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Research Papers

Damage Development During Superplasticity of Light Alloys

[+] Author and Article Information
R. Boissière, L. Salvo

 Institut National Polytechnique de Grenoble (INPG), SIMAP Laboratory∕GPM2 Group, CNRS∕UJF, BP 46, 38402 Saint-Martin d’Hères Cedex, France

J. J. Blandin

 Institut National Polytechnique de Grenoble (INPG), SIMAP Laboratory∕GPM2 Group, CNRS∕UJF, BP 46, 38402 Saint-Martin d’Hères Cedex, Francejean-jacques.blandin@inpg.fr

J. Eng. Mater. Technol 130(2), 021014 (Mar 13, 2008) (6 pages) doi:10.1115/1.2884335 History: Received July 26, 2007; Revised January 24, 2008; Published March 13, 2008

Superplastic forming (SPF) of metallic alloys allows the production of components with particularly complex shapes since in this regime, due to the predominance of grain boundary sliding (GBS), the material exhibits a high plastic stability. However, in many light alloys (i.e., Al or Mg alloys), superplastic deformation induces damage leading to premature fracture. Despite extensive work in the past, the mechanisms of damage induced by superplastic deformation remain under debate. In particular, due to the important contribution of GBS, voids with very irregular shapes frequently develop, resulting in a difficulty to obtain reliable experimental data from conventional quantitative metallography. It is the reason why the use of X-ray microtomography, providing 3D images of material bulk, is a particularly fruitful technique to investigate damage processes in superplastic materials. Thanks to this technique, damage development during superplastic deformation of Al and Mg alloys is investigated and the three main steps of damage development (nucleation, growth, and coalescence) are discussed.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) TEM observation of the ECAE processed UFG Al alloy; (b) OM observation of the as-received sheet of the FG Mg alloy

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Figure 2

Example of a volume obtained by X-ray microtomography showing cavities developed during superplastic deformation of the FG Al alloy (500°C, 10−4s−1, macroscopic tensile strain ≈1.3)

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Figure 3

3D images showing (a) the complex shapes of cavities developed when the FG Mg alloy is deformed inside the superplastic domain (400°C, 6×10−4s−1, macroscopic tensile strain ≈1.3); (b) the elongated shapes of cavities developed when the FG Mg alloy is deformed outside the superplastic domain (250°C, 6×10−4s−1, macroscopic tensile strain ≈0.7)

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Figure 4

Comparison of experimental cavity distributions with values predicted from growth model (assuming no nucleation or coalescence between macroscopic tensile strains of 0.8 and 1.3): One can show that the growth model cannot predict the experimental cavity distribution at ε=1.3

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Figure 5

3D image showing the connection between cavities and particles in the FG Al alloy

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Figure 6

Variation with CV of both the number of cavities NV per unit volume (3D data) and of the coalescence parameter (CP) for the FG Al alloy

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