Synthesis, properties and XPS analysis of Cu-, Ni-, Ti-based metallic glasses

Transcription

1 Celebrating the th Anniversray 44 Vol. No.4 July 4 Synthesis, properties and XPS analysis of Cu, Ni, Tibased metallic glasses *Qin Chunling, Wang Lijuan, Wang Zhifeng, Ding Jian, Zhao Weimin, and Akihisa Inoue. School of Materials Science and Engineering, Hebei University of Technology, Tianjin, China;. School of Materials Science and Engineering, Tianjin University, Tianjin 7, China Abstract: Metallic glasses (MG) represent an interesting group of materials as they possess outstanding physical, chemical and mechanical properties compared to their crystalline counterparts. This paper reviews the synergistic influence of Ni and Nb elements on thermal stability of supercooled liquid and corrosion resistance of ascast CuZr(Hf)TiNiNb bulk metallic glasses (BMGs). Additionally, insitu second phase reinforced Cubased BMG composites with high corrosion resistance and excellent mechanical properties are investigated. On the other hand, this paper reports the development of ultrahigh corrosion resistant Nibased metallic glasses at high temperatures for their potential applications. Corrosion resistance and XPS analysis of the Nifree Tibased BMG are also introduced. Key words: metallic glasses; supercooled liquid region; corrosion resistance; mechanical properties; XPS analysis CLC numbers: TG46 Document code: A Article ID: 6764(4)447 A glass is a disordered (or amorphous) solid which does not possess the long range periodicity as present in a typical crystal. The best known natural glass is obsidian, formed by solidification after volcanic activity []. The production of a metallic glass by rapid cooling of the molten Au 7 Si (compositions given in at.% throughout this paper) eutectic alloy in 96 is generally considered to be the beginning of the era of metallic glasses []. Since then, a number of metallic glasses have been developed in Ni, Fe, Pd, Al, Zr, Cu and Tibased alloy systems [, 6]. More importantly, metallic glasses exhibit unique properties that are superior to the conventional crystalline metals and alloys, such as high strength, high elastic strain limit, high hardness, good soft magnetic properties, and good corrosion resistance [, 6]. Additionally, the metallic glasses with different compositions can be fabricated in the form of powders, thin ribbons, fine wires, plates and bulk form with large dimensional size, for advanced structural and functional materials. Figure shows regular, wide ribbon samples and bulk glassy alloys prepared by melt spinning, arc melting and copper mold casting. The purpose of this paper is to present an overview of the Cu, Ni and Tibased metallic glasses with useful properties. This paper includes three sections. The first section outlines the glass forming ability (GFA), mechanical properties and corrosion behavior of the ascast CuZr(Hf)Tibased bulk metallic glasses (BMGs) and their composites. Insitu Ta/ Nbreinforced CuZr(Hf)Ti BMG composites with increased plasticity and enhanced corrosion resistance are also reviewed. The second section reviews the development of ultrahigh corrosion resistant Nibased metallic glasses at high temperature. The third section introduces the Nifree Tibased BMG with good corrosion resistance and its XPS analysis. * Qin Chunling Born in 966. She received her Ph.D degree in Materials Science and Engineering from Tohoku University in. From Nov. to Mar. 4 and May 4 to Sep. 7, she worked as a postdoctoral researcher at Institute for Materials Research (IMR) of Tohoku University and Japan Science and Technology Agency (JST), respectively. During Nov. 7 to Sep., she was employed as an assistant professor at World Premier International (WPI) Research Center, Advanced Institute for Materials Research (AIMR) of Tohoku University, Japan. She is currently a professor at Hebei University of Technology. Her research interests include metallic glasses and nanoporous metals that exhibit unique useful physical, chemical and mechanical properties. Her academic research has led to the publication of more than papers. In, she was recognized by the " Talents Project" of Hebei Province. chunlingqin@6.com Received: 46 Accepted: 46 47

