Effects of Metallographic Structures on the Properties ofHigh-Performance Phosphor Bronze
by Kuniteru Mihara * , Tatsuhiko Eguchi * , Takashi Yamamoto * 2 and Akihiro Kanamori * 2As electronic equipment such as mobile phones become more compact, higher
in mounting density and lower in profile in recent years, connector materials
for these equipment are required to be higher in strength and better in bending workability. In
response to such requirements, Furukawa Electric has developed F5218 and F5248 alloys, newversions of phosphor bronze for springs with smaller grain size, achieving improved strengthand bending workability 1). In this work, the following conclusions were reached: the grain sizeand strength in recrystallized structures can be described by the Hall-Petch relationship; thestrength and bending workability improve as grain size is reduced; irrespective of their small
grain size, degradation in the stress relaxation is suppressed due to Fe-(Ni)-P precipitates; andtheir grain growth rates are lower than those of C5210 and C5240 due to the contribution of P
precipitates that suppress the growth.

ABSTRACT1.

INTRODUCTION

As electronic equipment such as mobile phones become
more compact, higher in mounting density and lower
in profile in recent years, connector materials for theseequipment are required to be higher in strength andbetter in bending workability.
Phosphor bronze has been widely used in connectors
and relay contacts thanks to its low Young’s modulus
and high spring characteristics, i.e. strength. However,phosphor bronze generally degrades in bending
workability when heavily worked for work hardening.
To improve such characteristics, Furukawa Electric
developed F5218 and F5248 alloys, new versions ofphosphor bronze for springs with smaller grain size,
provided with improved strength and bending workability.
In this work, we continued the study on reducing thegrain size, and succeeded in combining high strengthand superior bending workability. This report will describe
the effects of small grain on the alloy properties as well as
the structure control technology.

2.

EXPERIMENTAL METHODOLOGY

Table 1 shows the compositions of F5218 and F5248
developed here, and those for C5210 and C5240 are
included for comparison. As shown, the alloys developed
here have a small addition of Fe and Ni to the basecopper alloys containing respectively 8 % and 10 % of

* Metals Research Center, R&D Division* 2 Metals Co.

Sn. The items and methods of characteristics evaluationof these alloys will be described below.
Table 1

Chemical compositions of the alloys. (mass%)
Sn

P

Fe

Ni

Cu

F5218

8

0.04

0.1

0.05

Balance

F5248

10

0.04

0.1

0.05

Balance

C5210

8

0.15

—

—

Balance

C5240

10

0.15

—

—

Balance

2.1 Mechanical Characteristics
The tests were carried out using an Instron type testingmachine and JIS No. 5 test specimens, under theconditions of gage length: 50 mm and cross head speed:
10 mm/min at room temperature to obtain tensile strength(TS), 0.2 % yield strength (YS) and elongation (El).
2.2 Measurement of Grain Size
Grain size was measured in cross sections perpendicular
to the rolling direction in conformity with JIS H 0501,
whereby the specimen was mechanically polished andsurface etched, and photographs were taken using an
optical microscope and a scanning electron microscope(SEM) to evaluate the grain size using the method ofsection. More than 100 crystal grains were measured andthe averaged grain size was obtained.
2.3 Bending Workability
90-degree W-bend tests without lubrication were carriedout using a 1-ton pressing machine. Specimens were cut
perpendicular to the rolling direction to effect “bad-way
bending,” and the minimum bending radius R at which
no cracking occurred was evaluated to calculate the

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Effects of Metallographic Structures on the Properties of High-Performance Phosphor BronzeTitle

2.4 Stress Relaxation Test
The tests were carried out by the cantilever method in
conformity with EMS-3003, the standard of the Electronic
Materials Manufacturers Association of Japan. The
loading stress was set to YS x 0.8, and heating was doneat 150°C in ambient atmosphere using an air bath.
2.5

Transmission Electron Microscope (TEM)
Observation
TEM specimens were prepared by the twin-jet polishingmethod, and observations were made using JEM-3010
equipped with an energy dispersive X-ray spectrometer(EDS) at an acceleration voltage of 300 kV.

3.

EXPERIMENTAL RESULTS ANDDISCUSSIONS

3.1 Grain Size and Hall-Petch Relationship
Figure 1 shows the grain size dependence of mechanicalcharacteristics (TS and YS) of annealed F5218. The
mechanical characteristics show a linear dependence
on d−1/2, where d represents the grain size, and it canbe seen that the smaller the grain size, the higher thestrength, i.e. the Hall-Petch relationship. Moreover, when
the gradients of dependence are compared between TS
and YS, YS is seen to be higher in the gradient indicatinga higher susceptibility on grain size. Meanwhile, sucha relationship was confirmed with F5248, C5210 andC5240. In this study, various characteristics will becompared among the three grain sizes of d−1/20.31(d10 µ m), d−1/2
0.46 (d4.7 µ m) and d−1/20.75(d1.8 µ m).

