Scripta Materialia 54 (2006) 1931–1935
www.actamat-journals.com
Effect of Ag on the aging characteristics of Cu–Fe in situ composites
Haiyan Gao, Jun Wang, Da Shu, Baode Sun
*
The State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, China
Received 14 December 2005; received in revised form 11 January 2006; accepted 2 February 2006
Available online 3 March 2006
Abstract
Aging characteristics of Cu–12Fe and Cu–11Fe–6Ag in situ composites were investigated. Curves of Vickers hardness vs. temperature, and electrical conductivity vs. temperature for the aged composites were plotted. Cu–11Fe–6Ag exhibits special hardness and conductivity peaks at about 350 °C. Microstructure investigations by scanning electron microscopy and transmission electron microscopy
reveal that the presence of Ag can harden the composite at temperatures lower than 350 °C and accelerate the precipitation kinetics
of Fe in the copper matrix.
Ó 2006 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
Keywords: Cu–Fe–Ag; In situ composites; Microstructure; Aging; Vickers hardness; Electrical conductivity
1. Introduction
Binary deformation processed Cu–Fe in situ composite
has attracted considerable attention during the past two
decades because of the low cost of iron [1–12]. However,
the Cu–Fe systems have a lower conductivity than other
copper base in situ composites such as Cu–Nb, Cu–Ag.
The reason lies in (1) the relatively higher solubility of Fe
in Cu at high temperatures, coupled with the slow kinetics
of iron precipitation at lower temperatures; and (2) the particularly harmful effect on the conductivity of iron atoms in
solid solution [3]. Therefore, it is important to remove as
much iron from solid solution in the copper matrix as possible. Data in Hansen and Anderko [13] predicts an equilibrium solubility of less than 1 ppm Fe at 253 °C. If the
predicted solubility at 253 °C could be achieved, the
conductivity of the copper matrix would be reduced by
only 1%IACS (International Annealed Copper Standard,
17.241 nX m is defined as 100%IACS). Therefore, heat
treatment is essential to improve the conductivity of Cu–
Fe in situ composites. Thermal–mechanical treatments
have also been employed to optimize the strength and conductivity [3,4,6–9].
In recent years, efforts have been made to widen the
spectrum of in situ composites towards ternary copper base
alloys [6–9,11,14–16]. The development of ternary Cu–Fe
composites usually aims at further improving the strength
and conductivity of binary alloys, and reducing the costs
as well. Adding a third element allows a larger variety of
possible kinetic paths to be exploited, to attain a certain
strength–conductivity profile. Due to its similar electronic
structure, crystal structure and electronegativity to copper
plus its high conductivity, Ag has been chosen as the third
element in many research studies [6,11,12]. Gao et al. [11]
reported that the presence of Ag can reduce the maximum
solubility of Fe in Cu at high temperature, and refine the
Fe dendrites at the same time. Song et al. [12] reported that
Cu–9Fe–1.2Ag has a strength/conductivity combination of
939 MPa/56.2%IACS through thermal–mechanical treatment. In this study, the effect of Ag on the aging characteristics of Cu–Fe in situ composite was examined.
2. Experimental details
*
Corresponding author. Tel.: +86 21 62932914.
E-mail address: bdsun@sjtu.edu.cn (B. Sun).
Deformation processed Cu–12 wt.%Fe and Cu–11
wt.% Fe–6 wt.%Ag (denoted Cu–12Fe and Cu–11Fe–6Ag,
1359-6462/$ - see front matter Ó 2006 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
doi:10.1016/j.scriptamat.2006.02.006
1932
H. Gao et al. / Scripta Materialia 54 (2006) 1931–1935
respectively) in situ composites were investigated. Hemispherical ingots of about 20 mm in diameter were separately
prepared from electrolytic Cu, commercial Fe and Ag with
at least 99.9 wt.% purity using tungsten arc-melting in an
atmosphere of ultra high purity argon. The ingots were
hot forged, cold rolled and then drawn to a diameter of
2 mm with a draw ratio of g = 4.1, where g = ln(A0/Af),
A0 is the initial sectional area obtained after hot forging
and Af is the final sectional area. Details of the materials
preparation can be found elsewhere [11].
