Journal ArticleDOI

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

01 Mar 1989--Vol. 20, Iss: 3, pp 453-462

AbstractCavitation 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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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.

91 citations

Journal ArticleDOI
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.

19 citations

Journal ArticleDOI
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

Journal ArticleDOI
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

Journal ArticleDOI
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.

6 citations

##### References
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Book
01 Jan 1965

4,036 citations

Book
01 Jan 1965
Abstract: Part I General introduction. Formal geometry of crystal lattices. The theory of reaction rates. The thermodynamics of irreversable processes. The structure of real metals. Solids solutions. The theory of dislocations. Polycrystalline aggregates. Diffusion in the solid state. The classical theory of nucleation. Theory of thermally activated growth. Formal theory of transformation kinetics. Part II Growth from the vapour phase. Solidification and melting. Polymorphic Changes. Precipitation from supersaturated solid solution. Eutectoidal transformations. Order-disorder transformations. Recovery recrystalisation and grain growth. Deformation twinning. Characteristics of martensic transformations. Crystallography of martensitic transformations. Kinetics of martensitic transformations. Rapid solidification. Bainite steels. Shape memory alloys.

3,395 citations

Journal ArticleDOI
Abstract: The kinetic problem of intergranular fracture at elevated temperatures by the nucleation and growth of voids in the grain boundary is analysed in detail. Diffusional transport accounts for the void growthrat in the analysis, and the nucleation-rate is obtained by using the concepts of classical nucleation theory. The two are compounded to calculate the time-to-fracture. The influence of grain size, strainrate, temperature, second phase particles and interface energies is taken into account. Particular attention is given to the presence of inclusions in the boundary; the role of the stress concentration at the interface between the inclusion and the matrix, and the energy of this interface is investigated.

745 citations

Journal ArticleDOI
Abstract: A kinetic approach is used, to explain the nucleation of cavities in grain boundaries at elevated temperature. Under the influence of a tensile stress, vacancies cluster together and form cavities. However a free energy barrier exists for the nucleation of a cluster which is stable. Heterogeneous nucleation, especially at the junctions of grain boundaries and second phase particles, is favored. Also an incubation time is required to form a vacancy cluster of critical size. Its duration depends upon the volume of the cluster and the diffusivity of vacancies. In this manner a stress and time condition for the nucleation of cavities in grain boundaries is obtained.

222 citations

Journal ArticleDOI
Abstract: In a creeping solid holes may grow by vacancy condensation or by the action of the applied stress producing strains at the surface of the hole which cause it to grow. The latter mechanism does not involve a vacancy flux to the hole. A comparison of the two processes indicates the conditions under which hole growth without vacancy condensation is faster than hole growth by diffusion. Low values of the ratio σ/e, where σ is stress and e is the strain rate, as well as large voids favour the strain process. Such conditions usually arise in tertiary creep but may also occur earlier in the creep life. Experimental examples of cavitation in which vacancy condensation is shown to be the minor process are given, and the relevance of such a mechanism to hole growth in grain-boundary sliding and regions of localized flow is indicated.

211 citations