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A novel Cu-based BMG composite with high corrosion resistance and excellent mechanical properties
Acta Materialia 54 (2006) 3713–3719 www.actamat-journals.com
A novel Cu-based BMG composite with high corrosion resistance and excellent mechanical properties C.L. Qin a
a,*
, W. Zhang b, K. Asami b, H. Kimura b, X.M. Wang b, A. Inoue
b
Japan Science and Technology Agency, Institute for Materials Research, Katahira, Aoba-ku, Sendai 980-8577, Japan b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 18 December 2005; received in revised form 30 March 2006; accepted 2 April 2006 Available online 19 June 2006
Abstract A newly designed (Cu0.6Hf0.25Ti0.15)90Nb10 bulk metallic glass (BMG) composite with an excellent combination of high corrosion resistance and superior mechanical properties was successfully synthesized. The micrometer-sized ductile Nb-rich dendrite phase disperses homogeneously in the glassy matrix. Addition of Nb to the Cu–Hf–Ti alloy results in a significant decrease in the corrosion rates in the solutions examined. The high corrosion resistance of the (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite is attributed to the formation of Hf-, Ti-, and Nb-enriched highly protective surface films during immersion in acid- and chloride-ion-containing solutions. The composite alloy exhibits a high compressive true yield strength of 2073 MPa and true fracture strength of 2232 MPa together with a large true plastic strain of 14.1%. The embedded micrometer-sized dendrites in the glassy matrix retard inhomogeneous shear deformation and contribute to the large plastic strain. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Bulk metallic glass composite; Corrosion resistance; XPS measurements; Mechanical properties; Shear band
1. Introduction Recently, much worldwide research has been carried out on newly developed Cu-based bulk metallic glasses (BMGs) [1–5]. Due to their high strength, exceeding 2000 MPa, and high glass-forming ability, they are considered to have many potential applications as advanced engineering materials, such as in surgical instruments and bipolar plates in fuel cells. Unfortunately, like conventional amorphous alloys, Cubased BMGs fail by the formation of highly localized shear bands, which leads to catastrophic failure without much macroscopic plasticity [1,2]. This inhomogeneous deformation behavior has so far seriously limited the application of BMGs as engineering materials. To overcome this problem, there have been various attempts to produce reinforced BMGs [6–9], by partial crystallization, particle reinforcements or in situ formation of ductile body-centered cubic *
Corresponding author. Tel.: +81 22 215 3048; fax: +81 22 215 2413. E-mail address: [email protected] (C.L. Qin).
(bcc)-phase precipitates. As a result, a new class of materials, i.e. BMG composites, combining ceramic-like strength with metal-like ductility has been produced. Recent research has reported that in situ Cu–Zr(Hf)–(Ta,Nb) alloy composites exhibit markedly improved plasticity [10–12]. However, it should be realized that one major drawback has been ignored, i.e. Cu-based BMGs [13,14] suffer strongly from corrosion in many aggressive environments, such as acidic and salt solutions, at ambient temperature and in humid air. It is difficult to use Cu-based alloys in practical service because of their poor corrosion resistance, especially in chloride-ion-containing solutions. With the aim of improving the corrosion resistance as well as ductility, a design strategy for proper composition and suitable casting conditions is very important. Nb was chosen as an addition to Cu–Hf–Ti BMG for two reasons: (1) Nb has a positive heat of mixing with the constituent elements, Cu, Hf, and Ti, which can facilitate the precipitation of a micrometer-sized ductile phase [15] and (2) Nb has a strong passivating ability for metallic glasses. In our prior
1359-6454/$30.00 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.04.005
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works [14], the addition of Nb up to 8 at.% to Cu–Hf–Ti BMG was effective for enhancing the corrosion resistance. However, at best the plastic strain in uniaxial compression rose to only 2.8%. Considering industrial and environmental applications, we have focused on the development of new Cu–Hf–Ti-based BMG matrix composites by the adjustment of appropriate Nb content, and have succeeded in finding compositions with excellent mechanical properties and good corrosion resistance. In this paper, we report the first successful synthesis of in situ formed (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite containing ductile phase with the best combination of strength, plasticity and corrosion resistance. The effects of Nb on the microstructure, thermal properties, corrosion behavior and mechanical properties were investigated. The origin of the good corrosion resistance of the Nb-reinforced alloy is also discussed. 2. Experimental Master alloys with nominal compositions of (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) were prepared from pure elemental Cu, Hf, Ti, and Nb of 99.9 mass% purity by arc melting under an argon atmosphere using a water-cooled Cu hearth. At first the binary alloy of Hf and Nb was prepared. This binary alloy was then melted with Cu and Ti. Remelted ingots were ejected into a copper mold to produce a 50 mm long cylindrical rod with a diameter of 2 mm. The microstructures of ascast rods were characterized using X-ray diffraction (XRD; Rigaku diffractometer) with Cu Ka radiation at 40 kV and scanning electron microscopy (SEM; JSM 6330FS). The compositions of the dendrite components were determined using electron probe microanalysis (EPMA). Thermal stability was examined using differential scanning calorimetry (DSC) with a Seiko DSC6300U calorimeter at a heating rate of 0.67 K s 1. Corrosion behavior of the alloys was evaluated by weight loss and electrochemical measurements. Prior to the 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 N HCl, 3 mass% NaCl, 1 N H2SO4, and 1 N H2SO4 + 0.01 N NaCl solutions open to air were used at room temperature (about 298 K). The corrosion rates were estimated from the weight loss after immersion in the solutions for 1 or 2 weeks. The weight loss for each alloy was measured three times and the average value was used for corrosion rate estimation. Electrochemical measurements were conducted in a three-electrode cell using a platinum counter electrode and an Ag/AgCl reference electrode. Potentiodynamic polarization curves were measured at a potential sweep rate of 50 mV min 1 after open-circuit immersion for about 20 min when the opencircuit potential became almost steady. X-ray photoelectron spectroscopy (XPS) measurements for surface analysis of the specimens before and after
immersion in the solutions were performed using a SSI SSX-100 photoelectron spectrometer with monochromatized Al Ka excitation (hm = 1486.6 eV). The composition of the surface film and the composition of the underlying alloy surface were quantitatively determined with a previously proposed method using the integrated intensities of photoelectrons under the assumption of a three-layer model of an outmost contaminant hydrocarbon layer of uniform thickness, a surface film of uniform thickness and an underlying alloy surface of infinite thickness as regards X-ray photoelectrons [16,17]. Mechanical properties, including Young’s modulus, yield strength, fracture strength, and plastic strain, were measured at room temperature, on samples with gauge dimensions of 4 mm long and 2 mm diameter, using an Instron 5581 mechanical testing machine and an Electronic Instruments strain gauge. The strain rate was 5.0 · 10 4 s 1. The yield strength was taken to be the true stress at a plastic strain of 0.002. 3. Results and discussion 3.1. Synthesis, microstructure and thermal properties From the XRD patterns of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) rods with a diameter of 2 mm, only a single glassy phase was observed for the Nb-free alloy. The XRD pattern of the Nb-containing alloy exhibited a broad halo peak superimposed on the sharp diffraction peaks, indicating that some crystalline phases precipitated from the glassy matrix. The position and intensity of the crystalline peaks exactly coincided with those of bcc-Nb crystalline phase. This result indicated that Nb-rich solid solution precipitated in the glassy matrix. Fig. 1 shows the microstructure of the as-cast 2 mm diam-
Fig. 1. SEM image of the central region in the transverse cross section of the as-cast (Cu0.6Hf0.25Ti0.15)90Nb 10 alloy rod with a diameter of 2 mm.
