Microstructure and Electrochemical Behavior of Pd­Cu­Ni­PBulk Metallic Glass and Its Crystallized Alloys

F. X. Qin+, T. Wada, G. Q. Xie, S. L. Zhu and A. Inoue

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

The microstructure and electrochemical behavior of the Pd42.5Cu30Ni7.5P20 as-quenched bulk metallic glass and its crystallized alloys afterheat-treatment at 623 and 723K were investigated. The results revealed that the Pd42.5Cu30Ni7.5P20 bulk metallic glass and its crystallized alloyswere spontaneously passivated in 0.144M NaCl and Hanks’ solution with a low passive current density and a wide passive region. Thetranspassive potential of the fully crystallized alloy heat-treated at a higher temperature was lower than those of the as-quenched bulk metallicglass and partially crystallized alloy. The difference in the microstructure of the as-quenched bulk metallic glass, the partially crystallized alloyand the fully crystallized alloy was responsible for their different transpassive potentials. The wider passive region of the alloys in Hanks’solution is attributed to the presence of HCO3

¹, HPO42¹, SO4

2¹ and H2PO4¹ ions in the solution, which act as corrosive inhibitors.

[doi:10.2320/matertrans.MBW201120]

(Received December 12, 2011; Accepted February 28, 2012; Published April 18, 2012)

Keywords: microstructure, corrosion, palladium, bulk metallic glass

1. Introduction

The recent rapid developments in bulk metallic glassesreflect the high expectations for their applications asfunctional materials in many engineering fields. The highestglass-forming ability of bulk metallic glasses was obtainedin the Pd­Cu­Ni­P alloy system with a critical cooling rateof 0.067K/s for glass formation and a maximum samplediameter of 75mm, respectively.1­3) The thermal stabilityand mechanical properties4) of Pd­Cu­Ni­P bulk metallicglasses as well as their porosity counterparts5­7) have beenextensively investigated. It was found8) that the nucleationbehavior of the as-cast Pd-based bulk metallic glasseschanged from an internal nucleation mode to a surfacenucleation mode due to the optimization of the eutecticcomposition. This change is attributed to the eliminationof heterogeneity, resulting in a longer incubation time.In addition, the porous Pd-based bulk metallic glasses witha lower Young’s modulus, a lower yield strength, a muchhigher absorption energy, was also investigated, and nodifference in thermal stability was recognized between theporous alloys and the pore-free alloys. Porous bulk metallicglass rods can be prepared by water quenching the mixtureconsisting of Pd­Cu­Ni­P liquid plus NaCl solid, followedby leaching of the NaCl solid in water,6) as well as by thehigh hydrogen pressure melting-water quenching method.9)

However, research on the electrochemical behavior ofPd-based BMG is relatively rare.10) It is known that Pdalloys have been utilized as dental materials. The Pd-basedbulk metallic glasses are expected to have good corrosionresistance and biocompatibility due to their chemical andstructural homogeneity. In this research, the microstructureand electrochemical behavior of Pd42.5Cu30Ni7.5P20 bulkmetallic glass and its crystallized alloys were investigatedin NaCl and Hanks’ solutions.

2. Experimental Procedures

The mother alloy with a nominal atomic composition ofPd42.5Cu30Ni7.5P20 was prepared by arc melting a mixture ofPd­P pre-alloy, pure Pd, Cu and Ni metals in an argonatmosphere. In order to eliminate the heterogeneous nucleidue to oxide contamination, a B2O3 flux treatment wascarried out in a highly purified Ar atmosphere during alloypreparation. The Pd42.5Cu30Ni7.5P20 bulk metallic glasseswith a diameter of 6mm and a length of 50mm wereproduced by a water quenching technique in a 0.1MPa argonatmosphere. The as-quenched bulk metallic glass was heat-treated in a vacuum of about 10¹3 Pa at each temperature of623K (between Tg, glass transition temperature and Tx, onsettemperature of crystallization) and 723K (50K above thecrystallization peak temperature) for 10min. The structurewas examined by X-ray diffraction (XRD) with Cu K¡radiation. The microstructure was investigated by conven-tional transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM, JEOL2010). The samples for TEM observation were prepared bythe ion milling method. Thermal stability was characterizedby differential scanning calorimetry (DSC) under an argonatmosphere with a heating rate of 0.67K/s. The corrosionbehavior of the alloys was evaluated by electrochemicalmeasurements in a three-electrode cell using a platinumcounter electrode and a saturated calomel reference electrode(SCE). The electrolytes used were 0.144M NaCl solutionwith pH 6.0 and Hanks’ solution with pH 7.4 at roomtemperature open to air, which was prepared from reagentgrade chemicals and deionized water. The compositionof Hanks’ solutions (g/L) is 8.00 NaCl, 0.40 KCl, 0.35NaHCO3, 0.19 CaCl2·2H2O, 0.09 Na2HPO4·7H2O, 0.2MgSO4·7H2O, 0.06 KH2PO4 and 1.00 Glucose.11)