2 Report Special Vol. No.4 July 4 Celebrating the th Anniversray 44 (c) cm Fig. : Ribbon samples prepared by melt spinning, bulk glassy alloys prepared by arc melting and copper mold casting, and wide ribbon samples fabricated by melt spinning (c) Ascast Cubased BMGs/ composites with good properties. Preparation and corrosion behavior of CuZr(Hf)TiNiNb bulk glassy alloys Cubased BMGs are particularly interesting and show commercial potential as structural materials in some fields due to the combination of high strength of exceeding GPa, high glass forming ability (GFA) and good wear resistance. This includes the discoveries of unusual glassforming ability (GFA) and high strength in ternary CuZr(Hf)Ti(Al) [79] and quaternary CuZr(Ti, Al)Ag [,] alloys. These bulk metallic glasses (BMGs) as an engineering material are now attracting attention for potential applications as advanced engineering materials in many areas such as surgical instruments and bipolar plates in fuel cells. However, the Cubased BMGs exhibit low corrosion resistance in corrosive environments, especially, when the solutions contain chlorideions []. In order to develop BMGs with a larger supercooled liquid region and good viscous flow workability as well as higher corrosion resistance, BMGs with critical diameters of.4. mm were synthesized in the (Cu.6Zr.Ti.)xyNiyNbx (x = 6at.% and y = 7at.%) and (Cu.6Hf.Ti.)xyNiyNbx (x = 6at.% and y = at.%) alloy systems by the copper mold casting method [67]. Figure shows DSC curves of the CuZr(Hf)TiNiNb glassy rods with a diameter of. mm [6,7], where Tg and Tx correspond to glass transition temperature and onset temperature of crystallization, respectively. The addition of Ni element causes an extension of a supercooled liquid region (ΔTx=TxTg) from 4 K for Cu6ZrTi to 6 K for (Cu.6Zr.Ti.)9Ni and from 6 K for Cu6HfTi to 7 K for (Cu.6Hf.Ti.)9Ni, accompanying the change in the crystallization mode from two stages to a single stage. It is concluded that the addition of at.% Ni is effective for the increase in thermal stability of the supercooled liquid before crystallization. The large ΔTx values of exceeding 4 K and the high reduced glass transition temperature (Tg/Tl, where Tl is the liquidus temperature examined by differential thermal analysis) values of above.6 have enabled the formation of bulk glassy alloys with a critical diameter for glass formation (dc) in the range from. mm to 4. mm in the CuZr(Hf)TiNiNb alloy systems over a wide composition range by copper mold casting. The thermal properties Tg, Tx, ΔTx and critical diameter dc for glass formation are summarized in Table. (Cu.6Zr.Ti.)89NiNb6 (Cu.6Zr.Ti.)9NiNb4 Exothermic (a.u.) Exothermic (a.u.) (Cu.6Zr.Ti.)89NiNb6 (Cu.6Zr.Ti.)9NiNb (Cu.6Zr.Ti.)9Ni7 (Cu.6Zr.Ti.)9Ni Tg 6 (Cu.6Zr.Ti.)9NiNb (Cu.6Zr.Ti.)9Ni Cu6HfTi Cu6ZrTi (Cu.6Zr.Ti.)9NiNb4 Tg Tx Temperature (K) 6 Tx Temperature (K) Fig. : DSC curves of ascast (Cu.6Zr.Ti.)xyNiyNbx and (Cu.6Hf.Ti.)xyNiyNbx glassy rods with a diameter of. mm at.67 K s 48

3 Celebrating the th Anniversray 44 Vol. No.4 July 4 Table : Thermal properties and critical diameters (d c ) of glassy alloys Alloy T g (K) T x (K) ΔT x (K) T l (K) T g /T l d c (mm) Cu 6 Zr Ti (Cu.6 Zr. Ti. ) 9 Ni (Cu.6 Zr. Ti. ) 9 Ni (Cu.6 Zr. Ti. ) 9 Ni Nb (Cu.6 Zr. Ti. ) 9 Ni Nb (Cu.6 Zr. Ti. ) 89 Ni Nb Cu 6 Hf Ti (Cu.6 Hf. Ti. ) 9 Ni (Cu.6 Hf. Ti. ) 9 Ni Nb (Cu.6 Hf. Ti. ) 9 Ni Nb (Cu.6 Hf. Ti. ) 89 Ni Nb Figure shows anodic polarization curves for the ascast. mm (Cu.6 Zr. Ti. ) xy Ni y Nb x and (Cu.6 Hf. Ti. ) xy Ni y Nb x glassy alloys in mass% NaCl solution at 98 K [6,7]. It is clearly seen that the alloys with and without Ni exhibit different polarization behaviors. The alloys without Ni suffer general corrosion. However, the alloys containing Ni are spontaneously passivated with low passive current density, although they suffer pitting corrosion by anodic polarization. Moreover, their pitting corrosion potentials are nobler with an increase in Ni and Nb contents. In particular, the simultaneous addition of Ni and Nb to the alloys is beneficial for enhancing the pitting corrosion potential against localized corrosion in chlorideionscontaining solutions. Accordingly, the coexistence of Ni and Nb elements leads to the reduction of pitting susceptibility and the improvement of pitting corrosion resistance of the CuZr(Hf)TiNiNb alloys. It is believed that (Cu.6 Hf. Ti. ) 89 Ni Nb 6 alloy exhibits the highest pitting corrosion potential in comparison with the other Cubased BMGs, such as CuZr(Hf)Ti(Mo, Nb, Ta) and CuZrTiNi Nb alloys [4,6,7]. XPS measurements revealed that the highly protective Zr, Ti and Nbenriched surface film is formed by the rapid initial preferential dissolution of Cu and Ni, which is able to separate the bulk of the alloy from the corrosive solutions. x=, y= x=, y= x=, y= 7 x=, y= x=, y= x=, y= x= 4, y= x= 6, y= x=, y= x=, y= x= 4, y= x=, y= x= 6, y= x=, y= x= 4, y= Fig. : Anodic polarization curves of ascast (Cu.6 Zr. Ti. ) xy Ni y Nb x and (Cu.6 Hf. Ti. ) xy Ni y Nb x glassy rods in mass% NaCl solutions at 98 K. Cubased BMG composites with high corrosion resistance and excellent mechanical properties Due to their high strength in excess of MPa and high glass forming ability, Cubased BMGs are considered to have many potential