700 MPa and 750 MPa at each grain size were plotted at
appropriate points, and then the points belonging to thesame YS values were connected by a single line. It canbe seen that the bending workability of an F5218 material
with YS
750 MPa shown in Figure 2 (a), for example,
exhibits improvements such that the R/t value decreases
as the grain size becomes smaller over the course of
d−1/2
0.31, d−1/2
0.46 and d−1/2
0.75. This tendency
can be identified in the other YS values and with F5248
alloy as shown in Figure 2 (b), indicating that reducing
the grain size results in the improvement of bending
workability.
That is to say, reducing the grain size can achieve
combining high strength and good bending workability.
It is reported that improvements in bending workability
by reducing the grain size result from a process suchthat the traveling distances of dislocations shorten whenthe material is subjected to bending, so that stressconcentration due to the accumulation of dislocations is
reduced thereby improving the fracture toughness 2). An
alternative explanation is that the grain boundary areas
are enlarged due to the reduction of grain size, thus
enhancing the effects to disperse the stress caused bybending.
5

Bending workability, R/t

minimum bending radius/thickness ratio expressed by
R/t, an indicator of bending workability.

4

3

2

1

0

0

0.2

Grain size (µm)
10

5

3

0.4

0.6

0.8

1.0

Grain size, d --1/2 (µm--1/2)
2(a) F5218

1

550
5

450

Bending workability, R/t

Strength (MPa)

500

400
TS

350

YS

300
250
200

0

0.2

0.4

0.6

0.8

3

2

1

1
0

Grain size, d --1/2 (µm--1/2)

Figure 1

4

0

0.2

0.4

0.6

0.8

1.0

Grain size, d --1/2 (µm--1/2)

Grain size dependence of strength of F5218.

(b) F5248

3.2

Relationship between Grain Size and Bending
Workability
Figure 2 shows the evaluated results of the bending
workability represented by R/t of F5218 and F5248
with three different grain sizes. In the Figure, the R/t ofmaterials having YS of approximately about 650 MPa,

Figure 2

Relationship between bending workability and grain
size of selected alloys.

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Effects of Metallographic Structures on the Properties of High-Performance Phosphor Bronze

3.3 Stress Relaxation Characteristics
Figures 3 (a) and 3 (b) show the relationship between the
stress relaxation and the grain size represented by d−1/2 of
F5218 and C5210, and of F5248 and C5240, respectively.
The stress relaxation characteristics of every alloy
deteriorate as the grain size reduces. The explanation forthis commonly known tendency is that the contribution ofdiffusive creep grows in the thermally activated state, andthat the mean free path of moving dislocations shortensdue to the large areas of boundaries 3).
However, when the stress relaxation characteristicsof F5218 are compared with C5210, and F5248 with
C5240, for the same grain size, it can be seen that
F5218 and F5248 show better performance than theircounterparts. When the grain size dependency of eachalloy is approximated using a straight line, the gradientsfor F5218 and F5248 are less steep than those forC5210 and C5240, indicating lower susceptibilities to thechanges in grain size.
The reason why F5218 and F5248 have superiorstress relaxation characteristics may be explained bya hypothesis such that the small amount of Fe and Ni
additives form Fe-(Ni)-P precipitates thus pinning down themoving dislocations in the thermally activated process 1).
60

Stress relaxation (%)

C5210
50

F5218

40
30

industrial technique to obtain small grain size in a stablemanner.
Specimens of F5218, F5248, C5210 and C5240 wereheat treated under an equi-temporal heat treatment(batch-type heat treatment) of 300~500°C for 2 hr toinvestigate their recrystallization behavior. As a result,it was confirmed that the primary recrystallization
temperatures for F5218 and F5248 and those for C5210
and C5240 were 360°C and 340°C, respectively. Figure
4 shows the results of investigation on the grain size
after the specimens were heat treated at temperatureshigher than the recrystallization temperatures. It can beseen from Figure 4 that there is much difference in thegrowth rate of grain size between F5218 and C5210, andbetween F5248 and C5240. Specifically, it was confirmedthat C5210 grew in grain size at temperatures higher than
the recrystallization temperature resulting in the increasein the grain size, whereas F5218 was suppressed in grain
growth thus maintaining the small grain size even at atemperature higher than the recrystallization temperatureby 50°C.
TEM observations were made to investigate thereason of suppression of grain growth in F5218 andF5248. Figure 5 (a) shows the structure of recrystallized
F5218. From this TEM image, precipitates of 50 nm
in size (indicated by the arrows) are formed on therecrystallization boundaries thereby suppressing the
growth of grains. The precipitates were analyzed using
the EDS equipped on the TEM, and it was confirmed thatthey are Fe-(Ni)-P precipitates as illustrated in Figure 5
(b).