3. Results and Discussion

Figure 1 shows the XRD patterns of the Pd42.5Cu30Ni7.5P20as-quenched bulk metallic glass and its crystallized alloys+Corresponding author, E-mail: fxqin@imr.tohoku.ac.jp

Materials Transactions, Vol. 53, No. 5 (2012) pp. 936 to 939©2012 The Japan Institute of Metals

after heat-treatment at 623 and 723K. It is clear that only ahalo peak appears in the XRD pattern of the Pd42.5Cu30-Ni7.5P20 as-quenched bulk metallic glass, indicating that theas-quenched alloy is in a glassy state. Although some smallpeaks appear in the XRD pattern after heat-treatment at623K, the peaks do not indicate which phase they are from.However, some obvious diffraction peaks appearing in theXRD pattern of the alloy heat-treated at 723K were identifiedas Pd6P, Pd7P3 and Ni2Pd2P phases. Figure 2 shows the DSCcurves of the Pd42.5Cu30Ni7.5P20 as-quenched bulk metallicglass and its crystallized alloys after heat-treatment at 623and 723K. The as-quenched bulk metallic glass shows anobvious glass transition temperature (Tg = 577K) and anonset temperature of crystallization (Tx = 671K), followedby one crystallization peak.5) When the as-quenched bulkmetallic glass is heat-treated at 623K for 10min, thecrystallization peak becomes weaker, indicating the occur-rence of partial crystallization. The volume fraction ofcrystallization can be evaluated by comparing the crystal-lization enthalpy of the exothermic peak of the as-quenchedalloy with that of the heat-treated alloys, with the formula asVcrst ¼ ð�Has-quenched ��HcrystallizedÞ=�Has-quenched.12) Aftercalculation, the crystallization volume fraction is about 7%for the alloy after heat-treatment at 623K. When the heat-

treatment temperature is increased to 723K, the crystalliza-tion peak completely disappears, and the crystallizationvolume fraction is 100%.

The potentiodynamic polarization curves of the Pd42.5Cu30-Ni7.5P20 as-quenched bulk metallic glass and its crystallizedcounterparts are measured in 0.144M NaCl and Hanks’solutions at room temperature, as shown in Fig. 3. In 0.144MNaCl solution (Fig. 3(a)), all alloys exhibit similar corrosionbehavior. They are spontaneously passivated with a passivecurrent density between 10¹2 and 10¹1 A/m2 in anodicpolarization curves. As the anodic potential increases,passivity breakdown occurs. At the same time, the trans-passive potential of the fully crystallized alloy is about100mV lower than that of the as-quenched and the partiallycrystallized alloy in 0.144M NaCl solution. The results inHanks’ solution (Fig. 4(b)) are similar to that in 0.144MNaCl solution. All the samples are passivated in the anodicpolarization in Hanks’ solution, while the passive regions ofthe Pd42.5Cu30Ni7.5P20 as-quenched bulk metallic glass andits crystallized alloys in Hanks’ solution are about 150mVwider than that in 0.144M NaCl solution.

The TEM and HRTEM images and SAD patterns (Fig. 4)of the Pd42.5Cu30Ni7.5P20 as-quenched bulk metallic glass andits crystallized alloys reveal that the as-quenched alloy has aglassy structure, with only halo rings in the correspondingSAD pattern. After heat-treatment at 623K, several ringpatterns are superimposed on the diffuse halo rings in the

Fig. 1 XRD patterns of the as-quenched Pd42.5Cu30Ni7.5P20 bulk metallicglass and its crystallized alloys heat-treated at 623 and 723K for 10min.

Fig. 2 DSC curves of the as-quenched Pd42.5Cu30Ni7.5P20 bulk metallicglass and its crystallized alloys heat-treated at 623 and 723K for 10min.

(a)

(b)

Fig. 3 The potentiodynamic polarization curves of the Pd42.5Cu30Ni7.5P20as-quenched bulk metallic glass and its crystallized alloys heat-treated at623 and 723K in 0.144M NaCl (a) and Hanks’ solutions (b).

