Materials Transactions, Vol. 45, No. 6 (2004) pp. 1958 to 1961
#2004 The Japan Institute of Metals
RAPID PUBLICATION
New Cu-Zr-Al-Nb Bulk Glassy Alloys with High Corrosion Resistance
Chunling Qin* , Wei Zhang, Hisamichi Kimura, Katsuhiko Asami and Akihisa Inoue
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
New Cu-based bulk glassy alloys with good mechanical properties were formed in Cu-Zr-Al-(Nb) system by copper mold casting. They
exhibit a large supercooled liquid region (Tx ) of 50 K-74 K and high glass-forming ability with maximum diameters of 2.0 mm-3.0 mm. The
addition of Nb to the glassy alloys is effective for improving the corrosion resistance in all the solutions. The corrosion rate of the 5 at% Nb alloy
decreases by one order of magnitude compared to those of the 0 at% Nb alloys in 3 mass% NaCl solution. In 0.5 moldm3 H2 SO4 solution, the
Cu-Zr-Al-(Nb) alloys are passivated in wide passive region and low passive current density of the order of 102 Am2 , indicating high
corrosion resistance in this solution.
(Received February 13, 2004; Accepted April 19, 2004)
Keywords: bulk glassy alloy, copper-based alloy, glass-forming ability, corrosion resistance, polarization curve
1.
Introduction
Recently, bulk glassy alloys have attracted increasing
interest because of the novelty of materials science as well as
the importance of new engineering materials with unique
characteristics.1–3) The synthesis of bulk glassy alloys by
conventional casting processes was achieved for the first time
in La-Al-Ni system in 19894) and their mechanical strength
values were much higher than those for the corresponding
crystalline alloys. Since then, a number of bulk glassy alloys
were prepared in multicomponent systems such as Mg-,5)
Zr-,6,7) Ti-,8) Hf-,9) Fe-,10) Pd-Cu-,11) Co-,12) Ni-13) based
alloys. Among these bulk glassy alloys, high tensile fracture
strength combined with good ductility has been obtained only
for the La-, Zr-, Hf-, Pd-Cu-based bulk glassy alloys. The
tensile strength level has been reported to be 1200 MPa for
the La-based alloys14) and 1500 to 1700 MPa for the Zr-, Hfand Pd-Cu-based alloys.14) The search for a new bulk glassy
alloy system with high tensile strength has been one of major
research subjects because it leads to extension of application
fields.
Very recently, it has been succeeded in finding new Cubased bulk glassy alloys in Cu-Zr-Ti,15) Cu-Hf-Ti,16) Cu-ZrHf-Ti,17) Cu-Zr-Ti-Y18) and Cu-Zr-Ti-B19) systems. These
Cu-based bulk glassy alloys exhibit high tensile fracture
strength of 2000 to 2500 MPa.15–19) The better mechanical
properties for the Cu-based bulk glassy alloys also imply high
reliability of the bulk glassy alloys as engineering materials.
Especially, Cu-Zr-Ti-based bulk glassy alloys with large
plastic elongation and low cost have attracted much attention.
However, the disadvantage point for these bulk glassy alloys
is low viscous flow workability in the supercooled liquid
region because their Tx is below 50 K.15) Great efforts have
been devoted to synthesize a Cu-based bulk glassy alloy with
larger supercooled liquid region and good viscous flow
workability.
More recently, the Cu-based glassy alloys with the large
supercooled liquid region exceeding 70 K have been formed
in the Cu-Zr-Al20) system and the bulk glassy alloys also
exhibit good mechanical properties. The supercooled liquid
region Tx was 72 K for Cu50 Zr45 Al5 and 74 K for
*Japan
Science and Technology Agency, Sendai 980-8577, Japan
Cu55 Zr40 Al5 .20) In the supercooled liquid region with such
large Tx values, the bulk glassy alloy can be deformed to
various useful shapes in the maintenance of good mechanical
properties. However, there have been no data on the
corrosion resistance for these Cu-Zr-Al bulk glassy alloys,
though they are expected to cause significant extension of
application fields of bulk glassy alloys. Therefore, it is of
great importance to clarify their chemical stability in the
environments and subsequently to improve their corrosion
resistance. The 5 at% Nb alloy with large Tx value of 50 K
was also prepared because niobium is an effective alloying
element in enhancing the corrosion resistance of alloys in
aggressive environments.21,22) This paper intends to examine
the corrosion resistance of these new Cu-Zr-Al bulk glassy
alloys and clarify the influence of additional element Nb on
the glass formation and corrosion behavior of the Cu-Zr-Al
alloys.
2.
