The effect of grain size and temperature on the superplastic deformation behavior of a 7075 Al alloy

The high-temperature deformation behavior of a 7075 Al alloy has been investigated within the framework of a recently proposed internal-variable theory for structural superplasticity (SSP). The flow curves were obtained by performing a series of load relaxation tests for specimens with various grain sizes, at temperatures ranging from 445 °C to 515 °C. The overall flow curves were then separated into two parts, according to the respective physical mechanisms, viz., the grain-boundary sliding (GBS) and the accommodating dislocation glide processes, contrary to the conventional approach which uses a single power-law relation. These individual curves were then analyzed based on the internal-variable theory. Much valuable information has been obtained in this way, providing new physical insight as well as a more comprehensive understanding of SSP. The GBS curve could be described as a Newtonian viscous flow, signified by the power-index value of M _g =1.0 for this alloy. The unresolved issue of threshold stress is also clarified and identified as a critical stress required for the GBS. The role of grain refinement is found to shift the grain-matrix deformation (GMD) curve into a higher stress and strain-rate region, while the GBS curve into a lower stress and higher strain-rate region along the respective characteristic scaling line to bring both curves into a common flow-stress region, in which the GMD and GBS can operate simultaneously, resulting in the usual superplastic deformation behavior.     

Cavitation behavior of an Al−Cu eutectic alloy during superplastic deformation

Cavitation behavior upon deformation of an Al−Cu eutectic alloy was studied by densitometry and quantitative microscopy. Tensile specimens were strained to different strain levels at constant strain rates and temperatures over the range of 10⁻⁵ to 10⁻² s⁻¹ and 400° to 540 °C, respectively. The cavity volume increased with increasing strain and strain rate but decreased with increasing temperature. The increase in cavity volume occurred through an increase in both the number and size of cavities. The cavities were spherical in most of the cases, which was attributed to the diffusion-controlled cavity growth mechanism and its modification when the cavity size reaches the size of a grain. The number and volume of cavities were used to evaluate the nature of the cavity nucleation rate and the level of pre-existing cavities. B. P. KASHYAP, formerly Research Associate with the Metallurgical Sciences Laboratory, Engineering Faculty, University of Manitoba, Winnipeg, MB, Canada R3T 2N2     

A fractal model for cavity damage and fracture of materials during superplastic deformation

A fractal model for cavity damage and fracture of materials during superplastic deformation is proposed in this paper. The formula which shows the effect of gauge dimensions and grain size on total superplastic elongation was inferred. The formula shows that the finer the grain size, the larger the elongation of specimen and a larger elongation can be obtained by increasing the gauge width and thickness or by decreasing the gauge length of specimen under the same deformation condition. The prediction of the new formula is consistent with most experimental results observed up to now.     

The influence of grain junctions and boundaries on superplastic deformation

The crystallography of the grain boundary/grain junction network has been analysed in an aluminium—lithium alloy after 1100% superplastic deformation. The grain junctions were classified as “I-lines” or “U-lines” as defined by an extension to the O-lattice model. Overall there were 38% I-lines in the sample population which compares to a predicted 4% for random generation. It was shown that the grain orientation distribution alone was not responsible for the high proportion of I-lines. The occurrence of intergranular cavities in the microstructure was associated with high angle boundaries and U-lines. Cavities arrested at I-lines, which highlights the structural differences between the two junction types. With regard to the mechanisms of superplastic flow, it was argued that the two recognised competing mechanisms for grain accommodation, diffusion creep and dislocation creep, are associated with U- and I-lines respectively, which accounts for the observed proportions of these defects.     

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

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

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10⁻⁵ s⁻¹ to 1 × 10⁻² s⁻¹, with a maximum value of 0.65 at 5 × 10⁻⁴ s⁻¹ and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C. R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan     

Sources of acoustic emission generated during the plastic deformation of 7075 aluminum alloy

The plastic deformation of crystalline solids will in general result in the generation of acoustic emission. Identifying the actual sources of the acoustic emission is difficult since the data may be comprised of emissions from several sources which may be operating either independently or cooperatively. By using a series of testing procedures and simultaneously measuring the dislocation damping while testing, it has been possible to identify the major sources of acoustic emission in 7075-T6 and 7075-T651 aluminum alloys. For the 7075-T651 alloy essentially all of the emission is from the fracture of small intermetallic precipitates dispersed within the matrix. For the alloy in the T-6 temper there is additional acoustic emission occurring at the onset of plastic deformation due to break-away and subsequent motion of dislocation line segments from their pinning points. formerly graduate student at the University of Denver, Denver, CO.     

