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RESEARCH PAPERS

The New Grain Formation During Warm and Hot Deformation of Copper

[+] Author and Article Information
Agnieszka M. Wusatowska-Sarnek

 Pratt and Whitney Co., 400 Main Street, M/S 114-40, East Hartford, CT 06108agnieszka.wusatowska-sarnek@pw.utc.com

J. Eng. Mater. Technol 127(3), 295-300 (Mar 16, 2005) (6 pages) doi:10.1115/1.1925284 History: Received August 13, 2004; Revised March 16, 2005

Initial stage of dynamic recrystallization (DRX) under warm and hot deformation was investigated in polycrystalline copper. At the onset of DRX at intermediate deformation temperatures (i.e., high values of Zener–Hollomon parameter Z) large orientation gradients are developed near grain boundaries, and conversely smaller gradients at high temperatures (i.e., low Z values). The frequency of thermally activated grain boundary motion coupled with various levels of local orientation gradients lead to the operation of different mechanisms of DRX nucleation. At warm deformation equiaxed grains with moderate- to high-angle boundaries are evolved at near the pre-existed boundary as a result of increasing density of geometrically necessary boundaries and progressive rotation of subgrains with little accompanying boundary migration, while at hot deformation grain boundary bulging accompanied with twinning takes place.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

True stress–true strain curves of 6N Cu at a strain rate of 1.6×10−1s−1 and 4N Cu at 1.6×10−3s−1 obtained by compression at various temperatures

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Figure 2

Optical micrographs of microstructures developed in 4N Cu (a, c) and 6N Cu (b, d) after compression to a strain around 1.4. (a) 4N Cu, T=473K, ε̇=1.6×10−3s−1; (b) 6N Cu, T=473K, ε̇=1.6×10−1s−1; (c) 4N Cu, T=573K, ε̇=1.6×10−3s−1; (d) 6N Cu, T=573K, ε̇=1.6×10−1s−1.

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Figure 3

Grey-level OIM map taken at initial grain boundary of 6N Cu deformed to a strain 1.4 at 473K, showing inhomogeneous deformation. Gray lines correspond to boundaries of misorientation >1.3deg, thin, black lines of >5deg, and thick, black lines of >15deg, respectively. Point-to-point misorientations (●) and cumulative disorientations (엯) were measured at along the lines m1 and m2. G.B. means grain boundary.

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Figure 4

TEM micrographs showing warm deformed substructures evolved in 6N Cu at T=473K and ε=1.4. (a) A colony of fine grains separated by medium- to high-angle boundaries evolved near the grain boundary (indicated by line of dots). (b) A substructure developed in grain interior. Numbers indicate the boundary misorientations in deg.

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Figure 5

TEM micrograph showing warm deformed substructure evolved in 4N Cu at T=523K and ε=1.4. Numbers indicate the boundary misorientations in degrees; initial grain boundaries indicated by line of dots.

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Figure 6

(a) Grey-level OIM map of necklace DRX in 6N Cu deformed to a strain of 1.3 at 573K. Gray lines correspond to boundaries of misorientation >1.2deg, thin, black lines of >4deg, and thick, black lines of >15deg, respectively. (b). Boundary map complementary to (a); twin boundaries shown as thick, gray lines. C.A. means compression axis.

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Figure 7

Grey-level OIM map taken at initial grain boundary of 4N Cu deformed to a strain 0.2 at 723K, showing nuclei of new grains evolved along serrated grain boundary. Gray lines correspond to boundaries of misorientation >0.8deg, thin, black lines of >4deg, and thick, black lines of >15deg, respectively. Twin boundaries are shown as thick, gray lines. C.A. means compression axis.

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Figure 8

TEM micrograph showing hot deformed structure evolved in 4N Cu deformed to a strain 0.2 at 723K. A nucleus of new grain, separated from matrix by twin boundary, evolved at bulged portion of brain boundary. Numbers indicate the boundary misorientations in degrees; cumulative disorientations (ΣΘ) measured towards grain interior, indicated by arrows, are less than 8deg.

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Figure 9

Relationship between peak flow stress (σp) and dynamic grain size (DS, circles) or transverse cell size (lT, triangles) for 4N (full symbols) and 6N Cu (open symbols) under warm and hot deformation

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Figure 10

Schematic drawing of the nucleation of new grains under DRX. For high Z values (left-hand side) development of strain-induced geometrically necessary boundaries assisted with local lattice rotations followed by progressive transformation into high angle boundaries leads to the formation of new grains. For low Z values (right hand side) high boundary mobility leads to significant boundary serrations followed by separation of bulged parts by twinning.

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