Experimental
Multi-component Cu-based Cu-Zr-Al-(Nb) alloy ingots
were prepared by arc melting the mixtures of pure Cu, Zr, Al
and Nb metals in a Ti-gettered argon atmosphere. The purity
of metals was over 99.9 mass%. Alloy ingots were re-melted
five times to ensure chemical homogeneity. The mass losses
were measured for each ingot after melting and were less than
0.1 mass%. Bulk alloys in a rod form with a length of about
50 mm and diameters up to 3 mm were produced by copper
mold casting. The structure of the specimens was examined
by X-ray diffraction (XRD) and the absence of micrometer
scale crystalline phase was confirmed by optical microscopy
(OM). Thermal stability associated with glass transition,
supercooled liquid and crystallization for the glassy alloys
was investigated by differential scanning calorimetry (DSC)
at a heating rate of 0.67 K/s.
Corrosion behavior of the glassy alloys was evaluated by
weight loss and electrochemical measurements. Prior to
corrosion tests, the specimens were mechanically polished in
cyclohexane with silicon carbide paper up to grit 2000,
degreased in acetone, washed in distilled water, dried in air
and further exposed to air for 24 h for good reproducibility.
Electrolytes of 1 moldm3 HCl, 3 mass% NaCl and
0.5 moldm3 H2 SO4 solutions open to air were used at
New Cu-Zr-Al-Nb Bulk Glassy Alloys with High Corrosion Resistance
1959
Fig. 3 Corrosion rates of the Cu-Zr-Al-(Nb)
bulk glassy alloys in 3 mass% NaCl and
0.5 moldm3 H2 SO4 solutions at 298 K
open to air.
Fig. 1 X-ray diffraction patterns of the cast
Cu-Zr-Al-(Nb) rods with a diameter of
1.5 mm.
Fig. 2 DSC curves of the cast Cu-ZrAl-(Nb) rods with maximum diameters of 2.0 mm–3.0 mm.
298 K. Corrosion rates were estimated from the weight loss
after immersion in the solutions for a period of time.
Electrochemical measurements were conducted in a threeelectrode cell using a platinum counter electrode and a Ag/
AgCl reference electrode. Potentiodynamic polarization
curves were measured at a potential sweep rate of
50 mVmin1 after open-circuit immersion for about 20 min
when the open-circuit potential became almost steady. After
the corrosion tests, surfaces of the alloys were observed by
scanning electron microscopy (SEM).
3.
Results and Discussion
Figure 1 shows the X-ray diffraction patterns of the cast
Cu55 Zr40 Al5 , Cu50 Zr45 Al5 and Cu50 Zr40 Al5 Nb5 rods with a
diameter of 1.5 mm. Their XRD patterns exhibit a main halo
peak at 2 40 with a width of about 6.0 at the position of
the half maximum and no diffraction peaks from crystalline
phases are detected, indicating that the alloys consist of a
single glassy phase.
Figure 2 shows DSC curves of the bulk glassy alloys with
diameters of 2 to 3 mm, Tg and Tx correspond to glass
transition temperature and onset temperature of crystallization, respectively. All the alloys exhibit a distinct glass
transition, followed by a supercooled liquid region and then
exothermic reactions due to crystallization. Although the
replacement of Cu by 5 at% Nb for Cu55 Zr40 Al5 glassy alloy
causes a decrease in the temperature interval of supercooled
liquid region (Tx ¼ Tx Tg ) from 74 K at 0 at% Nb to 50 K
at 5 at% Nb by decreasing Tx , the 5 at% Nb alloy still keeps
high glass-forming ability. The maximum diameters (tmax )
for glass formation were 2 mm for the Cu55 Zr40 Al5 and
Cu50 Zr40 Al5 Nb5 alloys and 3 mm for the Cu50 Zr45 Al5 alloy,
respectively. The outer surfaces were smooth, and no trace of
precipitation of crystalline phase was seen.
Then, we discuss the reason for the decrease of GFA of the
Cu-Zr-Al alloy containing Nb. It has been reported20) that the
bulk glassy alloys with a maximum diameter of 3.0 mm can
be obtained in the Cu-Zr-Al alloy system which satisfies the
following three empirical component rules,2,23,24) i.e., (1)
multi-component consisting of more than three elements, (2)
significant atomic size mismatches above 12% among the
main three elements, and (3) suitable negative heats of
mixing among the main elements. However, the substitution
by Nb for a portion of Cu in the Cu-Zr-Al glassy alloy
generates the bonding pairs of Cu-Nb and Zr-Nb with
positive and nearly zero heats of mixing. The partial
existence of the atomic pairs with the repulsive nature causes
the disturbance of the formation of a highly dense random
packed structure through the unsatisfaction of the three
empirical component rules2,23,24) for the achievements of
stabilization of supercooled liquid and bulk glass formation.
