Advanced Materials Research
ISSN: 1662-8985, Vols. 26-28, pp 569-572
doi:10.4028/www.scientific.net/AMR.26-28.569
© 2007 Trans Tech Publications, Switzerland
Online: 2007-10-02
Microstructure and Properties of Cu-3.2Ni-0.75Si-0.3Zn
Alloy for lead frame
Yi Zhang1,a, P.Liu2,b, B.H.Tian3,c, D.M.Zhao3,c, S.G.Jia3,c, X.H.Cheng 3,c
1
.School of Materials Science and Engineering, Xi’an University of Technology, Xi’an710048, China
2
. Institute of electric functional Materials, University of ShangHai for Science and Technology,
ShangHai ,200093, China
3
.School of Materials Science and Engineering, Henan University of Science and Technology,
Luoyang, 471003, China
a
[email protected], [email protected], [email protected]
Keywords: Cu-3.2Ni-0.75Si-0.3Zn alloy;aging; cold rolling; precipitation, the transformation kinetics
Abstract. The effect of aging temperature and aging time on properties of Cu-3.2Ni-0.75Si-0.3Zn
alloy were studied. The alloys were isochronally or isothermally aged after solution treatment. The
cold rolling prior to the aging treatment was used to increase the precipitation rate .The microstructure
of the alloy was studied by means of transmission electron microscope (TEM). The results show that
the fine and dispersed precipitates are fully coherent with the Cu matrix and make the alloy possesses
higher hardness and conductivity after the alloy was solution at 1173K and then aged at different time.
The precipitates responsible for the age-hardening effect was Ni2Si.The transformation kinetics were
studied by analyzing the electrical resistance variation of the solution Cu-3.2Ni-0.75Si-0.3Zn alloy in
the process of aging.
Introduction
Copper-based alloys possess high strength and high electrical conductivity. As a result, they are
potential materials for the application as lead frames and connectors in electrical and electronic
industries.The ideal lead-frame materials should reach the electrical conductivity which is
80%IACS,tensile strength of 600MPa and microhardness of 180HV[1-7].
The current work was designed to investigate the aging behavior of Cu-3.2Ni-0.75Si-0.3Zn
alloy.The microstructure and properties of the alloy aged at various temperatures for various times
were studied.Due to the sensitivity of electrical resistance to the precipitation,it is possible to study
the phase transformation and the precipitate kinetics by analysing the variation of the electrical
resistance ratio.
Experiment procedures
The alloys used in this investigation were prepared by melting copper of 96% purity together with Ni
and Si in a 10 kg medium frequency furnace at the vacuum of about 10-5 Pa.The chemical
composition of the alloy was Cu-3.2wt%Ni-0.75 wt%Si-0.3 wt%Zn.The ingots were homogenized at
1125 K for 2 h and subsequently rolled at this temperature from a thickness of 22 mm to a 2.0 mm
thick strip. The material was solution heat-treated for 1 h at 1173K in the RJX-2.5-10 tube electrical
resistance furnace and water quenched. The aging treatment was performed at various temperatures in
the SRJX-3-12 tube electrical resistance furnace under a fluid atmosphere of nitrogen.
The microhardness was carried out on a HVS-1000 digital microhardness tester under an indenting
load of 50g and holding for 30s. The TEM examinations were carried out using H-800 transmission
electron microscope operating at 200 kV.
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Advanced Materials and Processing
Results and discussion
Effects of aging on hardness and electrical conductivity. The three-dimensional mesh graphs of
the microhardness、aging time and aging temperature for the Cu-3.2Ni-0.75Si-0.3Zn alloy by using
the two-dimensional spline interpolation method was shown in Fig.1(a). The peak hardness is about
254 HV at an aging time for 120 min at 773K.
Fig.1Changes in the microhardness and electrical conductivity of Cu-3.2Ni-0.75Si-0.3Zn alloy after
aging at various time and temperatures
Fig.1(b) shows the effects of aging time on electrical conductivity of the alloy aging at various
temperatures. Fig.1(b) reveals that a maximum conductivity of 38% IACS is achieved after aging the
sample at 873K for 480 min.
The effect of cold rolling before aging on aging properties. Fig. 2(a) shows the effect of aging time
on hardness of Cu-3.2Ni-0.75Si-0.3Zn alloy with 60% deformation and without deformation before
aging at 723K. The hardness of the alloy after cold rolling 60% and aging has reached a peak after
aging at 723K for 1 h and then decreases with increasing aging time above 1h.The highest hardness is
about 268HV.
Fig.2 The effect of cold deformation before aging onproperties of the alloy aged at 723K
The conductivity of the alloy increased with aging time. The conductivity of the alloy aged with
60% deformation was higher than without deformation and its conductivity was 40%IACS after aging
for 8h .