applications as advanced engineering materials, such as surgical instruments and bipolar plates in fuel cells [7]. Unfortunately, like conventional amorphous alloys, Cubased BMGs fail due to the formation of highly localized shear band, which leads to catastrophic failure without much macroscopic plasticity. This inhomogeneous deformation behavior has seriously limited the application of BMGs as an engineering material so far. To solve this problem, there have been various attempts to produce reinforced BMGs [8], by partial crystallization, particle reinforcements or in situ formation of ductile bodycentered cubic (bcc)phase precipitates. As a result, a new class of materials, i.e. BMG composites, combining ceramiclike strength with metallike ductility has 49

4 Report Special Vol. No.4 July 4 Celebrating the been produced. Considering industrial and environmental applications, the (Cu.6Hf.Ti.) 94Ta 6 [] and (Cu.6Hf.Ti.) 9Nb BMG composite [4] with excellent combination of high corrosion resistance and superior mechanical properties was successfully synthesized. The XRD pattern [Fig. 4] of the Ta/Nbcontaining alloy exhibited a broad halo peak superimposed with the sharp diffraction peaks, indicating that Ta/Nbrich solid solution Intensity (a.u.) precipitated in the glassy matrix. Figure 4 and (c) show the microstructure of the ascast Φ mm (Cu.6Hf.Ti.)94Ta6 and (Cu.6Hf.Ti.)9Nb alloys. The Ta/Nbrich 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 % and µm, respectively, for the (Cu.6Hf.Ti.)94Ta6 alloy, and about 8% and µm, respectively, for (Cu.6Hf.Ti.)9Nb alloys. (Cu.6Hf.Ti.)94Ta6 Ta(bcc) Nb(bcc) (Cu.6Hf.Ti.)94Ta6 th Anniversray 44 (Cu.6Hf.Ti.)9Nb (c) (Cu.6Hf.Ti.)9Nb Cu6HfTi 4 6 θ(degrees) Cu Ka 7 8 μm μm Fig. 4: XRD patterns of ascast mm diameter CuHfTi(Ta/Nb) glassy alloys and SEM images of central region in transverse cross section of the ascast (Cu.6Hf.Ti.)94Ta6 and (Cu.6Hf.Ti.)9Nb (c) alloy rods Figure shows compressive stressstrain curves of the ascast Φ mm (Cu.6Hf.Ti.)xTax (x = and 6at.%) alloys and (Cu.6Hf.Ti.)9Nb in a uniaxial compression at room temperature (98 K) []. Young s modulus (E), compressive yield strength (σc,y), compressive fracture strength (σc,f) and compressive plastic strain (εc,p) are 4 GPa, MPa, 6 MPa and.6%, respectively, for the Tafree alloy [6], 4 GPa, MPa, MPa and 4%, respectively, for the Tareinforced alloy [] and 6 GPa, 48 MPa, 6 MPa and.%, respectively, for Nbreinforced alloy [,4]. Apparently, for the Ta and Nbreinforced alloys, the combination of strength and plasticity is superior to that for the Cu6HfTi glassy single phase alloy. Figure shows the outer shape and deformation structure 4 4 on the outer surface of the ascast Φ mm (Cu.6Hf.Ti.)94Ta6 composite alloy after compression test. It is seen that the final fracture occurs along the maximum shear stress plane which is declined by about 4 to the direction of applied load (as inset in Fig. b), and a large number of the shear bands distribute homogeneously throughout the surface of the specimen, reflecting the significantly improved plastic strain in the composite. 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 CuHfTi(Ta/Nb) mixed structure composite with ductile dendrites, during the propagation of shear bands under loading, μm 4 ε 4 μm Fig. : Compressive stressstrain curves of ascast mm diameter CuHfTi(Ta/Nb) glassy alloys, and inserted outer shape and high magnification of shear bands throughout the outer surface for ascast (Cu.6Hf.Ti.)94Ta6 composite alloy after compression testing

5 Celebrating the th Anniversray 44 Vol. No.4 July 4 the shear bands must interact with the microscale ductile dendrites and deflect their propagating direction, and result in the shear band branching. At the same time, the shear bands are also blocked or branched by other intersected shear bands. Thus, the increase in compressive straintofailure is due to the ductile dendrites restricting shear band propagation, promoting the generation of multiple shear bands. The average corrosion rates of the ascast (Cu.6 Hf. Ti. ) x Ta x (x = 6 at.%) and (Cu.6 Hf. Ti. ) 9 Nb alloys immersed in N HCl, mass% NaCl and N H SO 4 +. N NaCl solutions, respectively, at 98 K open to air for one week are shown in Fig. 6 []. The corrosion rates of an industrial brass (6wt.% Cu + 4wt.