C.L. Qin et al. / Acta Materialia 54 (2006) 3713–3719
eter (Cu0.6Hf0.25Ti0.15)90Nb10 alloy. The Nb-rich solid solution phase with the dendritic structure disperses homogeneously in the glassy matrix. The volume fraction and mean size of the dendrite bcc phase are about 8% and 5–10 lm, respectively. The average compositions of six data points in the dendrite and remaining glassy matrix phases examined using EPMA were Cu2.6Hf5.4Ti11.6Nb80.4 and Cu56Hf22.9Ti14.2Nb6.9, respectively. The Nb-rich and Cu-poor compositions of the dendritic phase revealed that the Nb-rich phase precipitated as the primary dendrite phase because of the immiscible nature against Cu-rich liquid phase and much higher melting temperature of the Nb-rich phase. Thus, this new type of (Cu0.6Hf0.25Ti0.15)90Nb10 composite material has a micrometer-sized dendritic Nb-rich solid solution dispersed in a glassy matrix. Fig. 2 shows DSC curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys with a diameter of 2 mm using a heating rate of 0.67 K s 1. In this figure Tg and Tx correspond to the glass transition temperature and onset temperature of crystallization, respectively. Both samples exhibit an endothermic heat event characteristic of the glass transition followed by three exothermic heat releases indicating the transformation from the metastable supercooled liquid state into crystalline phases. With the addition of Nb to the Cu–Hf–Ti alloy, Tg remains almost constant, while Tx shifts to about 25 K lower, while the accompanying first peak also shifts to about 17 K lower. In addition, the heat release of the first crystalline peak decreases from 27.8 J g 1 for Nb-free BMG to 12.7 J g 1 for Nb-reinforced composite. This indicates that Nb has almost no effect on the glass transition temperature, but significantly changes the crystallization transformation
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process of the glassy phase. To put it simply, the addition of Nb to the Cu–Hf–Ti BMG promotes the formation of dendrite phase, but suppresses glass formation. 3.2. Corrosion resistance The average corrosion rates of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys immersed in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions at 298 K open to air for 1 week are shown in Fig. 3. The corrosion rates of an industrial brass (60 wt.% Cu + 40 wt.% Zn) in those solutions are also displayed for comparison. The corrosion rate less than 1 · 10 3 mm year 1 is below the detection limits for the present measurements, and hence is taken as zero in Fig. 3. In 1 N HCl solution, the Cu60Hf25Ti15 alloy without Nb dissolves actively showing a high corrosion rate of 0.34 mm year 1 due to the formation of CuCl2 complex anion [18]. However, the composite alloy containing Nb exhibits a much lower corrosion rate of 0.011 mm year 1, being about one order of magnitude lower than that of the Nb-free alloy. In addition, in the NaCl solution, it also appears that the average corrosion rate of the Nb-containing alloy decreases significantly with addition of Nb alloying element. The corrosion rate of the Nb-free alloy is about 0.10 mm year 1, while that of the Nb-reinforced alloy is less than the measurable value. The (Cu0.6Hf0.25Ti0.15)90Nb10 alloy maintained its previous metallic luster after 1 week of immersion in 3 mass% NaCl. Both alloys show an undetectable weight loss when immersed for 2 weeks in 1 N H2SO4 solution. So we chose 1 N H2SO4 containing a small amount of chloride ions as a corrosive solution, i.e. 1 N H2SO4 + 0.01 N NaCl. By comparison with the Nb-free alloy, the (Cu0.6Hf0.25Ti0.15)90Nb10 composite has a markedly increased corrosion resistance, indicating a more inhibited chloride attack due to the presence of Nb. In all solutions examined, regardless of strongly acidic
as-cast φ 2 mm 0.67 K/s
(Cu0.6Hf0.25Ti0.15)100-xNbx
Corrosion rate, R / mm year
Exothermic (a. u.)
-1
0.35
x=10
x=0 Tg
Cu60Hf25Ti15 (Cu0.6Hf0.25Ti 0.15)90Nb10 Industrial brass φ 2 mm cast rod, 298 K, 168 h, open to air
0.30 0.25 0.20 0.15 0.10 0.05
Tx
0.00 600
700
800
900
1000
1100
Temperature, T / K Fig. 2. DSC curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloy rods with diameters of 2 mm.
1 N HCl
3 mass% NaCl
1 N H2SO4+0.01 N NaCl
Fig. 3. Average corrosion rates of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys and an industrial brass in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions at 298 K open to air.
