Effects of strain hardenability and strain-rate sensitivity on the plastic flow and deformation homogeneity during equal
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The effects of strain hardenability and strain rate sensitivity on the plastic flow and deformation inhomogeneity during equal channel angular pressing were studied using a finite element method analysis. In this study, perfect plastic nonhardening and rate-insensitive materials, and rate-sensitive materials were considered. In case of the nonhardening and rate-insensitive materials, the deformed geometry was predicted to be quite uniform and homogeneous. Deformation inhomogeneity developed, however, in materials with finite work-hardening exponent and strain-rate sensitivity. The corner gap formed in strain-hardening materials whereas the upper and lower channel gaps formed in strain-rate-sensitive materials. The deformation inhomogeneity was strongly dependent on the relative effects of strain-hardening exponent and strain-rate sensitivity. The predictions on the deformation inhomogeneity and the formation of corner and channel gaps were compatible with the experimental data published in the literature.
I. INTRODUCTION
It has been well known for a long time that grain refinement enhances mechanical properties (both strength and ductility) in polycrystalline materials. The strengthening of materials by grain size refinement is commonly represented by the Hall–Petch relationship.1,2 In particular, bulk ultrafine-grained materials with a grain size < 1 m (including nanocrystalline materials having a grain size of typically 0.
冉
冊
冉
⌿ ⌽ ⌿ ⌽ + ⌿ cosec + + 2 2 2 2
冊
.
According to Eq. (2), the equivalent strain decreases with the die corner angle ⌿. Figure 2 shows the strain developed according to Eq. (2) with corner angle at various
J. Mater. Res., Vol. 16, No. 3, Mar 2001
http://journals.cambridge.org
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(2)
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H.S. Kim et al.: Effects of strain hardenability and strain-rate sensitivity on the plastic flow and deformation homogeneity
FIG. 2. Calculated strain according to Eq. (2).
channel angles. It can be shown that the effective strain during ECAP can decrease from a maximum of 1.15 to a minimum of 0.907 while changing the corner angle from ⌿ ⳱ 0° to ⌿ ⳱ 90° when the channel angle is fixed at ⌽ ⳱ 90°. Although it is apparent that the die channel angle has more influence on the strain generated during ECAP than the corner angle, there is no reason to use the curved corner die, which generates inhomogeneous deformation9 and less overall shear strain. Due to the industrial prospects of ECAP, significant progress has been made not only in the understanding of fundamental properties (strength,11 fatigue,12 superplasticity,13 diffusivity,14 elasticity,15 internal friction,16 and corrosion17) of the equal channel angular pressed materials but also in analyzing the process itself. Because the evolution of microstructures and mechanical properties of the deformed material are directly related to the amount of plastic deformation, understanding the phenomenon associated with the strain development is very important in ECAP. The effects of various factors such
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