20

16

10
0 0

0.2

0.4

0.6

0.8

Grain size (µm)

14

1.0

Grain size, d --1/2 (µm--1/2)(a) F5218 and C5210
60

Stress relaxation (%)

50

F5248

2
0

Figure 4

20

F5248
F5218

0

20

40

60

80

100

Re-crystallization temperature +∆ Temp. (K)Relationship between grain size and heat treatment
temperature.

3.5

10

0

0.2

0.4

0.6

0.8

1.0

Grain size, d --1/2 (µm--1/2)(b) F5248 and C5240
Figure 3

6
4

30

C5210
C5240

8

C5240

40

0

12
10

Stress relaxation characteristics of selected alloys.

Evaluation of Effects of Secondary Phase Using
Classical Recrystallization Rate Equation
It has been shown that the Fe-(Ni)-P precipitates
suppress grain growth achieving small grains in a stablemanner. Then we proceeded to evaluate the effects ofsecondary phase using the classical recrystallization
equation. The following calculating formulas were used 4).

3.4 Grain Growth and Fe-(Ni)-P Precipitates
As was described in Section 3.2, reducing grain size canrealize good alloy performance combining strength andbending workability, but it is necessary to establish an

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Effects of Metallographic Structures on the Properties of High-Performance Phosphor BronzeTitle

having the secondary phase.
It is thought that the reason the reduction in grain size
is brought about under the existence of secondary phaseis that: 1) after primary recrystallization is completed,the secondary phase pins down the grain boundaries
to suppress grain growth; and 2) new recrystallization
nuclei are generated in the regions where grain growth issuppressed. And it is estimated that by this mechanism
F5218 and F5248 having secondary phase achieve grain
size reduction rather easily.
7

200nm

6

Grain size (µm)(a) Bright image with the arrows indicating precipitates

C5210

5

F5218

4
3
2
1
0

1

100

10

1000

10000

1000

10000

Time (s)(a) Grain growth(b) EDS chart of precipitates
Figure 5

1

TEM image and EDS analysis of F5218.

C5210

Re-crystallization

0.8

1) Rate of change in recrystallized grain size R
dR/dt = MV (∆ G + 3γ 1/R – 3fγ 2/2r) + J/N (Rc – R + δ R/2)
(1)
where: ∆ G is the driving force for recrystallization,γ 1 and γ 2 are the interfacial energy for different
phases of recrystallization, f is the volume fractionof precipitate, Rc is the critical nucleus size, δ R
is fluctuation of the critical nucleus size identicalto 1.1 Rc, M is the mobility of grain boundary,
V is mole volume, t is time, and r is the size ofrecrystallized grain.

dN/dt = J(2)

where: J is the generation rate of recrystallization
nucleus in the classical theory.
3) Change in recrystallization ratiodln (1 − X) /dt = −4/3π · J · R3

(3)

Figure 6 (a) and 6 (b) show the calculated results at
400°C (673 K) of grain growth and the recrystallization
ratio over time of F5218 and C5210, respectively, whereparameters γ 1= 0.6 J/m2 and γ 2=0.85 J/m2 were used 4).
Comparing the grain growth between C5210 and F5218
having the secondary phase, the latter is seen to be slow
in the grain growth rate. Moreover, from Figure 6 (b)where the recrystallization ratio is shown, it is suggested
that the recrystallization behavior is delayed in F5218

0.4

0.2

0

1

10

100

Time (s)(b) Recrystallization ratio
Figure 64.

2) Change in recrystallized grain density N

F5218
0.6

Calculated grain growth and recrystallization ratio.

CONCLUSION

Effects of metallographic structures on the properties ofF5218 and F5248, i.e. new phosphor bronze for springs
with a small addition of Fe and Ni, were studied, and thefollowing conclusions were reached.
1) The grain size and strength in recrystallized
structures can be described by the Hall-Petch
relationship, and by this correlation it is shown thatthe yield strength is more susceptible to grain size
than the tensile strength is.
2) By reducing the grain size, F5218 and F5248 alloys
with high strength and good bending workability
have been developed.
3) Grain growth rate after primary recrystallization of
F5218 and F5248 is lower than that of C5210 andC5240, and it is confirmed that phosphor precipitates
contribute to suppressing and retarding the grain
growth.

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Effects of Metallographic Structures on the Properties of High-Performance Phosphor Bronze

4) F5218 and F5248 with phosphor precipitates show
little degradation in stress relaxation characteristics
irrespective of reduced grain size, which is also
shown to be attributable to contributions ofphosphor precipitates.

REFERENCES
1) K. Mihara: Transactions of Japan Copper and Brass Association, 41,
210 (2002). (in Japanese)
2) Y. Murakami: Journal of The Japan Institute of Metals, 17, 190(1978). (in Japanese)
3) J. Miyake: Transactions of Japan Copper and Brass Workshop, 32,
63 (1993). (in Japanese)
4) H. Fujiwara: Journal of The Japan Institute of Metals, 40, 14 (1999).

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