JOURNAL OF MATERIALS SCIENCE LETTERS 22, 2003, 1731 – 1734
Cyclic softening of Zr
55
Al
10
Ni
5
Cu
30
bulk amorphous alloy
Q. S. ZHANG, Y. F. DENG, H. F. ZHANG
∗
,B.Z.DING, Z. Q. HU
National Lab. for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, People’s Republic of China
E-mail: hfzhang@imr.ac.cn
In the last decade, Zr-based [1, 2] amorphous alloy
systems exhibiting a wide supercooled liquid region
combined with a distinct glass transition have attracted
considerable interest, for these types of alloys allow
the production of large-scale bulk amorphous speci-
mens by the conventional casting process at low cool-
ing rates of 1–10 k/s. The bulk amorphous alloys make
it possible to conduct mechanical testing under a much
wider range of conditions compared with earlier as-
quenched metallic glasses. Recently, several studies of
mechanical properties of bulk amorphous alloys, in-
cluding fracture toughness and fatigue-crack propaga-
tion, have been done [3–8]. In contrast to oxide glasses,
bulk amorphous alloys can exhibit fracture toughness
as high as ∼55 MPa
√
m, and also show fatigue-crack
growth properties comparable to those of high-strength
steels and aluminum alloys. Conversely, fatigue life-
times are much shorter in the amorphous alloy as com-
pared with the steels and exhibit a far lower dependence
on the applied stress range. The low fatigue strengths
have been considered to be due to the inhomogeneous
slip distribution, and no microstructure stabilizes the
crack growth for these amorphous alloys. However, in
the partially or fully crystallized condition, the bulk
amorphous alloys become brittle and display a severe
drop in fracture toughness [3, 8].
Although a few studies have been made on frac-
ture and crack propagation, there is little information
available on the symmetrically cyclic deformation be-
havior of the bulk amorphous alloys with R =−1,
(R = σ
min
/σ
max
, the ratio of the minimum load to
maximum load). Consequently, in the present study,
the symmetrically cyclic deformation behavior of the
Zr
55
Al
10
Ni
5
Cu
30
bulk amorphous alloy was studied,
with the objective of examining the relationship be-
tween cyclic deformation and fatigue failure. Moreover,
the effect of quenched-in crystallites on fatigue behav-
ior was also investigated for comparison.
AZr
55
Al
10
Ni
5
Cu
30
(nominal composition in at.%)
alloy was chosen in the present study for its exceptional
glass-forming ability [9]. Ingots of Zr
55
Al
10
Ni
5
Cu
30
alloy were prepared by arc melting a mixture of pure
zirconium, aluminum, nickel and copper in an argon
atmosphere, followed by casting into a copper mold.
The resulting ingots were 100 mm long, 6 mm wide
and 4 mm high. In order to produce fully amorphous as
well as partially crystalline samples, the ejection tem-
∗
Author to whom all correspondence should be addressed.
peratures were changed by adjusting the voltage of the
high-frequency inductor [10]. Fatigue samples were cut
from the ingots using electrodischarge machining, and
were ground with emery paper, then electrolytically
polished at a voltage of 16–20 V DC in a solution of
70% perchloric acid in acetic acid for about 5 min at
room temperature.
Cyclic fatigue tests were performed on a computer
controlled MTS 810 series servohydraulic-testing ma-
chine with a constant force amplitude of 4.5 kN. The fa-
tigue samples had a gauge length of 16 mm and a gauge
width and thickness of 3 mm. In the fatigue testing a
sinusoidal waveform with zero mean force [R =−1]
was adopted at a frequency of 1 Hz. The tensile and
compressive peak strains were continuously recorded
for every cycle throughout the test by an extensometer
with an 8-mm gauge length, clamped to the specimen.
All the samples finally failed under cyclic deformation.
The outer surfaces of fatigued specimens were investi-
gated in an Oxford scanning electron microscope op-
erating at 20 kV. Secondary electron imaging was used
exclusively in the present study.
Fig. 1a shows the backscattered SEM image of the
fully amorphous Zr
55
Al
10
Ni
5
Cu
30
sample, in which a
uniform microstructure with no high contrast phases to
be seen. The microstructure of the partially crystalline
amorphous alloy specimen with a lower ejection tem-
perature is shown in Fig. 1b. Quenched-in crystallites
with an average size of about 2 µm and an area fraction
of about 8% were mainly distributed in the amorphous
matrix. With much lower ejection temperature, crys-
tallites with an average size of 5–20 µm precipitated
from the melt as shown in Fig. 1c. According to the
XRD pattern, the crystallite was indexed as the Zr
2
Cu
phase.
Fig. 2a shows the dependence of the total strain am-
plitude ε
t
(ε
t
= ε
max
-ε
min
, where ε
max
and ε
min
are
the maximum and minimum values of strain at each
cycle respectively) on the number of cycles for the
fully amorphous specimen. The material neither soft-
ens nor hardens during the first 200 cycles. Then a grad-
ual softening occurs during the later cycles. After this,
a plateau region is reached for the final cycles. The
hardening/softening curves of the amorphous specimen
with crystallites are shown in Fig. 2b–c. For the amor-
phous specimen with fine crystallites, the material soft-
ened gradually during the initial 500 cycles, and then
0261–8028
C
2003 Kluwer Academic Publishers
1731