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

Tensile ductility of superplastic ceramics and metallic alloys

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.

Cavitation during grain-boundary-sliding deformation in an AZ61 magnesium alloy

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.

The characteristics of cavitation in superplastic metals and ceramics

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.

Grain growth behaviour of the Al-Cu eutectic alloy during superplastic deformation

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

Experimental Investigation of Cavitation Behavior in AZ61 Magnesium Alloy

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.

The variation in flow stress and microstructure during superplastic deformation of the Al-Cu eutectic

Abstract: Stress-strain curves have been obtained for the superplastically deformed Al-Cu eutectic tested in tension under constant true strain-rate conditions. It is shown that constant flow stress conditions do not obtain and that, after an initial transient, the flow stress is linearly related to natural tensile strain. Optical metallography has been employed to follow the variation of both inter-phase particle separation and α-Al grain size with strain and it is concluded that the observed strain hardening is due mainly to grain coarsening.

Fracture of a superplastic ternary brass

Abstract: A study has been made of flow and fracture in a strain-rate sensitive ternary brass at 400–800°C over the strain-rate range 10 −1 −10 −3 min −1 . The material is superplastic with an optimum ductility at 600°C. Plastic flow is accompanied by the continuous development of intergranular and interphase cavities. Under these conditions failure occurs without measurable external necking. The instability of plastic flow is analyzed in terms of the development of internal bifurcations (analogous to diffuse, multiple, external necks for non-cavitating superplastic alloys) and the linking of cavities by internal necking (analogous to rapid preferential growth of one external neck). It is concluded that for precise analysis of void-linkage a dynamic model is required as opposed to the current quasi-static models. An experimental basis for this work is provided.

Microstructural observations of superplastic cavitation in fine grained 7475-Ai

Abstract: Intergranular cavitation is an important consideration in the successful development of a commercially viable superplastic forming process for the high strength aluminum alloy, 7475. This work examined the microstructural features involved in the initiation stages of cavity formation. The observations suggest that, with the optimum superplastic deformation conditions, cavity nucleation is generally the rate determining step in the overall development of cavitation with strain. Cavities do not generally form at even the largest of the common single phase inclusion particles unless forming conditions are such that the flow stress significantly increases. It appears that, as well as local stress concentrations, additional effects are required, such as temperature induced particle decohesion and internal gas evolution, in order that cavities may grow to stable sizes. Such conditions may exist at certain two phase inclusion particles in the 7475 Al alloy. Suitable modifications to the standard alloy processing may therefore be devised which result in even lower rates of cavitation at the optimum superplastic forming conditions.

Structural Changes during Superplastic Deformation of the Al–Cu Eutectic Alloy

Abstract: The microstructural changes that occur during superplastic deformation of the Al–Cu eutectic alloy are considered. It is shown that considerable modification occurs to the shapes and distribution of the inter-metallic CuAl2 particles during deformation. This effect may be understood in terms of sliding and diffusion-controlled shape changes associated with grain boundaries within the intermetallic. The roles of interface sliding and of diffusion creep in superplasticity are discussed.

An analysis of cavity growth during superplasticity

Abstract: 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,rc. The value ofrc 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.