% Zn) in those solutions are also displayed for comparison. The corrosion rate less than mm y is out of detectable limits for the present measurements, and hence is taken as zero in the figure. It can be clearly seen that the addition of Ta or Nb to the CuHf Ti alloy results in a significant decrease in the corrosion rates in all the solutions examined, despite the fact that the BMG composites consist of micrometersized dendrite phase in glassy matrix. Moreover, the corrosion resistance of the Ta/ Nbcontaining Cubased BMG composites is better than that of industrial brass. Therefore, the Cubased BMG composite with its good ductility and high strength together with high corrosion resistance will be highly promising as new advanced engineering materials. Corrosion rate (mm year ) Cu6HfTi ( Cu. Hf Ti ) Nb ( Cu6. Hf.Ti.) 94Ta6 Industrial brass NHCI mass% NaCI NHSO+ 4. N NaCI Fig. 6: Average corrosion rates of ascast CuHfTi(Ta/Nb) alloys and an industrial brass in N HCl, mass% NaCl, and N H SO 4 +. N NaCl solutions at 98 K open to air Development of ultrahigh corrosion resistant Nibased glassy alloys at high temperature The NiNbbased bulk glassy alloys with high strength of over MPa and large supercooled liquid region ΔT x and excellent corrosion resistance in aggressive acids have been developed in [78]. In a supercooled liquid region, they can be fabricated into intricate shapes with a high level of dimensional accuracy. Hence, the high corrosion resistance together with easy forming ability in the viscous state lead to outstanding application potentials in place of traditional crystalline metals in some fields.. High corrosion resistant Nibased glassy alloys as bipolar plate materials in fuel cell environments High fabrication cost of conventional graphite bipolar plates of fuel cells is pushing the automobile industry to seek for alternatives. For this purpose, Ni 6 Nb x Ta x Ti Zr (x =,, and at.%) [9] and Ni 6 Nb x Ta x Ti (x =,, and at.%) glassy alloys with high thermal stability and high corrosion resistance were developed successfully using the melt spinning technique. The DSC curves of the meltspun Ni 6 Nb x Ta x Ti Zr and Ni 6 Nb x Ta x Ti alloys are shown in Fig. 7. In this figure, T g and T x correspond to glass transition temperature and onset temperature of crystallization, respectively. All the Nibased glassy alloys show a distinct glass transition, followed by a large supercooled liquid region prior to crystallization. The glass transition temperature (T g ) and crystallization temperature (T x ) increase with an increase in the Ta content of the alloys. At the same time, the supercooled liquid region (ΔT x = T x T g ) increases with an increase in tantalum content. Thus, the substitution of tantalum for niobium causes an extension of ΔT x from K for Ni 6 Nb Ti Zr to 67 K for Ni 6 Ta Ti Zr and from 47 K for Ni 6 Nb Ti to 8 K for Ni 6 Nb Ti Ta, respectively. It can be concluded that the addition of tantalum to NiNbTiZr and NiNbTi alloys is effective for increasing the thermal stability of the supercooled liquid phase. A larger supercooled liquid region and a higher stability of the supercooled liquid phase are useful for the superplastic processing of these alloys in the supercooled liquid state to produce, for example, bipolar plates for fuel cells. The fuel cell environment can be simulated by an H SO 4 solution with ph of and containing a small amount of chloride or fluoride of the order of ppm at a high temperature of K. All the Nibased glassy alloys are spontaneously passivated in wide passive regions and at low passive current densities in H SO 4 solution (ph = ) at K. Increasing Ta content of the alloys results in a decrease in the passive current density and thus an improvement in corrosion resistance. An example of the polarization curves of the Ni 6 Nb x Ta x Ti Zr alloys in H SO 4 solution (ph = ) at K is shown in Fig. 8 [9]. In addition, the NiNbTaTi(Zr) alloys maintain their high corrosion resistance even after the addition of ppm NaCl in the solution [9]. No pitting corrosion due to the existence of chloride ions is observed during the anodic polarization up to.4 V vs. Ag/AgCl. On the contrary, the polarization curve of SUS6L stainless steel undergoes an activepassive transition with a narrow passive region until the transpassive dissolution of chromium occurs. Therefore, NiNbTaTi(Zr) glassy alloys with nobler opencircuit

6 Celebrating the Vol. No.4 July 4 th Anniversray 44 Exothermic ( a. u.) x= x= x= x= Exothermic ( a. u.) x= x= x= x= T g T x T g T x Temperature ( K) Temperature ( K) Fig. 7: DSC curves of meltspun Ni 6 Nb x Ta x Ti Zr and Ni 6 Nb x Ta x Ti alloys at.67 K s SUS6L x= x= x= x= x= x= x= x= SUS6L Fig. 8: Anodic polarization curves of Ni 6 Nb x Ta x Ti Zr alloys and SUS6L stainless steel in ph H SO 4 and ph H SO 4 + ppm NaF solutions at K open to air potentials and a wider passive region exhibit much better corrosion resistance than SUS6L stainless steel in H SO 4 (ph = ) and H SO 4 + ppm NaCl (ph = ) solutions at K. On the other hand, in the H SO 4 + ppm NaF solution (ph = ) at K, the alloys without Ta and with at.% Ta suffer general corrosion, while the passive current densities decrease by about two orders of magnitude with the addition of at.