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.     

The mechanical properties of a superplastic quasi-single phase copper alloy

The mechanical properties of a superplastic quasi-single phase copper alloy, containing a cobalt-rich dispersion, are essentially identical to those of superplastic eutectic and eutectoid alloys. Points of similarity include a sigmoidal relationship between stress and strain rate dividing the behavior into three regions, activation energies which are similar to lattice and grain boundary diffusion in the low stress region I and the intermediate stress region II, respectively, and an inverse dependence on grain size raised to the power ~ 2.2–2.6 in regions I and II. In addition, experiments in the high stress region III indicate an approximately inverse dependence on grain size and activation energy which is intermediate between the values obtained in regions I and II. The results suggest that region III is due to an intragranular dislocation process with a concomitant contribution from grain boundary sliding. Identical creep curves are obtained in regions I and II with a very brief primary stage and an extended steady-state behavior, whereas the creep curves in region III exhibit a significantly longer primary stage.     

The mechanical properties of the superplastic AI- 33 Pct Cu eutectic alloy

Experiments were conducted to determine the mechanical properties of the superplastic Al-33 Pct Cu eutectic alloy at temperatures from 673 to 723 K. Specimens were tested in a well-annealed condition and there was no evidence for grain growth even at the lowest experimental strain rate of 6.7 × 1(10^-7 s^-1. It is shown that the stress-strain curves rapidly attain a steady-state value at strain rates below ′10^-4 s^-1, and there is a sigmoidal relationship between stress and strain rate which may be obtained using several different testing procedures. The maximum elongation to failure recorded in these experiments was 1475 Pct at an initial strain rate of 1.3 × 10^-5 s^-1. The true activation energy for plastic flow is 175 ±11 kJ mol^-1 in the superplastic region II, but it increases to 299 ± 18 kJ mol^-1 at low strain rates in region I. The exponent of the inverse grain size is 2.1 ±0.3 in region II. These results show that, when the grains size is stable, there is a genuine region I in the Al-33 Pct Cu alloy at initial strain rates below ∼10^-5 s^-1.     

The role of matrix dislocations in the superplastic deformation of a copper alloy

It is demonstrated that the densities of dislocations trapped in coherent twin boundaries may be used to provide a direct and quantitative comparison of the extent of intragranular slip in the three regions of behaviour associated with superplasticity. Measurements on a superplastic copper alloy, Coronze CDA 638, show that there is very little movement of matrix dislocations at low strain rates in region I, the movement of matrix dislocations increases with increasing strain rate in region II, and there are large numbers of mobile matrix dislocations at high strain rates in region III. Deformation in region II is considered to be controlled by grain boundary sliding occurring by the movement of grain boundary dislocations, while control of flow in region I is attributed to the rate at which grain boundary dislocations can bypass interfacial obstacles.     

A critical assessment of flow and cavity formation in a superplastic yttria-stabilized zirconia

Tensile tests were performed on specimens of high purity 3Y-TZP (tetragonal ZrO₂ stabilized with ∼ 3 mol% of Y₂O₃) at temperatures from 1623 to 1803 K. Superplastic-like flow was achieved with elongations of up to >400%. The experiments show that the stress exponent, n, is ∼ 3.0–3.4 and the activation energy for flow, Q, is ∼ 602±20 kJ mol⁻¹. Internal cavities developed in all specimens during flow, and quantitative measurements were taken of the cavity shapes and sizes over a range of experimental conditions. The results demonstrate that there is an increase in the extent of cavitation with (i) decreasing temperature at constant strain rate and (ii) increasing strain rate at constant temperature. The influence of strain rate on cavitation in 3Y-TZP is the opposite of most superplastic metals, and the difference arises because of an inhibition in the diffusion growth process in 3Y-TZP.     