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or chloride-ion-containing environments, the corrosion resistance of the Nb-containing composite is better than that of the industrial brass. Further examination was conducted using potentiodynamic polarization measurements. Figs. 4 and 5 show the potentiodynamic polarization curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys, an industrial brass and pure Nb in 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions, respectively, open to air at 298 K. In 3 mass% NaCl (Fig. 4), the (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys show different anodic polarization behavior, while the cathodic polarization behavior for oxygen and proton reduction is similar. The Nb-free alloy dissolves quickly by slight anodic polarization. In contrast, the Nb-reinforced alloy exhibits distinct anodic spontaneous passivation with low passive
10
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(Cu0.6Hf0.25Ti0.15 )90 Nb10
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φ 2 mm pure Nb metal
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3.3. XPS analysis of surface film
φ 2 mm cast rod 298 K, open to air in 3 mass% NaCl
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10
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-0.4
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Potential, E / V vs Ag/AgCl
Fig. 4. Potentiodynamic polarization curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys, an industrial brass and pure Nb in 3 mass% NaCl solution at 298 K open to air.
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Current density, I / Am
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φ 2 mm cast rod 298 K, open to air in 1 N H2SO4+0.01 N NaCl
brass
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current density, although it suffers pitting. The improvement of corrosion resistance for Nb-reinforced alloy is attributed to the high passivating ability of the Nb alloying element (see figure inset). At the same time, the brass shows an active state during anodization along with more activity in the cathodic reaction and ennoblement in open-circuit potential compared with the Cu-based alloys. In 1 N H2SO4 + 0.01 N NaCl solution (Fig. 5), the Nb-free alloy shows the active–passive transition and the current density corresponding to its active current peak is about 1.2 A m 2. Spontaneous passivation with a significantly low current density of the order of 10 2 A m 2 takes place for Nb-reinforced BMG composite, and the alloy undergoes two-step passivation during the potential range up to 2.0 V versus Ag/AgCl. No steep increase in current density due to pitting corrosion is seen for either alloys during anodic polarization. However, the current density of the industrial brass increases rapidly by slight anodic polarization, indicating high dissolution of the brass in the solution. Furthermore, although the pure Nb metal is spontaneously passivated in a wide passive region, its passive current density is much higher than that of the BMG composite. It can be concluded that the (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite exhibits high corrosion resistance under the strong corrosion conditions, resulting from the beneficial effect of Nb addition, despite the fact that this alloy consists of micrometer-sized dendrite phase in glassy matrix.
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential, E / V vs Ag/AgCl
Fig. 5. Potentiodynamic polarization curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys, an industrial brass and pure Nb in 1 N H2SO4 + 0.01 N NaCl solution at 298 K open to air.
In order to clarify the origin of the high corrosion resistance of the (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy, XPS analysis was carried out for the specimens exposed to air after mechanical polishing and those immersed in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions open to air for 1 week. The XPS spectra over a wide binding energy region exhibited peaks attributed to Cu, Ti, Hf, Nb, Cl, O, and C. The C 1s peaks arose from a contaminant hydrocarbon layer covering the specimen surface. The O 1s spectrum consisted of peaks originating from oxygen in metal–O–metal bonds, metal–OH bonds and bound water. The Cl 2p peak was assigned to Cl ion in the surface film, but the concentration of Cl ion was negligibly small. The XPS peaks of Cu 2p, Hf 4f, Ti 2p, Nb 3d together with O 1s and C 1s were used for quantitative determination of compositions. For the as-polished specimens, these peaks were composed of peaks of oxidized states and metallic states; the oxidized states and metallic states are assigned to signals from the surface film and underlying substrate, respectively. However, for the other specimens, those peaks showed only oxidic peaks because of the surface films on the specimens were much thicker than the escape depths of photoelectrons. The major cations in the surface film were Cu+, Hf4+, Ti4+ and Nb5+. Fig. 6 shows the surface film compositions of the (Cu0.6Hf0.25Ti0.15)90Nb10 alloy exposed to air and
C.L. Qin et al. / Acta Materialia 54 (2006) 3713–3719
Nb
Cationic concentration, (%)
80
Ti 60
40
Hf
20
Cu 0 nominal
as-polished
HCl
NaCl
H2SO4+NaCl
Fig. 6. Surface film compositions for the as-cast (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy exposed to air and samples immersed in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions open to air for 1 week after mechanical polishing.
immersed in the 1 N HCl, 3 mass% NaCl and 1 N H2SO4 + 0.01 N NaCl solutions after mechanical polishing. In the surface film formed by air exposure after mechanical polishing, Hf and Ti are enriched, while Cu is deficient and Nb is slightly deficient with respect to the alloy composition. This fact indicates that preferential oxidation of Hf and Ti occurs in the air-formed film. When the specimen was immersed in the 1 N HCl solution, it is found that the Cu content in the surface film significantly decreases, and further reduces after immersion in the 3 mass% NaCl or 1 N H2SO4 + 0.01 N NaCl solution. At the same time, Ti and Hf are largely concentrated on the alloy surface immersed in the 1 N HCl solution. The content of Ti increases more after immersion in the 3 mass% NaCl or 1 N H2SO4 + 0.01 N NaCl solution, while that of Hf slightly decreases. It is observed that the surface films of the alloy immersed in the solutions are enriched in Nb with respect to that in the as-polished film and the alloy Nb content. The increase in Nb content as compared with that of the as-polished film is about 44.8%, 69.1%, and 79.8% after immersion in the HCl, NaCl, and H2SO4 + NaCl solutions, respectively. Therefore, open-circuit immersion leads to the formation of Hf-, Ti-, and Nbenriched surface films as a result of preferential dissolution of Cu and the effect of Nb. In contrast, the concentrations of Cu in the surface film after immersion in HCl and NaCl solutions for the Cu60Hf25Ti15 BMG alloy [14] were very high in comparison with those of (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite alloy. Among all the constituent elements, Cu has a low corrosion resistance which easily dissolves in acidic and chloride-ion-containing solutions, whereas Hf, Ti, and Nb exhibit excellent corrosion resistance in the solutions examined in this work. So, a stable surface film