%at.% Ta to the Nibased glassy alloys [Fig. 8]. This indicates that the effect of the addition of tantalum (Ta) is more pronounced for improving the corrosion resistance in fluoride containing solutions than the other two types of solutions. Table summarizes the average passive current densities of the Ni 6 Nb x Ta x Ti Zr and Ni 6 Nb x Ta x Ti glassy alloys obtained from their anodic polarization curves in ph H SO 4, ph H SO 4 + ppm NaCl and ph H SO 4 + ppm NaF solutions at K. The changes in the corrosion behavior with tantalum content and solutions examined are clearly visible in this table.. High corrosion resistant Nibased glassy alloys in boiling nitric acid solutions for nuclear fuel reprocessing applications Reprocessing of spent nuclear fuel involves use of nitric acid of high concentrations and high temperatures. The boiling nitric acid also contains small amounts of other ions such as Cr 6+, Ag +, Ce 4+, Cr O 7, etc. Therefore, the materials for fabrication of reprocessing plant equipment should possess excellent corrosion resistance. Conventional materials such as AISI 4L stainless steel and even nitric acid grade stainless steel Uranus 6 do not have enough corrosion resistance []. This work presented high corrosion resistant Nibased glassy alloys in severe environments, i.e. boiling 6 N (or 9 N) HNO solutions with and without g L Cr 6+, for nuclear fuel reprocessing applications. Table lists the average corrosion rates of the glassy alloys immersed in boiling 9 N HNO and boiling (9 N HNO

7 Celebrating the th Anniversray 44 Vol. No.4 July 4 Table : Average passive current densities of Nibased glassy alloys determined from their anodic polarization curves in different solutions at K Alloys Average passive current densities (A m ) ph H SO 4 ph H SO 4 + ppm NaCl ph H SO 4 + ppm NaF Ni 6 Nb Ti Zr * Ni 6 Nb Ta Ti Zr.6.49.* Ni 6 Nb Ta Ti Zr..7. Ni 6 Ta Ti Zr.7..7 Ni 6 Nb Ti.8.4.8* Ni 6 Nb Ta Ti...46* Ni 6 Nb Ta Ti Ni 6 Ta Ti..6.4 *Current density Table : Average corrosion rates (mm y ) of glassy alloys in boiling 9 N HNO and boiling (9 N HNO + g L Cr 6+ ) solutions at 97 K open to air Samples Boiling Boiling 9 N HNO (9 N HNO + g L Cr 6+ ) Ni 6 Cr P 6 B Ni 6 Cr P 6 B 4 Ta..4 Ni 6 Nb Zr..7 Ni 7 Nb 9 Zr 9 Ta.. Ni 6 Nb Zr Ti Ta..4 Fe 4 Cr 6 Mo 6 C B Fe 4 Cr Mo 6 C B Ta g L Cr 6+ ) solutions, open to air for week []. The Fe based glassy alloys dissolved rapidly within h in boiling 9 N HNO solution, while the Febased alloys dissolved completely after immersion in boiling 9 N HNO + g L Cr 6+ solution for 47 h. On the other hand, the Nibased glassy alloys exhibit much better corrosion resistance as compared to the Febased glassy alloys. For the NiCrmetalloid and Nivalve metal alloy systems, it is found that the addition of at.% Ta is effective in improving corrosion resistance in such severe environments. In addition, Nivalve metal alloys containing Ta element show the best corrosion resistance. Then, we focused on the Ni 7 Nb 9 Zr 9 Ta and Ni 6 Nb Zr Ti Ta glassy alloys to further investigate their corrosion resistance and surface characteristics in boiling 6 N HNO solutions. Figure 9 shows the corrosion rates of the Ni 7 Nb 9 Zr 9 Ta and Ni 6 Nb Zr Ti Ta glassy alloys immersed in boiling 6 N HNO and boiling (6 N HNO + g L Cr 6+ ) solutions, open to air for weeks []. For comparison, the stainless steel NAR Nb, which is known for high corrosion resistance in nitric acid, is also examined in the same solutions. The temperature in both solutions is as high as about 8 K. After immersion in boiling 6 N HNO solution for weeks, both of the Nibased glassy alloys maintained their previous metallic luster, whereas some cracks resulting from grain boundary attack were observed in the corroded surface for NARNb stainless steel. The Corrosion rate (mm y ) Ni Nb Zr Ti Ta Ni7Nb9Zr9Ta NARNb Boiling 6NHNO Boiling( 6NHNO +g L Cr ) Fig. 9: Average corrosion rates of Nibased glassy alloys and NARNb stainless steel in boiling 6 N HNO and boiling (6 N HNO + g L Cr 6+ ) solutions at 8 K open to air corrosion rates of the Ni 7 Nb 9 Zr 9 Ta and Ni 6 Nb Zr Ti Ta alloys are about.48 and.4 mm y, respectively, which are about one order of magnitude lower than that of NARNb stainless steel,. mm y. In boiling Cr 6+ containing solution, the corrosion rate for the Ni 7 Nb 9 Zr 9 Ta alloy is. mm y and decreases to about. mm y for the Ni 6 Nb Zr Ti Ta alloy. On the other hand, NARNb stainless steel dissolves completely within 44 h, which corresponds to a very high corrosion rate of.74 mm y. These facts clearly indicate the superiority of the present Nibased glassy alloys. Therefore, it can be concluded that Nibased glassy alloys have much higher corrosion resistance than NARNb stainless steel in nitric acid of high concentration at high temperature. In particular, the Ni 6 Nb Zr Ti Ta glassy alloy with easy formability in the supercooled liquid region [9] and superior corrosion resistance shows great potential as a new engineering material for nuclear fuel reprocessing applications. It is important to clarify the surface compositions together with the chemical states for elucidation of the origin of the high corrosion resistance for these Nibased glassy alloys. XPS analysis was applied to the Ni 6 Nb Zr Ti Ta specimens aspolished mechanically in cyclohexane or immersed in

