in the fabrication of complex shaped components which are otherwise technically difficult or economically costly to form by conventional methods. The ability of some titanium alloys to undergo superplastic deformation coupled with their diffusion bonding capability (SPF/DB) provides excellent opportunities to fabricate intricate parts in a single operation resulting in significant cost and weight savings, particularly in the manufacture of aerospace structures. In the present work, experimental studies to characterize the superplastic behaviour of an as-received titanium Ti-6Al-4V alloy sheet commonly used in aerospace structural applications are reported. Tensile test coupons prepared from the alloy sheet were subjected to high temperature tensile tests in the temperature range of 1123 K (850°C) to 1223 K (950°C) and strain rate range of 10^?4 s^?1 to 10^?2 s^?1 in order to characterize the superplastic deformation behaviour. Suitable dies, for superplastic forming of 80 mm × 80 mm square components to depths of 43 and 50 mm, were designed and fabricated. Components were superplastically formed at a temperature of 1200 K (927°C) and 0.7 MPa constant argon pressure. The components were characterized for their thickness distribution, mechanical and metallurgical properties and the results are presented.     

High-strain-rate superplasticity in bulk cryomilled ultra-fine-grained 5083 Al

The ductility and creep of bulk ultra-fine-grained (UFG) 5083 Al (grain size ∼440 nm) processed by gas atomization, cryomilling, and consolidation were studied in the temperature range 523 to 648 K. Also, the creep microstructure developed in the alloy was examined by means of transmission electron microscopy (TEM). The ductility as a function of strain rate exhibits a maximum that shifts to higher strain rates with increasing temperature. An analysis of the experimental data indicates that the true stress exponent is about 2, and the true activation energy is close to that anticipated for boundary diffusion in 5083 Al. These creep characteristics along with the ductility behavior of 5083 Al are a reflection of its creep behavior as a superplastic alloy and not as a solid-solution alloy. In addition, the observation of elongations of more than 300 pct at strain rates higher than 0.1 s⁻¹ is indicative of the occurrence of high-strain-rate (HSR) superplasticity. Microstructural evidence for the occurrence of HSR superplasticity includes the retention of equiaxed grains after deformation, the observation of features associated with the occurrence of grain boundary sliding, and the formation of cavity stringers. Grain size stability during the superplastic deformation of the alloy is attributed to the presence of dispersion particles that are introduced during gas spraying and cryomilling. These particles also serve as obstacles for dislocation motion, which may account for the threshold stress estimated from the creep data of the alloy.     

An investigation of superplasticity in a thermomechanically processed U-2Mo (α + δ) alloy

A uranium-2 molybdenum (U-2Mo) alloy was shown to exhibit superplastic behavior over the β + γ two-phase field temperature regime and over a limited temperature span in the α + γ field. At Oak Ridge, two distinct processes were developed that evolved microstructures conducive to superplasticity. These microstructures were shown to exhibit superplasticity (elongations >500 pct) over a broad range of strain rates, from 2.5 × 10^-4 to 1 × 10^-2 s^-1. A maximum value of 700 pct elongation was reached at 695 °C and a true constant strain rate of 2.5 × 10^-3 s^-1. This study details the processing sequences, microstructures, strain-rate sensitivity, and maximum elongation data generated to characterize the superplastic U-2Mo alloy. In addition, the fracture and cavitation analyses conducted on constant strain-rate tensile test specimens are discussed.     

Superplastic deformation,cavitation, and fracture of microduplex Cu-Ni-Zn alloys

A study has been made of the superplastic behavior during tensile straining of two α/β Cu-Ni-Zn alloys (nickel silvers). Cavitation occurred during deformation and has been studied using metallographic and density techniques. Cavities nucleated at α/β boundaries and triple points involving two phases, and cavity growth and interlinkage led to brittle superplastic fracture. Density studies showed that the volume of cavities increased with increasing strain, but was relatively independent of strain rate and temperature. The results were consistent with a high rate of cavity nucleation in the early stages of deformation, followed by a grain boundary sliding mechanism of growth.     

An analysis of cavitation occurring in near-&gg titanium aluminide during superplastic deformation

In order to understand the cavitation behavior of near-&gg titanium aluminide alloys under superplastic forming conditions, the uniaxial hot-tension behavior of a Ti-45.5Al-2Cr-2Nb (at. pct) rolled sheet material containing a microduplex structure was determined. Three initial microstructures were examined: as-rolled, and two coarser-grained rolled-and-heat-treated conditions (1177 °C/4 h or 1238 °C/ 2 h). The cavitation behavior was analyzed after isothermal constant-strain-rate tests were conducted at temperatures between 900 °C and 1200 °C and strain rates in the range of 10⁻⁴ to 10⁻² s⁻¹. Interrupted tests and strain-to-failure tests were conducted in order to track cavity growth with time. After testing at a given temperature and strain rate, as-rolled specimens developed fewer large-size cavities than heat-treated specimens, possibly due to the finer grain size in the as-rolled material. Cavity growth was found to be plasticity controlled; the largest cavity size and density of cavities increased with increasing strain or strain rate and decreasing temperature. Since the number of finest-sized cavities examined did not decrease with strain, it is believed that continuous cavity nucleation occurred. For all three initial microstructures, the optimum sheet-forming temperature in the regime examined was identified as 1200 °C, at which the lowest cavity growth rates and highest ductilities were observed.