cannot be formed for the Nb-free alloy immersed in HCl and NaCl solutions, which is consistent with the results of corrosion rates, as shown in Fig. 3. Generally, the corrosion resistance of metallic glasses is expected to be better than that of their crystalline counterparts. However, one must realize that the corrosion resistance of all materials is mainly dominated by the basic chemical composition, rather than by structural factors. According to the results of corrosion rates and polarization curves for both alloys, as displayed in Figs. 3–5, the presence of Nb in the (Cu0.6Hf0.25Ti0.15)90Nb10 alloy is beneficial for enhancing the corrosion resistance and passivating ability of the alloy, in spite of the fact that the alloy consists of micrometersized crystalline phase in glassy matrix. It can therefore be said that the protective quality of the surface films is improved by the addition of Nb. The high-quality protective film can cover the entire surface of the composite alloy, which protects the alloy from corrosion in acid and chloride-containing solutions. Accordingly, the formation of highly protective surface films enriched in Hf, Ti, and Nb on the (Cu0.6Hf0.25Ti0.15)90Nb10 composite could be responsible for the high corrosion resistance. A detailed investigation of the influence of microscale crystalline phase on corrosion resistance is underway, and will be presented elsewhere. 3.4. Mechanical properties Fig. 7 shows the true stress–strain curves of the as-cast 2 mm diameter (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys in uniaxial compression at room temperature (298 K). Young’s modulus (E), compressive true yield strength (rc, y), compressive true fracture strength (rc, f) and compressive true plastic strain (ec, p) are 124 GPa, 2024 MPa, 2088 MPa, and 1.6%, respectively, for the Nbfree alloy and 106 GPa, 2073 MPa, 2232 MPa, and
2500
Compressive true stress, σ / MPa
100
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2000 (Cu0.6Hf0.25Ti0.15)90Nb10 1500
1000
Cu60Hf25Ti15
φ 2 mm cast rod
500
-4
-1
Strain rate = 5 x 10 s 0 0
2
4
6
8
10
12
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True strain, ε (%) Fig. 7. Compressive true stress–strain curve of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys.
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C.L. Qin et al. / Acta Materialia 54 (2006) 3713–3719
(a)
(c)
(b)
Compression direction
(c)
500 µm 10 µm
10 µm
Fig. 8. Outer shape (a), shear bands developed on the outer surface (b) and high magnification of the dashed circle part (c) for the as-cast (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy after compression testing.
14.1%, respectively, for the Nb-reinforced alloy. Apparently, for the Nb-reinforced alloy, the combination of strength and plasticity is superior to that for the Cu60Hf25Ti15 alloy. It is believed that the embedded micrometer-sized dendrites in the Nb-reinforced alloy retard inhomogeneous shear deformation and contribute to the large plastic strain. Since the volume fraction of the dendritic phase is not large (about 8%) in the glassy matrix, the strength of the composite alloy is still dominated by the glassy matrix. As mentioned above, the glassy matrix consists of 6.9 at% Nb, and the presence of Nb in the residual glassy matrix enhances the bonding forces among Cu, Hf, Ti, and Nb elements [14], which results in higher strength. Fig. 8 shows outer shape and deformation structure on the outer surface of the as-cast 2 mm diameter (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy after compression testing. It is seen that the final fracture occurs along the maximum shear stress plane which is declined by about 42 to the direction of the applied load (Fig. 8(a)), and a large number of the shear bands distribute homogeneously throughout the surface of the specimen (Fig. 8(b)), reflecting the significantly improved plastic strain in the composite. A high-magnification image (Fig. 8(c)) shows more detailed information about the dashed circle part in Fig. 8(b). The shear bands observed in the composite sample are characterized as being straight, wavy or twisting. In general, plastic deformation of glassy single-phase alloys is highly localized into shear bands, followed by the rapid propagation of these shear bands and sudden fracture. For the Cu–Hf–Ti–Nb mixed structure composite with ductile dendrites, during the propagation of shear bands under loading, the shear bands must interact with the microscale ductile dendrites and have to deflect their propagating direction, resulting in shear band branching. At the same time, the shear bands are also blocked or branched by other intersected shear bands. Thus, the formation of a large number of the branching shear bands causes the large increase in the plasticity of the alloy.
4. Conclusions A newly designed (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite with excellent combination of high corrosion resistance and superior mechanical properties was successfully synthesized. The Nb-rich solid solution phase with the dendritic structure disperses homogeneously in the glassy matrix. The volume fraction and mean size of the dendrite bcc phase are about 8% and 5–10 lm. The (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy exhibits much higher corrosion resistance in acidic and chloride-containing solutions as compared with the Nb-free alloy. The addition of Nb is favorable for the alloys in forming Hf-, Ti-, and Nb-enriched highly protective surface films with higher chemical stability in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions. The composite alloy exhibits a high compressive true yield strength of 2073 MPa, true fracture strength of 2232 MPa and large true plastic strain of 14.1%. The increase in compressive strain-to-failure is due to the ductile dendrites restricting shear band propagation, promoting the generation of multiple shear bands. The new Cu-based BMG composite with good ductility and high strength together with high corrosion resistance is very promising as a new advanced engineering material. References [1] Inoue A, Zhang W, Zhang T, Kurosaka K. Acta Mater 2001;49:2645. [2] Inoue A, Zhang W, Zhang T, Kurosaka K. J Mater Res 2001;16:2836. [3] Inoue A, Zhang W, Zhang T, Kurosaka K. Mater Trans 2001;42:1805. [4] Wang D, Li Y, Sun BB, Sui ML, Lu K, Ma E. Appl Phys Lett 2004;84:4029. [5] Das J, Tang MB, Kim KB, Theissmann R, Baier F, Wang WH, Eckert J. Phys Rev Lett 2005;94:205501. [6] Fan C, Takeuchi A, Inoue A. Mater Trans JIM 1999;40:42. [7] Conner RD, Choi-Yim H, Johnson WL. J Mater Res 1999;14:3292. [8] Hays CC, Kim CP, Johnson WL. Phys Rev Lett 2000;84:2901. [9] Fan C, Ott RT, Hufnagel TC. Appl Phys Lett 2002;81:1020.