The corrosion rates of the bulk Cu-Zr-Al-(Nb) glassy
alloys with a diameter of 1.5 mm in different solutions at
298 K were measured. Corrosion rates were obtained as mean
values by immersion after 24 h in 1 moldm3 HCl solution,
168 h in 3 mass% NaCl solution and 336 h in 0.5 moldm3
H2 SO4 solution, respectively. In 1 moldm3 HCl solution,
the Nb-free alloys were corroded severely showing high
corrosion rate of 29.8 mmy1 for Cu55 Zr40 Al5 , 15.6 mmy1
for Cu50 Zr45 Al5 , respectively. In addition, the corrosion
resistance of the alloy containing Nb was much improved,
and the 5 at% Nb alloy exhibited much lower corrosion rate
of 0.12 mmy1 . The corrosion rates of the Cu-Zr-Al-(Nb)
alloys in 3 mass% NaCl and 0.5 moldm3 H2 SO4 solutions
are shown in Fig. 3. In 3 mass% NaCl, the corrosion rate for
the Cu55 Zr40 Al5 alloy is 0.20 mmy1 and decreases to about
0.12 mmy1 for the Cu50 Zr45 Al5 alloy. Furthermore, the
effect of additional element Nb to the Cu-Zr-Al alloy is
significant. The corrosion rate of the Nb-containing alloy
decreases greatly from the order of 101 mmy1 for the CuZr-Al alloy to the order of 102 mmy1 for the Cu-Zr-Al-Nb
alloy. In 0.5 moldm3 H2 SO4 solution, all the alloys possess
high corrosion resistance, especially the Cu50 Zr45 Al5 and
Cu50 Zr40 Al5 Nb5 alloys show undetectable weight loss,
indicating a corrosion rate of less than 1 103 mmy1
which is the reproducibility limit for the present measurement. Moreover, the surfaces of the specimens exposed to air
1960
C. Qin, W. Zhang, H. Kimura, K. Asami and A. Inoue
Fig. 5 Potentiodynamic polarization curves of the Cu-Zr-Al-(Nb) bulk
glassy alloys in 3 mass% NaCl solution at 298 K open to air.
Fig. 4 SEM micrographs of the surfaces of the Cu55 Zr40 Al5 and
Cu50 Zr40 Al5 Nb5 bulk glassy alloys with a diameter of 1.5 mm before
and after immersion in 3 mass% NaCl solution for 168 h at 298 K.
after mechanical polishing and those immersed in 3 mass%
NaCl solution for one week were further examined by SEM.
The SEM micrographs are shown in Fig. 4. The alloy without
additional element Nb after immersion suffers uneven serious
corrosion, and its surface is covered with deep red-colored
corrosive products. The 5 at% Nb alloy still keeps the
previous metallic luster and almost no changes in its surface
are seen before and after immersion. The results obtained
from SEM micrographs indicate that the addition of Nb to the
alloys is effective for suppressing the corrosion of the glassy
Cu-Zr-Al-Nb alloys during open-circuit immersion in the
chloride containing environment, which is in agreement with
the corrosion rates of these bulk glassy alloys.
For further understanding of the corrosion behavior, it is
necessary to measure the polarization curves of the alloys.
Figure 5 shows their polarization curves in 3 mass% NaCl
solution open to air at 298 K. It is seen that the Cu55 Zr40 Al5
and Cu50 Zr45 Al5 glassy alloys exhibit similar polarization
behavior to each other, although the corrosion rate of
Cu50 Zr45 Al5 glassy alloy is about half of that of Cu55 Zr40 Al5
glassy alloy. At potentials of 0:39 V versus Ag/AgCl for
the Nb-free alloys and 0:29 V versus Ag/AgCl for the Nbcontaining alloys pit nucleation appears and their anodic
current densities rapidly increase, although their pitting
potentials are not so clearly determined. The formation of pits
with small and round shape was further confirmed by SEM
observation after anodic polarization. On the other hand, the
Nb-containing alloy shows nobler pitting potential and lower
anodic current density, leading to the reduction of pitting
susceptibility and the improvement of pitting corrosion
resistance. Their potentiodynamic polarization curves in
0.5 moldm3 H2 SO4 solution are also shown in Fig. 6. These
alloys show active-passive transition although their active
current peaks are very small. The active current peak
decreases about one order of magnitude by addition of Nb.
Their passive current densities are of the order of 102 Am2
in the wide passive region up to the potential of about 1.0 V
versus Ag/AgCl and then gradually increase. These results
indicate the high corrosion resistance of the bulk Cu-Zr-Al(Nb) glassy alloys in 1 N H2 SO4 solution. On the other hand,
the Cu50 Zr45 Al5 glassy alloy exhibits higher corrosion
resistance in comparison with the Cu55 Zr40 Al5 glassy alloy.
Furthermore, replacing of a portion of Cu by Nb is more
effective for decreasing the passive current density and hence
enhancing the corrosion resistance of the Cu-Zr-Al glassy
alloy.