8 Celebrating the Vol. No.4 July 4 th Anniversray 44 boiling 6 N HNO and boiling (6 N HNO + g L Cr 6+ ) solutions for weeks. The XPS spectra of the specimens over a wide binding energy region exhibited peaks of Ni p, Nb d, Zr d, Ti p, Ta 4f, Cr p, O s, C s etc []. Figure shows cationic concentration (%) of surface films formed on the Ni 6 Nb Zr Ti Ta alloy aspolished and immersed in boiling 6 N HNO and boiling (6 N HNO + g L Cr 6+ ) solutions for weeks []. The surface film on the specimen aspolished was formed by air exposure after mechanical polishing. The Ti and Zr cations are enriched in the surface film, whereas Ni cations are largely deficient. On the other hand, the concentrations of Nb and Ta cations slightly change with respect to the alloy composition. This fact reveals that the preferential oxidation of Ti and Zr takes place during air exposure whereas Ni is scarcely oxidized. The major cation in the aspolished surface film is Ti 4+ and followed by Zr 4+, Nb +, Nb +, Ta + and Ni +. When the specimen was immersed in boiling 6 N HNO solution, it is interesting to find that the cations of the passive surface are composed exclusively of Nb + and Ta +. This result revealed that the airformed film was not stable in boiling 6 N HNO and converted to the stable passive film that consisted only of Nb and Ta passivating elements, as a consequence of preferential dissolution of Ni, Zr and Ti elements during the immersion. After immersion in Cr 6+ containing solution, we also found a certain amounts of Cr + ions in the surface which are from the solution, in addition to Nb and Ta cations. Consequently, the origin of high corrosion resistance for the present alloy in such severe environments is explained by formation of the highly protective surface film composed exclusively of Nb +, Ta + or Cr +, which is able to separate the alloy from the corrosive solutions. Cationic concentration (%) Ta TI Zr Nb TI4+ NI Zr 4+ Nominal Ta + Nb + Nb + NI + Ta + Cr + Ta + Nb + Nb + Aspolished NHO NHO Cr + 6+ Fig. : Cationic concentration (%) of surface film formed on Ni 6 Nb Zr Ti Ta alloy exposed to air and samples immersed in boiling 6 N HNO and boiling (6 N HNO + g L Cr 6+ ) solutions open to air for weeks after mechanical polishing Electrochemical properties and XPS analysis of a Nifree Tibased BMG Among BMG systems developed so far, Tibased BMGs are particularly interesting and show commercial potential as structural and functional materials in some fields due to the combination of superior strength, low Young s modulus, and excellent corrosion resistance as well as relatively low cost of fabrication. During 998 to 4, several Tibased BMGs have been developed, including TiNiCuSn [] and TiZr CuNi(Sn, Zr, Be) BMGs systems [,4], etc. However, all the alloys contained Ni because only the addition of the Ni helped to improve their glassforming ability. It has been noticed that pure Ni is a toxic element and causes Nihypersensitivity. So, it was a big challenge to develop a Nifree Tibased BMG for biomedical applications. In 7, success was achieved in the development of a new Nifree Tibased BMG, i.e. Ti 4 Zr Pd Cu Sn 4 []. The Ti 4 Zr Pd Cu Sn 4 BMG possessed high strength of 97 MPa and low Young s modulus of 9 GPa, together with good corrosion resistance in simulated body fluids [4]. In addition, the Ti 4 Zr Pd Cu Sn 4 BMG shows promise in applications in the industrial field as well as in biomaterials. For application in industrial use, the alloy must have a good corrosion resistance and electrochemical stability in industrial environments where the conditions are more severe in comparison to biomaterial environments. So, this work aimed to investigate the corrosion behavior of the alloy in various corrosive solutions in order to obtain basic knowledge for industrial application. Xray photoelectron spectroscopy is used to clarify the origin of high corrosion resistance of the alloy.. Corrosion rates in various solutions The average corrosion rates of the ascast. mm diameter Ti 4 Zr Pd Cu Sn 4 BMG immersed in different solutions open to air at 98 K were measured. Those for SUS6L and SUS4 stainless steel samples