C.L. Qin et al. / Acta Materialia 54 (2006) 3713–3719 [10] Bian Z, Ahmad J, Zhang W, Inoue A. Mater Trans 2004;45:2346. [11] Qin CL, Zhang W, Kimura H, Inoue A. Mater Trans 2004;45:2936. [12] Lee JC, Kim YC, Ahn JP, Kim HS, Lee SH, Lee BJ. Acta Mater 2004;52:1525. [13] Qin CL, Asami K, Zhang T, Zhang W, Inoue A. Mater Trans 2003;44:749.
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[14] Qin CL, Zhang W, Asami K, Ohtsu N, Inoue A. Acta Mater 2005;53:3903. [15] Boer FR, Boom R, Mattens WCM, Miedema AR, Niessen AK. Cohesion in metals. Amsterdam: North-Holland; 1988. [16] Asami K, Hashimoto K, Shimodaira S. Corros Sci 1977;17:713. [17] Asami K, Hashimoto K. Corros Sci 1984;24:83. [18] Naka M, Hashimoto K, Masumoto T. J Non-Cryst Sol 1978;30:29.
A novel Cu-based BMG composite with high corrosion resistance and excellent mechanical properties C.L. Qin a
a,*
, W. Zhang b, K. Asami b, H. Kimura b, X.M. Wang b, A. Inoue
b
Japan Science and Technology Agency, Institute for Materials Research, Katahira, Aoba-ku, Sendai 980-8577, Japan b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 18 December 2005; received in revised form 30 March 2006; accepted 2 April 2006 Available online 19 June 2006
Abstract A newly designed (Cu0.6Hf0.25Ti0.15)90Nb10 bulk metallic glass (BMG) composite with an excellent combination of high corrosion resistance and superior mechanical properties was successfully synthesized. The micrometer-sized ductile Nb-rich dendrite phase disperses homogeneously in the glassy matrix. Addition of Nb to the Cu–Hf–Ti alloy results in a significant decrease in the corrosion rates in the solutions examined. The high corrosion resistance of the (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite is attributed to the formation of Hf-, Ti-, and Nb-enriched highly protective surface films during immersion in acid- and chloride-ion-containing solutions. The composite alloy exhibits a high compressive true yield strength of 2073 MPa and true fracture strength of 2232 MPa together with a large true plastic strain of 14.1%. The embedded micrometer-sized dendrites in the glassy matrix retard inhomogeneous shear deformation and contribute to the large plastic strain. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Bulk metallic glass composite; Corrosion resistance; XPS measurements; Mechanical properties; Shear band
1. Introduction Recently, much worldwide research has been carried out on newly developed Cu-based bulk metallic glasses (BMGs) [1–5]. Due to their high strength, exceeding 2000 MPa, and high glass-forming ability, they are considered to have many potential applications as advanced engineering materials, such as in surgical instruments and bipolar plates in fuel cells. Unfortunately, like conventional amorphous alloys, Cubased BMGs fail by the formation of highly localized shear bands, which leads to catastrophic failure without much macroscopic plasticity [1,2]. This inhomogeneous deformation behavior has so far seriously limited the application of BMGs as engineering materials. To overcome this problem, there have been various attempts to produce reinforced BMGs [6–9], by partial crystallization, particle reinforcements or in situ formation of ductile body-centered cubic *
Corresponding author. Tel.: +81 22 215 3048; fax: +81 22 215 2413. E-mail address: [email protected] (C.L. Qin).
(bcc)-phase precipitates. As a result, a new class of materials, i.e. BMG composites, combining ceramic-like strength with metal-like ductility has been produced. Recent research has reported that in situ Cu–Zr(Hf)–(Ta,Nb) alloy composites exhibit markedly improved plasticity [10–12]. However, it should be realized that one major drawback has been ignored, i.e. Cu-based BMGs [13,14] suffer strongly from corrosion in many aggressive environments, such as acidic and salt solutions, at ambient temperature and in humid air. It is difficult to use Cu-based alloys in practical service because of their poor corrosion resistance, especially in chloride-ion-containing solutions. With the aim of improving the corrosion resistance as well as ductility, a design strategy for proper composition and suitable casting conditions is very important. Nb was chosen as an addition to Cu–Hf–Ti BMG for two reasons: (1) Nb has a positive heat of mixing with the constituent elements, Cu, Hf, and Ti, which can facilitate the precipitation of a micrometer-sized ductile phase [15] and (2) Nb has a strong passivating ability for metallic glasses. In our prior
1359-6454/$30.00 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.04.005
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works [14], the addition of Nb up to 8 at.% to Cu–Hf–Ti BMG was effective for enhancing the corrosion resistance. However, at best the plastic strain in uniaxial compression rose to only 2.8%. Considering industrial and environmental applications, we have focused on the development of new Cu–Hf–Ti-based BMG matrix composites by the adjustment of appropriate Nb content, and have succeeded in finding compositions with excellent mechanical properties and good corrosion resistance. In this paper, we report the first successful synthesis of in situ formed (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite containing ductile phase with the best combination of strength, plasticity and corrosion resistance. The effects of Nb on the microstructure, thermal properties, corrosion behavior and mechanical properties were investigated. The origin of the good corrosion resistance of the Nb-reinforced alloy is also discussed. 2. Experimental Master alloys with nominal compositions of (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) were prepared from pure elemental Cu, Hf, Ti, and Nb of 99.9 mass% purity by arc melting under an argon atmosphere using a water-cooled Cu hearth. At first the binary alloy of Hf and Nb was prepared. This binary alloy was then melted with Cu and Ti. Remelted ingots were ejected into a copper mold to produce a 50 mm long cylindrical rod with a diameter of 2 mm. The microstructures of ascast rods were characterized using X-ray diffraction (XRD; Rigaku diffractometer) with Cu Ka radiation at 40 kV and scanning electron microscopy (SEM; JSM 6330FS). The compositions of the dendrite components were determined using electron probe microanalysis (EPMA). Thermal stability was examined using differential scanning calorimetry (DSC) with a Seiko DSC6300U calorimeter at a heating rate of 0.67 K s 1. Corrosion behavior of the alloys was evaluated by weight loss and electrochemical measurements. Prior to the 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 N HCl, 3 mass% NaCl, 1 N H2SO4, and 1 N H2SO4 + 0.01 N NaCl solutions open to air were used at room temperature (about 298 K). The corrosion rates were estimated from the weight loss after immersion in the solutions for 1 or 2 weeks. The weight loss for each alloy was measured three times and the average value was used for corrosion rate estimation. Electrochemical measurements were conducted in a three-electrode cell using a platinum counter electrode and an Ag/AgCl reference electrode. Potentiodynamic polarization curves were measured at a potential sweep rate of 50 mV min 1 after open-circuit immersion for about 20 min when the opencircuit potential became almost steady. X-ray photoelectron spectroscopy (XPS) measurements for surface analysis of the specimens before and after