Specimens for the aging treatment were sealed in quartz
tubes, which were evacuated to a vacuum of 104 Torr, to
avoid oxidation. Samples of each composite were annealed
at various temperatures between 150 °C and 700 °C for 1 h
and quenched into water. Longitudinal Vickers hardness
measurements were performed on an HXD-1000 microhardness tester using a load of 200 g. Each hardness value
was calculated from an average of at least ten indentations
with deviation being within 4%. Electrical resistivities of
samples were measured using a ZY9858 digital micro ohmmeter with precision of 1 lX at room temperature, and the
corresponding conductivity was evaluated according to the
definition of IACS. Microstructures of the samples were
investigated using a Sirion 200 field emission scanning electron microscope (FESEM), a JEM-2000EX transmission
electron microscope (TEM) and a JEM 2010F field emission transmission electron microscope (FETEM). TEM
specimens were prepared by mechanical thinning and ion
milling on a liquid nitrogen stage using an incidence angle
of 10° and 4° for enlarged thin areas.
3. Results
Vickers hardness vs. quenching temperature for the two
composites is plotted in Fig. 1. Ternary Cu–11Fe–6Ag
exhibits a different response to temperature from that of
binary Cu–12Fe. In the range 150–600 °C, the hardness
of Cu–12Fe decreases slowly except for a peak at about
425 °C, and for temperatures higher than 600 °C, the
decrease in hardness became quicker. However, Cu–
11Fe–6Ag shows a slight hardness increment at temperatures lower than 350 °C, and then it drops until 450 °C is
reached, where the hardness increases again and reaches
a peak at about 475 °C, above which there is another
decrease.
In general, the conductivity of the two composites
increases with the annealing temperature and reaches a
peak value at about 475 °C, and after 550 °C decreases
quickly. Careful observation revealed that in the increasing
stage, the conductivity of the Cu–11Fe–6Ag increases a bit
faster at temperatures lower than 400 °C, and exhibits a
small peak at about 350 °C. The conductivity difference
between the two composites became negligible after
500 °C. The results indicate that the proper aging temperature, in order to obtain higher conductivity, is between
450 °C and 500 °C for both the Cu–Fe and Cu–Fe–Ag
composites.
4. Discussion
According to Figs. 1 and 2, both the Vickers hardness
and conductivity of the as-drawn Cu–11Fe–6Ag are always
higher than that of the similarly processed Cu–12Fe. Our
previous work revealed that these differences could be largely attributed to the roles of Ag: reducing the maximum
solubility of Fe in Cu, refining iron dendrites of the as-cast
material and strengthening the copper matrix. More details
can be found in Ref. [11].
4.1. Aging characteristic of Cu–12Fe
The microstructure of the as-drawn composite is composed of copper matrix and elongated iron fibers, as shown
in Fig. 2(a). It is well accepted that: (1) the strength of the
deformation processed in situ composite follows the
famous Hall–Petch equation, i.e., r / k1/2, where r is
the strength and k the average spacing between the filaments; (2) the resistivity of copper base in situ composite
with aligned filaments can be evaluated using the parallelcircuit model [17]: q1C ¼ q1Cu fCu þ q1Fe fFe , where fCu and fFe
are volume fractions of Cu and Fe, respectively. According
to the model, about 97% composite resistivity results from
the matrix, which can be partitioned into the contribution
of four main scattering mechanisms: phonon, dislocation,
70
220
Conductivity, %IACS
Vickers hardness, HV
240
200
180
160
Cu-12Fe
Cu-11Fe-6Ag
140
Cu-12Fe
Cu-11Fe-6Ag
60
50
40
120
0
30
150
300
450
600
Annealing temperature,˚C
750
0
150
300
450
600
Annealing temperature,˚C
Fig. 1. Hardness and conductivity changes after isochronic aging treatment.
750
ID
1502899
Title
EffectofAgontheagingcharacteristicsofCu–Feinsitucomposites
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