The effect of fatigue deformation on microstructural evolution in a superplastic aluminum-zinc eutectoid alloy

The microstructure of a superplastic aluminum-zinc eutectoid alloy that had been fatigue tested at 100 °C and 200 °C was examined. At 100 °C, in the aluminum-rich phase, precipitate-free zones (PFZs) formed beside (Al)/(Zn) interphase boundaries because of interphase boundary migration. Interphase boundary migration was due to phase growth, which proceeded more rapidly during fatigue deformation than during annealing. At 100 °C and 200 °C, PFZs beside (Al)/(Al) grain boundaries were asymmetrical owing to grain boundary migration. The precipitation of the equilibrium zinc-rich phase in the aluminum-rich phase proceeded more rapidly during fatigue deformation than during annealing. J. W. BOWDEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON.     

Interface sliding,migration, and cracking during fatigue deformation of a superplastic aluminum-zinc eutectoid alloy

A superplastic aluminum-zinc eutectoid alloy was fatigue tested at 100 °C and 200 °C at different constant plastic strain amplitudes and strain rates. During fatigue deformation, the average peak stress increased with increasing strain rate and grain size and decreasing temperature. It was almost independent of the plastic strain amplitude. To detect interfacial sliding, interphase boundary migration, and intergranular cracking, selected areas on surfaces were examined before fatigue deformation and re-examined after fatigue deformation. Interface sliding, which was almost reversible, occurred on (Al)/(Al) and (Zn)/(Zn) grain boundaries and on (Al)/(Zn) interphase boundaries. Grains appeared to slide in groups. Cracks followed grain and interphase boundaries. Along an intergranular crack, most interfaces were (Zn)/(Zn) grain boundaries and (Al)/ (Zn) interphase boundaries. Grains deformed to accommodate interfacial sliding. The absence of slip lines suggested that diffusional creep made a significant contribution to deformation in grains of the zinc-rich phase. Deformation of the aluminum-rich phase involved the glide and climb of dislocations. J. W. BOWDEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto.     

A model for the evolution of grain size distribution during superplastic deformation

In a recent paper, we postulated that a grain size distribution in a polycrystal can result in mixed mode deformation during superplastic flow. Since diffusional flow is strongly grain size-dependent while power-law creep is not, it was inferred that large grains may deform by power-law creep, while concomitantly, the small grains deform by diffusional creep. Here, a first order model for dynamic change in the grain size distribution with strain is developed to explain the shape of the stress-strain curves obtained during superplastic deformation of aluminum alloys. The model is based on the simple assumption that regions deforming by diffusion creep suffer strain induced grain growth, while dislocation creep results in grain refinement. In spite of the approximation, the model correctly predicts the shape of the stress-strain curves. The possible significance of these concepts in classical dynamic recrystallization phenomena is also discussed.     

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.     

轧制变形量对Ti-45Al-7Nb-0.3W合金组织与性能的影响

利用X衍射分析(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、室温拉伸试验等手段,研究粉末冶金Ti-45Al-7Nb-0.3W(原子分数,%)合金包套轧制过程中的显微组织和力学性能的变化规律。结果表明:热等静压法态的Ti-45Al-7Nb-0.3W合金组织为近γ组织,主要由块状的γ相组成,同时包括少量的α2相及极少量的B2相。轧制后Ti Al合金板材为双态组织,B2相消失。随轧制变形量增加,合金板材强度增加,变形量为40%时,板材抗拉强度最大,达到955 MPa。继续增加变形量合金板材的力学性能有所降低。当变形量较小时,合金的塑性变形主要通过位错滑移和攀移来实现。随变形量增加,孪生和动态再结晶机制发挥作用。     

Influence of martensite formed during deformation on the mechanical behavior of Fe-Ni-C Alloys

Tension tests were performed on metastable austenitic Fe-Ni-C alloys to study the influence of martensite formed during straining on mechanical behavior. Although both stressassisted, plate martensite and fine, lathlike strain-induced martensite were formed, only stress-assisted martensite occurred in sufficient amounts to influence mechanical behavior. The formation of stress-assisted, plate martensite raised the flow stress and the strain-hardening rate, but only after 25 to 40 pct martensite had formed. A TRIP effect was obtained,i.e., ductility was enhanced by concurrent martensite formation, but this occurred over only a narrow temperature range because of the strong temperature dependence of the austenite-to-plate martensite reaction. This is in contrast to the behavior of high-Cr austenitic steels, which exhibit a TRIP effect over a wide temperature range because of the formation of large amounts of fine strain-induced martensite, whose formation is less temperature-sensitive than that of plate martensite. Formerly a graduate student at Stanford University