Cavitation behavior of an Al−Cu eutectic alloy during superplastic deformation
01 Mar 1989-Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science (Springer-Verlag)-Vol. 20, Iss: 3, pp 453-462
TL;DR: In this article, the cavities were spherical in most cases, which was attributed to the diffusion-controlled cavity growth mechanism and its modification when the cavity size reaches the size of a grain.
Abstract: 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.
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TL;DR: In this paper, the tensile ductility of superplastic ceramics and metallic alloys was investigated and the authors showed that tensile deformation is not only a function of the strain-rate-sensitivity exponent, but also a factor of the parameter ⋗e exp (Qc/RT), where Qc is the steady-state strain rate and RT is the activation energy for super-plastic flow.
Abstract: Superplastic ceramics and metallic alloys exhibit different trends in tensile ductility in the range where the strain-rate-sensitivity exponent, m, is high (m⩾0.5). The tensile ductility of superplastic metallic alloys (e.g. fine-grained zinc, aluminium, nickel and titanium alloys) is primarily a function of the strain-rate-sensitivity exponent. In contrast, the tensile ductility of superplastic ceramic materials (e.g. zirconia, alumina, zirconia-alumina composites and iron carbide) is not only a function of the strain-rate-sensitivity exponent, but also a function of the parameter ⋗e exp (Qc/RT) where ⋗e is the steady-state strain rate and Qc is the activation energy for superplastic flow. Superplastic ceramic materials exhibit a large decrease in tensile elongation with an increase in ⋗e exp (Qc/RT). This trend in tensile elongation is explained based on a “fracture-mechanics” model. The model predicts that tensile ductility increases with a decrease in flow stress, a decrease in grain size and an increase in the parameter (2γs−γgb), where γs is the surface energy and γgb is the grain boundary energy. The difference in the tensile ductility behavior of superplastic ceramics and metallic alloys can be related to their different failure mechanisms. Superplastic ceramics deform without necking and fail by intergranular cracks that propagate perpendicular to the applied tensile axis. In contrast, superplastic metallic alloys commonly fail by intergranular and transgranular (shearing) mechanisms with associated void formation in the neck region.
92 citations
15 Dec 2008-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, a relatively coarse-grained AZ61 alloy deformed under two conditions for which grain-boundary sliding (GBS) creep controlled plastic flow and which produced the same flow stress of 10MPa.
Abstract: Cavitation behavior has been investigated in a relatively coarse-grained AZ61 alloy deformed under two conditions for which grain-boundary sliding (GBS) creep controls plastic flow and which produce the same flow stress of 10 MPa. At a strain rate of 10−5 s−1 and a temperature of 573 K, GBS creep is rate controlled by grain-boundary diffusion, DGB. At a strain rate of 2 × 10−4 s−1 and a temperature of 648 K, GBS creep is rate controlled by lattice diffusion, DL. Tensile elongation is slightly greater when DGB accommodates GBS deformation. Despite accommodation of GBS by different diffusion mechanisms, cavity evolution under both deformation conditions is quite similar. Cavity volume percent increases similarly with strain under both conditions, as does the radius of the largest cavities. Cavity areal number density distributions are similar between the different deformation conditions when strain is a constant. All the features observed for cavitation indicate that cavity growth is plasticity controlled under both deformation conditions. The theory of plasticity-controlled cavity growth is in very good agreement with experimental data produced for this investigation.
20 citations
TL;DR: In this article, the size, shape, and configuration of internal cavities in superplastic metals and ceramics after deformation at high temperatures were investigated using a quantitative metallographic procedure and scanning video images.
Abstract: It is now well established that cavities are often formed during superplastic deformation. However, experimental investigations suggest important differences in the nature of the cavitation in typical superplastic metals and ceramics. These differences are demonstrated with reference to a superplastic Cu-based alloy and yttria-stabilized tetragonal zirconia (Y-TZP). By using a quantitative metallographic procedure and scanning video images, measurements are presented showing the size, shape, and configuration of internal cavities in these two materials after deformation at high temperatures.
11 citations
TL;DR: In this paper, the authors investigated the rate of cavitation with superplastic strain for a super-plastic AZ61 magnesium alloy at a strain rate of 2 × 10 - 4 s - 1 and temperature of 648 K, under the conditions of which an elongation of more than 250% has been found.