Microstructure. Fig. 3 shows the transmission electron micrograph of precipitates aging at 823K for
2h (a) and 8h (b). During isothermal aging at 823K for 2h and 8h, both grain boundary (Fig. 3(a)) and
intragranular (Fig. 3(b)) precipitates of δ - Ni2Si were observed.
Fig. 4 shows the microstructure of the alloy which was pre-aging at 723K for 8h,60% deformation,
subsequently aged at 723K for 1h The size of these precipitates are observed to be 200–250 nm.
Advanced Materials Research Vols. 26-28
Fig.3 Microstructure of Cu-3.2Ni-0.75Si-0.30Zn alloy aging
at 823K for 2h (a) and 8h (b)
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Fig.4Recrystallization microstructure of
Cu-3.2Ni-0.75Si-0.30Zn alloy after combined
treatment of 723K for 8h+60% deformation+ 723K
for 1h at 723K for 8h (a) and 823K for 8h (b)
The relationship of electrical conductivity and the volume fraction of precipitation. Due to the
sensitivity of electrical resistance to the precipitation of the second phase, the kinetics of precipitation
can be studied by measuring the changes in electrical resistance on aging. From the relationship
between the electrical conductivity and the volume fraction of precipitates,the time for transformation
can be cauculated at various aging temperatures.
The solute atoms precipitate from the supersaturated solid solution and form the second
phase.Because the solute atoms Ni, Si can not precipitate completely from the copper-matrix,the
volume fraction of precipitates,at any time t,can be defined by the expression:
f = V p / VBp
(1)
Where V B pis volume fraction of new phases per unit volume of the matrix while precipitation is
over;and V p is volume fraction of new phases formed per unit volume of the matrix at time t.At the
beginning of transformation, V p = 0 and f = 0 .The electrical conductivity of initial state is σ 0 .After
prolonged period of aging at a given temperature,the electrical conductivity hardly increases and
reaches to the maximum(σ max ).At this moment V p = VBp, f = 1. According to Matthissen-Fuliminge
rule,electrical resistance of the solid solution follows the the equation:
ρ s = ρ 0 + ap
(2)
In which, ρ0 is the electrical resistance of the solvent , α is the percentage of the solute atoms, p is
the change of resistance caused by the addition of one percent solute atoms.It can be seen from
equation(1) that there is a linearity between resistance and the percentage of the solute
atoms.Therefore,we can suppose that there could be a linearity between electrical conductivity and the
aging time,i.e,
σ = σ 0 + Af
(3)
As the transformation is finished, σ = σ maxand A = σ. max − σ 0 The volume fraction of precipitates can
be calculated at any time by the corresponding electrical conductivity is measured.In this way the
electrical conductivity and volume fraction of new phases precipitated in Cu-Ni-Si alloy are caculated
at an aging temperature at 723K(Tab.1).
Tab.1 Transformation ratio of precipitate varing with time aging at 723K
Time/min
0
15
30
60
120
240
480
σf(%IACS)
16.11
20.65
23.3
26.2
28.7
30.9
33.8
f（%）
0
0.257
0.406
0.570
0.712
0.836
1.000
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Advanced Materials and Processing
Volume fraction of precipitation integrality follows Avrami empirical formula for phase
transformation:
1 − f = exp(−bt n )
(4)
Where b and n are constant factors.In order to obtain the constants b and n,the equation can be
logarithmic transformation:
1
(5)
lg（ ln
) = lg b + n lg t
1− f
1
) − lg t shows a straight line (Fig. 5).It can be seen that
With the data of table 1,the curve of lg（ ln
1− f
n is the slope and lgb is intercept with n=0.646，lgb=-1.256，b=0.055.Therefor,transformation
kinetics equation of solid solute Cu-Ni-Si alloy aging at 723K can be described as:
(6)
1 − f = exp(−0.055t 0.646 )
With the data of equation(6), the electrical conductivity equation of Cu-3.2Ni-0.75Si-0.3Zn alloy
aging at 723K can be described as:
σ＝16.11＋17.69（
Fig.5
l g（ l n
1
) − l g t diagram
1− f
1 − e−0.055t
0.646
）
(7)
of the Cu-3.2Ni-0.75Si-0.3Zn alloyaged at 723K
Summary
(1) The microhardness and conductivity of the Cu-3.2Ni-0.75Si-0.3Zn alloy aged with 60%
deformation were higher than without deformation.
(2) More fine dispersed precipitates inside the Cu matrix make the Cu-3.2Ni-0.75Si-0.3Zn alloy lead
frame alloy possess higher hardness after aging.The precipitates responsible for age-hardening effect
in Cu-3.2Ni-0.75Si-0.3Zn alloy are δ - Ni2Si.
(3) The transformation kinetics equation and electrical conductivity equation of
Cu-3.2Ni-0.75Si-0.3Zn alloy aging at 723K can be described as:
0.646
(8)
1 − f = exp(−0.055t 0.646 ) ; σ＝16.11＋17.69（ 1 − e−0.055t ）
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