Microstructure and Electrochemical Behavior of Pd­Cu­Ni­P Bulk Metallic Glass and Its Crystallized Alloys 937

SAD pattern, indicating a mixture state of glassy matrix andnano crystallized particles. The nanoparticles are identified asa tetragonal Ni2Pd2P phase. By heat-treatment at 723K, it isseen that the residual glassy matrix is completely crystallized,as presented in Fig. 4(c). The insert diffraction pattern areidentified as a Pd6P phase with crystal planes of (2�30), (310)and (140).

As mentioned above, a similar corrosion behavior wasobserved for the Pd42.5Cu30Ni7.5P20 as-quenched bulk me-tallic glass and its crystallized alloys both in 0.144M NaCland Hanks’ solutions. The as-quenched bulk metallic glassand the partially crystallized alloy have a wider passiveregion than the fully crystallized alloy, i.e., the fullycrystallized alloy shows a lower transpassive potential in

both solutions. This means that the partially crystallized alloyand the as-quenched bulk metallic glass have a higherchemical stability. In the HRTEM images shown in Figs. 4(a)and 4(b), the morphology can be seen more like atomicclusters in a very fine distribution in a glassy matrix thannanoparticles. The as-quenched bulk metallic glass shows avery similar atomic cluster slightly smaller in size than that ofthe alloy heat-treated at 623K. It is not clear why the Pd-based as-quenched bulk metallic glass exhibits this type ofmorphology. Further investigation is necessary. It is believedthat the volume fraction of the grain boundary is as much as50% of the total crystal volume when the grain size is lessthan 100 nm.13) The existence of a large amount of grainboundary enhances the diffusion ability of the atoms, whichis beneficial for the formation of a protective oxide film.14,15)

In the present research, the interface between the atomicclusters and the glassy matrix exists with a large volumefraction in the as-quenched and the partially crystallizedsamples. Therefore, a low passive current density and a widepassive region are obtained for the two alloys in both0.144M NaCl and Hanks’ solution. In Fig. 1 and Fig. 4(c),the random distribution of several kinds of crystal particles(Pd6P, Pd7P3 and Ni2Pd2P) with sizes 20­200 nm in the fullycrystallized alloy results in the heterogeneity of the matrixand an inhomogeneous passive film. As such, the fullycrystallized alloy is susceptible to the formation of a micro-coupling cell which could trigger corrosion, and indicatedby the preferential adsorption of Cl¹ at some weak sites.Consequently, the transpassive potential of the fully crystal-lized alloys is about 100mV lower than that of two otheralloys in both NaCl and Hanks’ solutions. Furthermore, theconcentration 0.144M of the NaCl solution used in thisresearch is the same as the concentration of Cl¹ in Hanks’solution. Therefore, the wider passive region of the alloys inHanks’ solution is attributed to the presence of HCO3

¹,HPO4

2¹, SO42¹ and H2PO4

¹ ions in the solution, which act ascorrosive inhibitors.

4. Conclusions

The microstructure and corrosion behavior of thePd42.5Cu30Ni7.5P20 as-quenched bulk metallic glass and itscrystallized alloys were investigated. All the alloys show asimilar corrosion behavior both in the single Cl¹ containingsolution and the more complex Hanks’ solution. The maindifference is that the fully crystallized alloy exhibits atranspassive potential about 100mV lower than those of theas-quenched and partially crystallized alloys. The resultsindicated that the difference in the transpassive potentialamong the as-quenched bulk metallic glass, the partiallycrystallized alloy, and the fully crystallized alloy originatesfrom the differences in their microstructure. In addition, thepresence of HCO3

¹, HPO42¹, SO4

2¹ and H2PO4¹ ions in

Hanks’ solution can act as corrosive inhibitors which deferpitting corrosion.

Acknowledgments

This work is financially supported by Young Scientists (B)No. 22760530 from the Japan Society for the Promotion of

(a)

(b)

(c)

Fig. 4 TEM and HRTEM images as well as the corresponding SADpatterns of the as-quenched Pd42.5Cu30Ni7.5P20 bulk metallic glass (a) andits crystallized alloys heat-treated at 623 (b) and 723K (c).

F. X. Qin, T. Wada, G. Q. Xie, S. L. Zhu and A. Inoue938

Science (JSPS) and the Advanced Materials Developmentand Integration of Novel Structured Metallic and InorganicMaterials from the Ministry of Education, Culture, Sports,Science and Technology, Japan.

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