immersion in the solutions were performed using a SSI SSX-100 photoelectron spectrometer with monochromatized Al Ka excitation (hm = 1486.6 eV). The composition of the surface film and the composition of the underlying alloy surface were quantitatively determined with a previously proposed method using the integrated intensities of photoelectrons under the assumption of a three-layer model of an outmost contaminant hydrocarbon layer of uniform thickness, a surface film of uniform thickness and an underlying alloy surface of infinite thickness as regards X-ray photoelectrons [16,17]. Mechanical properties, including Young’s modulus, yield strength, fracture strength, and plastic strain, were measured at room temperature, on samples with gauge dimensions of 4 mm long and 2 mm diameter, using an Instron 5581 mechanical testing machine and an Electronic Instruments strain gauge. The strain rate was 5.0 · 10 4 s 1. The yield strength was taken to be the true stress at a plastic strain of 0.002. 3. Results and discussion 3.1. Synthesis, microstructure and thermal properties From the XRD patterns of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) rods with a diameter of 2 mm, only a single glassy phase was observed for the Nb-free alloy. The XRD pattern of the Nb-containing alloy exhibited a broad halo peak superimposed on the sharp diffraction peaks, indicating that some crystalline phases precipitated from the glassy matrix. The position and intensity of the crystalline peaks exactly coincided with those of bcc-Nb crystalline phase. This result indicated that Nb-rich solid solution precipitated in the glassy matrix. Fig. 1 shows the microstructure of the as-cast 2 mm diam-
Fig. 1. SEM image of the central region in the transverse cross section of the as-cast (Cu0.6Hf0.25Ti0.15)90Nb 10 alloy rod with a diameter of 2 mm.
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eter (Cu0.6Hf0.25Ti0.15)90Nb10 alloy. The Nb-rich solid solution phase with the dendritic structure disperses homogeneously in the glassy matrix. The volume fraction and mean size of the dendrite bcc phase are about 8% and 5–10 lm, respectively. The average compositions of six data points in the dendrite and remaining glassy matrix phases examined using EPMA were Cu2.6Hf5.4Ti11.6Nb80.4 and Cu56Hf22.9Ti14.2Nb6.9, respectively. The Nb-rich and Cu-poor compositions of the dendritic phase revealed that the Nb-rich phase precipitated as the primary dendrite phase because of the immiscible nature against Cu-rich liquid phase and much higher melting temperature of the Nb-rich phase. Thus, this new type of (Cu0.6Hf0.25Ti0.15)90Nb10 composite material has a micrometer-sized dendritic Nb-rich solid solution dispersed in a glassy matrix. Fig. 2 shows DSC curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys with a diameter of 2 mm using a heating rate of 0.67 K s 1. In this figure Tg and Tx correspond to the glass transition temperature and onset temperature of crystallization, respectively. Both samples exhibit an endothermic heat event characteristic of the glass transition followed by three exothermic heat releases indicating the transformation from the metastable supercooled liquid state into crystalline phases. With the addition of Nb to the Cu–Hf–Ti alloy, Tg remains almost constant, while Tx shifts to about 25 K lower, while the accompanying first peak also shifts to about 17 K lower. In addition, the heat release of the first crystalline peak decreases from 27.8 J g 1 for Nb-free BMG to 12.7 J g 1 for Nb-reinforced composite. This indicates that Nb has almost no effect on the glass transition temperature, but significantly changes the crystallization transformation
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process of the glassy phase. To put it simply, the addition of Nb to the Cu–Hf–Ti BMG promotes the formation of dendrite phase, but suppresses glass formation. 3.2. Corrosion resistance The average corrosion rates of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys immersed in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions at 298 K open to air for 1 week are shown in Fig. 3. The corrosion rates of an industrial brass (60 wt.% Cu + 40 wt.% Zn) in those solutions are also displayed for comparison. The corrosion rate less than 1 · 10 3 mm year 1 is below the detection limits for the present measurements, and hence is taken as zero in Fig. 3. In 1 N HCl solution, the Cu60Hf25Ti15 alloy without Nb dissolves actively showing a high corrosion rate of 0.34 mm year 1 due to the formation of CuCl2 complex anion [18]. However, the composite alloy containing Nb exhibits a much lower corrosion rate of 0.011 mm year 1, being about one order of magnitude lower than that of the Nb-free alloy. In addition, in the NaCl solution, it also appears that the average corrosion rate of the Nb-containing alloy decreases significantly with addition of Nb alloying element. The corrosion rate of the Nb-free alloy is about 0.10 mm year 1, while that of the Nb-reinforced alloy is less than the measurable value. The (Cu0.6Hf0.25Ti0.15)90Nb10 alloy maintained its previous metallic luster after 1 week of immersion in 3 mass% NaCl. Both alloys show an undetectable weight loss when immersed for 2 weeks in 1 N H2SO4 solution. So we chose 1 N H2SO4 containing a small amount of chloride ions as a corrosive solution, i.e. 1 N H2SO4 + 0.01 N NaCl. By comparison with the Nb-free alloy, the (Cu0.6Hf0.25Ti0.15)90Nb10 composite has a markedly increased corrosion resistance, indicating a more inhibited chloride attack due to the presence of Nb. In all solutions examined, regardless of strongly acidic
as-cast φ 2 mm 0.67 K/s
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Corrosion rate, R / mm year
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Cu60Hf25Ti15 (Cu0.6Hf0.25Ti 0.15)90Nb10 Industrial brass φ 2 mm cast rod, 298 K, 168 h, open to air
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Temperature, T / K Fig. 2. DSC curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloy rods with diameters of 2 mm.