Abstract: The rate of cavitation with superplastic strain was investigated for a superplastic AZ61 magnesium alloy at a strain rate of 2 × 10 - 4 s - 1 and temperature of 648 K, under the conditions of which an elongation of more than 250% has been found. Cavities initiated at grain boundaries. The cavitation showed a growth perpendicular to the applied stress direction after the initial strains. The subsequent growth and coalescence of cavities invariably leads to failure of the material. The experimental growth rates are in good agreement with the rate predicted by the plasticity-controlled growth mechanism.
8 citations
TL;DR: Grain growth behavior of the Al-Cu eutectic alloy was investigated as a function of strain (e), strain rate and deformation temperature (T) over as discussed by the authors, where grain size increases with increase in strain and temperature.
Abstract: Grain growth behaviour of the Al-Cu eutectic alloy was investigated as a function of strain (e), strain rate
$$(\dot \varepsilon )$$
and deformation temperature (T) over
$$\dot \varepsilon $$
= 10−2 s−1 and T=400 to 540°C The grain size increases with increase in strain and temperature Upon deformation to a fixed strain, the grain growth is generally seen to be more at lower strain rates The rates of overall grain growth
$$(\dot d_{\varepsilon ,t} )$$
and due to deformation alone
$$(\dot d_\varepsilon )$$
, however, increase with increasing strain rate according to
$$\dot d_{\varepsilon ,t} \propto \dot \varepsilon ^{086} $$
and
$$\dot d_{\varepsilon ,t} \propto \dot \varepsilon ^{064} $$
, respectively The increase in the grain growth rate with strain rate is attributed primarily to the shorter time involved at higher strain rate for reaching a fixed strain The activation energy for grain growth under superplastic conditions is estimated to be 79 kJ mol−1
7 citations
References
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TL;DR: In this article, a study was 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 CZN-Ni-Mn alloy.
Abstract: 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.
43 citations
TL;DR: In this paper, the contribution of lattice diffusion to the diffusional growth of cavities is analyzed in detail and the possible transitions in rate-controlling cavity growth mechanisms are considered.
Abstract: Superplastic alloys usually deform to very large extents but excessive cavitation can lead to premature cavitation failure in these materials. Several mechanisms can contribute to the growth of cavities during superplastic deformation although, generally, there is only one mechanism that controls cavity growth. Cavity growth mechanisms of relevance to superplastic materials are analysed in detail and the possible transitions in rate-controlling cavity growth mechanisms are considered. The contribution of lattice diffusion to the diffusional growth of cavities is included in the overall analysis of cavity growth. Cavity growth maps are constructed to show the dominant cavity growth mechanisms under different experimental conditions. Equations are developed to predict the appropriate transitions in cavity growth mechanisms with increasing cavity radii. Finally, it is demonstrated that the predictions of the cavity growth maps are consistent with the experimental results in several superplastic materials.
42 citations
TL;DR: In this paper, the authors studied cavitation during superplastic tensile flow in an α/β brass using metallography and precision density measurements and found that the level of cavitation increased as strain, strain rate and grain size were increased and as the temperature was decreased.
Abstract: Cavitation during superplastic tensile flow has been studied in anα/β brass using metallography and precision density measurements. Cavities nucleated primarily at triple points and were sometimes associated with small second phase particles. The level of cavitation increased as strain, strain rate and grain size were increased and as the temperature was decreased. The influence of these variables can be interpreted in terms of their effects on cavity nucleation and/or cavity growth rates.
39 citations
39 citations
TL;DR: In this article, the deformation and cavitation behavior of three microduplex α/β brasses with varying proportions of α and β phases were studied and it was observed that both strain-rate sensitivity and elongation-to-failure passed through a maximum when approximately equivolume proportions of the two phases were present.
Abstract: Studies have been made, using metallographic and precision density techniques, of the deformation and cavitation behaviour during superplastic tensile straining at 873 K of three microduplexα/β brasses which, as a consequence of varying composition, contained varying proportions ofα andβ phases. It was observed that both strain-rate sensitivity and elongation-to-failure passed through a maximum when approximately equivolume proportions of the two phases were present. Cavitation, on the other hand, decreased rapidly as the volume fraction ofβ phase was increased. The cavitation behaviour was attributed to the relative abilities of the phases to accommodate grain boundary sliding. When a high proportion ofα phase is present accommodation is minimal and cavity nucleation. occurs readily. Evidence is presented to show that grain-boundary sliding plays a predominant role in cavity growth. When a high proportion ofβ phase is present accommodation is almost complete and cavity nucleation is minimal.
34 citations