were also examined for comparison. In N HCl solution, a SUS4 stainless steel was dissolved actively, showing a very high corrosion rate of 4. mm y on average for a week immersion. The corrosion rate of a SUS6 stainless steel was about.8 mm y. On the other hand, the corrosion rate of the Tibased alloy was about.46 mm y, which was about one or two orders of magnitude lower than those of stainless steel samples. Accordingly, it is clear that the Tibased BMG has much higher corrosion resistance than stainless steels in strong corrosive environment. Furthermore, after immersion in mass% NaCl, N H SO 4 or N H SO 4 +. N NaCl solutions open to air at 98 K for months, the weight loss of all the Tibased BMG samples was undetectable, indicating that the Ti 4 Zr Pd Cu Sn 4 BMG possesses excellent corrosion resistance in mass% NaCl, N H SO 4 and N H SO 4 +. N NaCl solutions. 4

9 Celebrating the th Anniversray 44 Vol. No.4 July 4. Electrochemical properties in sulfuric acids Figure shows the anodic polarization curves of the ascast Ti 4 Zr Pd Cu Sn 4 BMG rods with a diameter of. mm and a SUS6L stainless steel in N H SO 4 and N H SO 4 +. N NaCl solutions, respectively, open to air at 98 K [6]. The polarization curves of the Cubased BMGs are also shown for comparison in those solutions at the same conditions. The alloy is passivated with a significant low current density of the order of A m and a wide passive potential region in sulfuric acids with and without Cl ions. According to their polarization curves, it is interesting to note that the Tibased BMG exhibits unique polarization characteristics in sulfuric acid solutions with and without. N NaCl as compared to the Cubased alloys and a SUS6L. One feature of interest is that the Tibased alloy demonstrates a very noble open circuit potential, which is related to an increase in the difficulty for anodic oxidation reaction of the alloy. The open circuit potentials of the alloy were about 6 mv, 4 mv and 48 mv (vs. Ag/AgCl) in NaCl, H SO 4 and H SO 4 + NaCl solutions, respectively. This is in accord with their high corrosion resistance during open circuit immersion of long duration. The presence of a noble element, Pd, in the alloy helps to ennoble the open circuit potential due to its high cathodic activity. Another interesting feature is that the Tibased alloy shows a highly stable passivation in high potential region, whereas the Cubased alloys are slowly dissolved and a SUS6L displays a high transpassive dissolution current density at the same potential region. Therefore, the Ti 4 Zr Pd Cu Sn 4 BMG with its high corrosion resistance and unique electrochemical properties has significant potential not only for human body implant applications, but also for structural uses in industry. CuZr4AI φ mm SUS6L Cu6ZrAI φ. mm TI4ZrPdCuSn4 Cu Hf TI 6 φ mm SUS6L φ. mm TI Zr Pd Cu Sn 4 4 (Cu Hf TI ) Nb Fig. : Anodic polarization curves of ascast. mm diameter Ti 4 Zr Pd Cu Sn 4 alloy rods measured in N H SO 4 and N H SO 4 +. N NaCl solutions open to air at 98 K. Polarization curves of SUS6L stainless and Cubased alloys are also shown for comparison. XPS analysis In order to clarify the surfacerelated chemical characteristics of the alloy, XPS analysis was performed for the specimens aspolished mechanically in cyclohexane or immersed for 68 h in N HCl, mass% NaCl, N H SO 4, and N H SO 4 +. N NaCl solutions. The XPS spectra of the specimens over a wide binding energy region exhibited peaks of Ti p, Pd d, Zr d, Cu p, Sn d, O s, C s etc. The deconvolution spectrum peaks from alloy constituents were composed of peaks of oxidized states and metallic states. The oxidized states (ox) and metallic states (m) are assigned to signals from the surface film and underlying alloy surface just beneath the surface film, respectively. By XPS analysis, the major cation in the immersed surface film is Ti 4+ and followed by Zr 4+, Cu +, Cu + and Sn 4+. Figure shows the cationic fraction of elements in the surface film and the atomic fraction of elements in the underlying alloy just below the surface film for the alloys after air exposure and immersion for 68 h in various solutions after mechanical polishing, respectively [6]. Immersion for 68 h in different solutions gives rise to the formation of a new film which is different from the airformed film. When the alloy is immersed in various solutions, the fractions of Ti and Zr cations in the surface film increase as compared with airformed surface composition, whereas the contents of Cu and Sn cations decrease. Therefore, the high corrosion resistance of the alloy is attributed to its chemically homogeneous single phase nature and the formation of Ti 4+ and Zr 4+ enriched highly protective thin surface film in corrosive solutions. The new Tibased BMG with excellent corrosion resistance and unique electrochemical properties is very promising as a new advanced engineering material. 4 Conclusions This work reviews the development of the Cu, Ni, Tibased metallic glasses/composites and their unique properties. The results obtained are summarized as follows: () Bulk metallic glasses (BMGs) with critical diameters of.4. mm were synthesized in the (Cu.6 Zr. Ti. ) xy Ni y Nb x (x = 6 at.% and y = 7 at.%) and (Cu.6 Hf. Ti. ) xy Ni y Nb x