1 N HCl
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1 N H2SO4+0.01 N NaCl
Fig. 3. Average corrosion rates of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys and an industrial brass in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions at 298 K open to air.
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or chloride-ion-containing environments, the corrosion resistance of the Nb-containing composite is better than that of the industrial brass. Further examination was conducted using potentiodynamic polarization measurements. Figs. 4 and 5 show the potentiodynamic polarization curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys, an industrial brass and pure Nb in 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions, respectively, open to air at 298 K. In 3 mass% NaCl (Fig. 4), the (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys show different anodic polarization behavior, while the cathodic polarization behavior for oxygen and proton reduction is similar. The Nb-free alloy dissolves quickly by slight anodic polarization. In contrast, the Nb-reinforced alloy exhibits distinct anodic spontaneous passivation with low passive
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3.3. XPS analysis of surface film
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Potential, E / V vs Ag/AgCl
Fig. 4. Potentiodynamic polarization curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys, an industrial brass and pure Nb in 3 mass% NaCl solution at 298 K open to air.
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current density, although it suffers pitting. The improvement of corrosion resistance for Nb-reinforced alloy is attributed to the high passivating ability of the Nb alloying element (see figure inset). At the same time, the brass shows an active state during anodization along with more activity in the cathodic reaction and ennoblement in open-circuit potential compared with the Cu-based alloys. In 1 N H2SO4 + 0.01 N NaCl solution (Fig. 5), the Nb-free alloy shows the active–passive transition and the current density corresponding to its active current peak is about 1.2 A m 2. Spontaneous passivation with a significantly low current density of the order of 10 2 A m 2 takes place for Nb-reinforced BMG composite, and the alloy undergoes two-step passivation during the potential range up to 2.0 V versus Ag/AgCl. No steep increase in current density due to pitting corrosion is seen for either alloys during anodic polarization. However, the current density of the industrial brass increases rapidly by slight anodic polarization, indicating high dissolution of the brass in the solution. Furthermore, although the pure Nb metal is spontaneously passivated in a wide passive region, its passive current density is much higher than that of the BMG composite. It can be concluded that the (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite exhibits high corrosion resistance under the strong corrosion conditions, resulting from the beneficial effect of Nb addition, despite the fact that this alloy consists of micrometer-sized dendrite phase in glassy matrix.
-1.0
-0.5
0.0
0.5
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Potential, E / V vs Ag/AgCl
Fig. 5. Potentiodynamic polarization curves of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys, an industrial brass and pure Nb in 1 N H2SO4 + 0.01 N NaCl solution at 298 K open to air.
In order to clarify the origin of the high corrosion resistance of the (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy, XPS analysis was carried out for the specimens exposed to air after mechanical polishing and those immersed in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions open to air for 1 week. The XPS spectra over a wide binding energy region exhibited peaks attributed to Cu, Ti, Hf, Nb, Cl, O, and C. The C 1s peaks arose from a contaminant hydrocarbon layer covering the specimen surface. The O 1s spectrum consisted of peaks originating from oxygen in metal–O–metal bonds, metal–OH bonds and bound water. The Cl 2p peak was assigned to Cl ion in the surface film, but the concentration of Cl ion was negligibly small. The XPS peaks of Cu 2p, Hf 4f, Ti 2p, Nb 3d together with O 1s and C 1s were used for quantitative determination of compositions. For the as-polished specimens, these peaks were composed of peaks of oxidized states and metallic states; the oxidized states and metallic states are assigned to signals from the surface film and underlying substrate, respectively. However, for the other specimens, those peaks showed only oxidic peaks because of the surface films on the specimens were much thicker than the escape depths of photoelectrons. The major cations in the surface film were Cu+, Hf4+, Ti4+ and Nb5+. Fig. 6 shows the surface film compositions of the (Cu0.6Hf0.25Ti0.15)90Nb10 alloy exposed to air and
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Nb
Cationic concentration, (%)
80
Ti 60
40
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Cu 0 nominal
as-polished
HCl
NaCl
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Fig. 6. Surface film compositions for the as-cast (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy exposed to air and samples immersed in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions open to air for 1 week after mechanical polishing.
