1. Properties and recrystallization behavior of heavily worked copper
J. Springs2, Y-T. Kao2, A. Srivastava1, Z. Levin2, R.E. Barber3, K.T. Hartwig1,2,3
1Materials Science & Engineering department, Texas A&M University, College Station, TX
2Department of Mechanical Engineering, Texas A&M University, College Station, TX
3Shear form, Inc. Bryan, TX 77801
Abstract
Results
Results
PROBLEM
Annealed copper has low strength and a superior conductivity, yet a stronger copper material is needed with still low resistivity. Could work hardening be the solution to a high strength low resistivity Cu? APPROACH
CDA 101, 110, and C182 copper were work hardened via the severe plastic deformation (SPD) process equal channel angular extrusion (ECAE). Subsequent testing revealed tensile and hardness properties, grains sizes, recrystallization behavior, conductivity, and low temperature residual resistivity. RESULTS
The highest strength copper was formed via ECAE + rolling with a tensile strength of 494 MPa. The highest tensile strength for ECAE only comes from 4 pass route B at 442MPa. Hardness and tensile strength saturates around 3-4 ECAE passes. CDA101 and CDA110 both had a 5% conductivity drop maximum compared to IACS when fully worked while the C182 has a 60% drop in conductivity. Recrystallization for pure Cu occurs at 225C for 8 pass samples. Route Bc produced the smallest grains for both as-worked and recrystallized conditions. CONCLUSIONS
Achieving a target tensile strength of 500MPa for copper may be possible with an improved SPD schedule and post processing treatment
Route Bc is a preferred method for creating the smallest grains
Alloying, while good for increasing mechanical properties, decreases conductivity substantially
Table 1: Microstructure for as-worked and recrystallized for CDA101 copper
Table 2: Summary of tensile strength data and hardness for CDA101 copper
Route
Accumulated Strain
Grain Size (micron)
Standard Deviation (micron)
Recrystallized Grain Size (micron) AR
29.5
14.4
N/A 1A
1.1
0.98
0.67
6.26
4.49 2A
2.3
0.75
0.24
4.44
2.72 4A
4.6
0.58
0.20
2.22
1.18 4B
0.49
0.11
2.48
1.56
4Bc
0.41
0.13
1.95
0.82 4E
0.47
0.14
2.14
1.23
8Bc
9.2
0.42
0.16
1.40
0.66 8E
0.53
1.66
1.15
16Bc
18.5
1.32
0.72
Route
Accumulated Strain
Vickers hardness (VH300)
Yield Strength (MPa)
Tensile Strength (MPa)
Strain to Failure AR
54 ± 1
181 ± 2
248 ± 2
0.38 ± .02 1A
1.1
126 ± 2
323 ± 5
349 ± 4
0.14 ± .01 2A
2.3
132 ± 5
364 ± 1
383 ± 4
0.14 ± .03 4A
4.6
137 ± 2
372 ± 1
397 ± 2
0.16 ± .01 4B
145 ± 4
399 ± 4
442 ± 4 4E
143 ± 3
402 ± 1
438 ± 2
0.19 ± .01
4Bc
144 ± 2
383 ± 17
421 ± 6
0.16 ± .02
8Bc
9.2
141 ± 4
373 ± 3
437 ± 6
0.20 ± .01 8E
145 ± 2
382 ± 2
427 ± 1
0.15 ± .01
16Bc
18.5
136 ± 2
357 ± 3
438 ± 3
0.18 ± .01
Figure 2: SEM images of a) annealed CDA at 400x b) 1A processed CDA101 at 20000x c) 8Bc processed CDA101 at 20000x a) b) c) a) b) c) a)
Table 3: Summary of conductivity for CDA101, CDA110, and C182 copper as well as RR and RRR values for CDA101 copper
C182
CDA110
CDA101
Processing
%IACS
RR (77K/4.2K)
RRR
(273K/4.2K)
Annealed
59.6
100.7
103.4
11.2
91.0 1A
41.2
97.0
98.2
7.40
51.6 2A
N/A
6.05
36.4 2B
41.4
97.2
96.9 4A
4.94
30.8
4Bc
40.4
96.5
95.6
4.14
24.5 4E
41.8
97.1
4.79
30.0
8Bc
39.1
95.4
3.64
21.1 8E
4.28
27.3
16Bc
3.48
19.2
Figure 3: Left-Stress strain curves for AR, 1A, 2A, 4A, 8A, and rolled 4A. Right-Conductivity of CDA101, CDA110, and C182 compared to the IACS value
TS= *RRR
R2=0.96
Deformation Processing
Figure 4: Left-Vickers hardness vs temperature of route 8Bc CDA101, CDA110, and C182 copper. Right-DSC curve for 8 pass CDA101 samples
Figure 1: Example ECAE 90˚ die and billet schematic
The different designation of routes in ECAE are determined by the rotation seen between successive passes. Route A has no rotation, Route B is rotated by 90º on even numbered passes and by 270º on odd numbered passes. Route C keeps the same orientation through all passes at a rotation of 180º. Route E is rotated 180º for all even numbered passes, and by 90º or 270º for the odd numbered passes. Finally route Bc, is rotated by 90º for all passes.
Figure 5: Tensile strength vs RRR correlation for various selected routes
Acknowledgments
Texas A&M University for the use of equipment and lab space
Michael Elverud for help in fabrication and testing