10 Celebrating the Vol. No.4 July 4 th Anniversray 44 Cation fraction in surface film (%) Sn Cu Pd Zr Ti Nominal As polished HCI Sn Cu Zr Ti NaCI ( a) Cation fraction in underlying surface (%) As polished Sn m Cu m Pd m HCI NaCI ( b) Fig. : Changes in cationic fractions in surface film and atomic fractions in underlying alloy surface for ascast Ti 4 Zr Pd Cu Sn 4 alloy exposed to air and those specimens immersed in N HCl, mass% NaCl, N H SO 4 and N H SO 4 +. N NaCl solutions open to air for 68 h after mechanical polishing (x = 6 at.% and y = at.%) alloy systems by the copper mold casting method. The addition of Ni element causes an extension of a supercooled liquid region (ΔT x =T x T g ) from 4 K for Cu 6 Zr Ti to 6 K for (Cu.6 Zr. Ti. ) 9 Ni and from 6 K for Cu 6 Hf Ti to 7 K for (Cu.6 Hf. Ti. ) 9 Ni. The simultaneous addition of Ni and Nb to the CuZr(Hf) Ti alloys is effective for synergistically improving the corrosion resistance of the alloys by remarkably decreasing the anodic current densities in chloride containing solutions and enhancing the pitting potential against the localized corrosion. () Insitu Ta/Nbreinforced (Cu.6 Hf. Ti. ) 94 Ta 6 and (Cu.6 Hf. Ti. ) 9 Nb BMG composites with excellent combination of high corrosion resistance and superior mechanical properties was successfully synthesized. The micrometersized ductile Ta/Nbrich dendrite phase disperses homogeneously in the glassy matrix. The addition of Ta/Nb to the CuHfTi alloy results in a significant decrease in the corrosion rates in the solutions examined. The Tareinforced composite alloy exhibits high compressive true yield strength of, MPa, fracture strength of MPa together with large plastic strain of 4%. () Ni 6 Nb x Ta x Ti Zr (x =,, and at.%) and Ni 6 Nb x Ta x Ti (x =,, and at.%) glassy alloys with high thermal stability and ultrahigh corrosion resistance in fuel cell environments at high temperature were developed successfully by the melt spinning technique. The substitution of tantalum for a portion of niobium in the alloys causes a significant increase in the supercooled liquid region. These alloys have great potential application as bipolar plate materials of fuel cells. (4) The Ni 7 Nb 9 Zr 9 Ta and Ni 6 Nb Zr Ti Ta metallic glasses demonstrate high corrosion resistance in boiling 6 N HNO solutions with and without Cr 6+ ions, which may be of great potential for nuclear fuel reprocessing applications. Xray photoelectron spectroscopy analysis reveals that the high corrosion resistance of the alloys is due to the formation of the passive film composed exclusively of Nb + and Ta + cations after immersion in the solution without Cr 6+ ions, and Nb +, Ta + and Cr + cations after immersion in the solution with Cr 6+ ions. () The Nifree Ti 4 Zr Pd Cu Sn 4 BMG exhibits excellent corrosion resistance after immersion in mass% NaCl, N H SO 4 and N H SO 4 +. N NaCl solutions. Xray photoelectron spectroscopy (XPS) measurements were used to analyze the changes of the elements on the alloy surface before and after immersion in various solutions. The formation of Ti and Zrenriched highly protective thin surface film is responsible for high corrosion resistance of the alloy in the corrosive solutions. References [] Greer A L, Ma E. Bulk metallic glasses: at the cutting edge of metals research. MRS Bulletin, 7, : 669. [] Klement W, Willens R H, Duwez P. Noncrystalline structure in solidified goldsilicon alloys. Nature, 96, 87: [] Inoue A. Stabilization of metallic supercooled liquid and bulk amorphous alloys. Acta Materialia,, 48: 796. [4] Johnson W L. Bulk glassforming metallic alloys: science and technology. MRS Bulletin, 999, 4(): 46. [] Telford M. The case for bulk metallic glass. Materials Today, March 4: 64. [6] Chen H S. Glassy metals. Reports on Progress in Physics, 98, 4: 4. [7] Inoue A, Zhang W, Zhang T, et al. Highstrength Cubased bulk glassy alloys in CuZrTi and CuHfTi ternary systems. Acta Materialia,, 49: 646. [8] Inoue A, Zhang W. Formation, thermal stability and mechanical properties of CuZrAl bulk glassy alloys. Materials Transactions,, 4: 99. [9] Das J, Tang M B, Kim K B, et al. Workhardenable ductile bulk metallic glass. Physical Review Letters,, 94: 4. 6

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