Testing the changes in open circuit potentials of the alloys
with immersion time gave further explanation for the
corrosion behavior of the Cu-Zr-Al-Nb bulk glassy alloys.
Figure 7 shows the changes in open circuit potentials for the
alloys with immersion time in 3 mass% NaCl solution open to
air at 298 K. The open circuit potentials for the Cu55 Zr40 Al5
and Cu50 Zr45 Al5 glassy alloys exhibit similar behavior to
each other during the immersion. Their potentials initially
drop down rapidly and then stay at highly negative potentials
of about 0:47 V versus Ag/AgCl. In addition, these alloys
free of Nb also show no increase in the open circuit potentials
during the immersion because their surface films are unable
to be stabilized in 3 mass% NaCl solution. For 5 at% Nb
alloy, the potential initially decreases quickly, and then goes
up slowly to reach constant value of about 0:33 V versus
Ag/AgCl. This fact reveals that the addition of Nb is
effective for the ennoblement of the open circuit potential
and improvement of stability of the surface film during the
immersion. The changes in open circuit potentials for the
alloys in 0.5 moldm3 H2 SO4 solution are shown in Fig. 8.
The open circuit potentials for all the alloys initially rise
sharply and further increase gradually towards the more
positive direction with immersion time, indicating their
surface films showing highly protective quality. These results
are in agreement with the fact that these glassy alloys have
high corrosion resistance in 0.5 moldm3 H2 SO4 as indicated by their corrosion rates and potentiodynamic polarization behavior. It is also clear that the open circuit
potentials of the alloys are much nobler with addition of
Nb, suggesting more stable surface film is formed on Nbcontaining alloy during immersion.
Corrosion behavior of an alloy primarily depends on the
characters of alloy constituents and solutions examined. It is
indicated that the alloy composition is of great importance for
corrosion behavior of the alloy. All the alloying elements, Cu
and Al are not corrosion resistance in acid and chloride
containing solutions, whereas Zr and Nb are corrosion
New Cu-Zr-Al-Nb Bulk Glassy Alloys with High Corrosion Resistance
Fig. 6 Potentiodynamic polarization curves
of the Cu-Zr-Al-(Nb) bulk glassy alloys in
0.5 moldm3 H2 SO4 solution at 298 K
open to air.
Fig. 7 Changes in the open circuit potentials with
immersion time for the Cu-Zr-Al-(Nb) bulk glassy
alloys in 3 mass% NaCl solution at 298 K open to
air.
resistance in the solutions examined in this work. Since Cu
element is a main constituent element in the alloy and not
stable in chloride containing solutions because of the
formation of a CuCl2 complex anion,25) the Cu-Zr-Al(Nb) alloys exhibit a lack of corrosion resistance in HCl and
NaCl solutions where the corrosion rate increases with the
rise of Cu content in the alloy. On the other hand, it is also
clarified that the alloys suffer pitting corrosion during anodic
polarization in NaCl solution due to the existence of Zr and
Al elements. This result is in agreement with previous work
that Zr-based glassy alloys26) and the alloys containing Zr27)
suffer pitting corrosion in chloride containing solutions.
Moreover, it is well known that Nb element is an effective
alloying element to provide a high passivating ability for the
metallic glasses. It has previously been reported that the
addition of Nb to the Cu-based glassy alloys decreased the
concentration of Cu in the surface film and increased those of
Zr and Nb, leading to the formation of Zr(Nb)-rich protective
surface film.28)
4.
Summary
The glassy Cu55 Zr40 Al5 , Cu50 Zr45 Al5 and Cu50 Zr40 Al5 Nb5
alloys can be synthesized in a bulk glassy form with
maximum diameters of 2.0–3.0 mm by copper mold casting,
although the glass-forming ability and thermal stability
decrease by the addition of Nb. The corrosion resistance of
the Cu-Zr-Al-(Nb) alloys is very low in 1 moldm3 HCl and
3 mass% NaCl solutions. By the addition of 5 at% Nb to the
Cu-Zr-Al alloy, the corrosion rate decreases by two orders of
magnitude in 1 moldm3 HCl solution and one order of
magnitude in 3 mass% NaCl solution in comparison with
those of the Cu-Zr-Al alloys. In 0.5 moldm3 H2 SO4
solution, all the bulk glassy alloys are passivated in wide
passive region and low passive current density of the order of
102 Am2 , indicating high corrosion resistance in this
solution. In addition, the addition of Nb to the Cu55 Zr40 Al5
alloy for replacing a portion of Cu is effective for decreasing
the passive current density and improving the corrosion
resistance.
1961
Fig. 8 Changes in the open circuit potentials
with immersion time for the Cu-Zr-Al-(Nb)
bulk glassy alloys in 0.5 moldm3 H2 SO4
solution at 298 K open to air.
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