immersed in the 1 N HCl, 3 mass% NaCl and 1 N H2SO4 + 0.01 N NaCl solutions after mechanical polishing. In the surface film formed by air exposure after mechanical polishing, Hf and Ti are enriched, while Cu is deficient and Nb is slightly deficient with respect to the alloy composition. This fact indicates that preferential oxidation of Hf and Ti occurs in the air-formed film. When the specimen was immersed in the 1 N HCl solution, it is found that the Cu content in the surface film significantly decreases, and further reduces after immersion in the 3 mass% NaCl or 1 N H2SO4 + 0.01 N NaCl solution. At the same time, Ti and Hf are largely concentrated on the alloy surface immersed in the 1 N HCl solution. The content of Ti increases more after immersion in the 3 mass% NaCl or 1 N H2SO4 + 0.01 N NaCl solution, while that of Hf slightly decreases. It is observed that the surface films of the alloy immersed in the solutions are enriched in Nb with respect to that in the as-polished film and the alloy Nb content. The increase in Nb content as compared with that of the as-polished film is about 44.8%, 69.1%, and 79.8% after immersion in the HCl, NaCl, and H2SO4 + NaCl solutions, respectively. Therefore, open-circuit immersion leads to the formation of Hf-, Ti-, and Nbenriched surface films as a result of preferential dissolution of Cu and the effect of Nb. In contrast, the concentrations of Cu in the surface film after immersion in HCl and NaCl solutions for the Cu60Hf25Ti15 BMG alloy [14] were very high in comparison with those of (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite alloy. Among all the constituent elements, Cu has a low corrosion resistance which easily dissolves in acidic and chloride-ion-containing solutions, whereas Hf, Ti, and Nb exhibit excellent corrosion resistance in the solutions examined in this work. So, a stable surface film
cannot be formed for the Nb-free alloy immersed in HCl and NaCl solutions, which is consistent with the results of corrosion rates, as shown in Fig. 3. Generally, the corrosion resistance of metallic glasses is expected to be better than that of their crystalline counterparts. However, one must realize that the corrosion resistance of all materials is mainly dominated by the basic chemical composition, rather than by structural factors. According to the results of corrosion rates and polarization curves for both alloys, as displayed in Figs. 3–5, the presence of Nb in the (Cu0.6Hf0.25Ti0.15)90Nb10 alloy is beneficial for enhancing the corrosion resistance and passivating ability of the alloy, in spite of the fact that the alloy consists of micrometersized crystalline phase in glassy matrix. It can therefore be said that the protective quality of the surface films is improved by the addition of Nb. The high-quality protective film can cover the entire surface of the composite alloy, which protects the alloy from corrosion in acid and chloride-containing solutions. Accordingly, the formation of highly protective surface films enriched in Hf, Ti, and Nb on the (Cu0.6Hf0.25Ti0.15)90Nb10 composite could be responsible for the high corrosion resistance. A detailed investigation of the influence of microscale crystalline phase on corrosion resistance is underway, and will be presented elsewhere. 3.4. Mechanical properties Fig. 7 shows the true stress–strain curves of the as-cast 2 mm diameter (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys in uniaxial compression at room temperature (298 K). Young’s modulus (E), compressive true yield strength (rc, y), compressive true fracture strength (rc, f) and compressive true plastic strain (ec, p) are 124 GPa, 2024 MPa, 2088 MPa, and 1.6%, respectively, for the Nbfree alloy and 106 GPa, 2073 MPa, 2232 MPa, and
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True strain, ε (%) Fig. 7. Compressive true stress–strain curve of the as-cast (Cu0.6Hf0.25Ti0.15)100 xNbx (x = 0 and 10 at.%) alloys.
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(a)
(c)
(b)
Compression direction
(c)
500 µm 10 µm
10 µm
Fig. 8. Outer shape (a), shear bands developed on the outer surface (b) and high magnification of the dashed circle part (c) for the as-cast (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy after compression testing.
14.1%, respectively, for the Nb-reinforced alloy. Apparently, for the Nb-reinforced alloy, the combination of strength and plasticity is superior to that for the Cu60Hf25Ti15 alloy. It is believed that the embedded micrometer-sized dendrites in the Nb-reinforced alloy retard inhomogeneous shear deformation and contribute to the large plastic strain. Since the volume fraction of the dendritic phase is not large (about 8%) in the glassy matrix, the strength of the composite alloy is still dominated by the glassy matrix. As mentioned above, the glassy matrix consists of 6.9 at% Nb, and the presence of Nb in the residual glassy matrix enhances the bonding forces among Cu, Hf, Ti, and Nb elements [14], which results in higher strength. Fig. 8 shows outer shape and deformation structure on the outer surface of the as-cast 2 mm diameter (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy after compression testing. It is seen that the final fracture occurs along the maximum shear stress plane which is declined by about 42 to the direction of the applied load (Fig. 8(a)), and a large number of the shear bands distribute homogeneously throughout the surface of the specimen (Fig. 8(b)), reflecting the significantly improved plastic strain in the composite. A high-magnification image (Fig. 8(c)) shows more detailed information about the dashed circle part in Fig. 8(b). The shear bands observed in the composite sample are characterized as being straight, wavy or twisting. In general, plastic deformation of glassy single-phase alloys is highly localized into shear bands, followed by the rapid propagation of these shear bands and sudden fracture. For the Cu–Hf–Ti–Nb mixed structure composite with ductile dendrites, during the propagation of shear bands under loading, the shear bands must interact with the microscale ductile dendrites and have to deflect their propagating direction, resulting in shear band branching. At the same time, the shear bands are also blocked or branched by other intersected shear bands. Thus, the formation of a large number of the branching shear bands causes the large increase in the plasticity of the alloy.
4. Conclusions A newly designed (Cu0.6Hf0.25Ti0.15)90Nb10 BMG composite with excellent combination of high corrosion resistance and superior mechanical properties was successfully synthesized. The Nb-rich solid solution phase with the dendritic structure disperses homogeneously in the glassy matrix. The volume fraction and mean size of the dendrite bcc phase are about 8% and 5–10 lm. The (Cu0.6Hf0.25Ti0.15)90Nb10 composite alloy exhibits much higher corrosion resistance in acidic and chloride-containing solutions as compared with the Nb-free alloy. The addition of Nb is favorable for the alloys in forming Hf-, Ti-, and Nb-enriched highly protective surface films with higher chemical stability in 1 N HCl, 3 mass% NaCl, and 1 N H2SO4 + 0.01 N NaCl solutions. The composite alloy exhibits a high compressive true yield strength of 2073 MPa, true fracture strength of 2232 MPa and large true plastic strain of 14.1%. The increase in compressive strain-to-failure is due to the ductile dendrites restricting shear band propagation, promoting the generation of multiple shear bands. The new Cu-based BMG composite with good ductility and high strength together with high corrosion resistance is very promising as a new advanced engineering material. References [1] Inoue A, Zhang W, Zhang T, Kurosaka K. Acta Mater 2001;49:2645. [2] Inoue A, Zhang W, Zhang T, Kurosaka K. J Mater Res 2001;16:2836. [3] Inoue A, Zhang W, Zhang T, Kurosaka K. Mater Trans 2001;42:1805. [4] Wang D, Li Y, Sun BB, Sui ML, Lu K, Ma E. Appl Phys Lett 2004;84:4029. [5] Das J, Tang MB, Kim KB, Theissmann R, Baier F, Wang WH, Eckert J. Phys Rev Lett 2005;94:205501. [6] Fan C, Takeuchi A, Inoue A. Mater Trans JIM 1999;40:42. [7] Conner RD, Choi-Yim H, Johnson WL. J Mater Res 1999;14:3292. [8] Hays CC, Kim CP, Johnson WL. Phys Rev Lett 2000;84:2901. [9] Fan C, Ott RT, Hufnagel TC. Appl Phys Lett 2002;81:1020.
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