Glassy Alloy Rods Research Articles

Two-stage-like glass transition and the glass-forming ability of a soft magnetic Fe-based glassy alloy

The structure, thermal stability, and crystalline behavior of (Fe0.9Co0.1)67.5Nb4Gd3.5B25 glassy alloy, which exhibits a two-stage-like glass transition phenomenon, were investigated using x-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The nanoscale metastable (Fe,Co)23B6 phase precipitated in the glassy matrix after annealing, while the two-stage-like glass transition disappeared, indicating the two-stage-like glass transition results from the overlap of the endothermic reaction for the glass transition with the exothermic reaction for the formation of the (Fe,Co)23B6 phase in the supercooled liquid region. The (Fe0.9Co0.1)67.5Nb4Gd3.5B25 glassy alloy exhibits high glass-forming ability, enabling the formation of glassy alloy rods with diameters exceeding 3.0 mm, rather high saturation magnetization of 0.91 T, low coercive force of 2.5 A/m, and high fracture strength of 3870 MPa.

Comments on the Paper “Production of Zr<SUB>55</SUB>Cu<SUB>30</SUB>Ni<SUB>5</SUB>Al<SUB>10</SUB> Glassy Alloy Rod of 30 mm in Diameter by a Cap-Cast Technique” (Yoshihiko Yokoyama, Enrico Mund, Akihisa Inoue and Ludwig Schultz, Mater. Trans. 48 (2007) 3190–3192.)

1. Summary of the Title Paper This paper describes a new casting technique and the required oxygen level for successful production of Zr55Cu30Ni5Al10 glassy alloy rod up to 30 mm in diameter. The casting technique named ‘‘cap-cast method’’ consisted of arc-melting the alloy followed by quick casting into water cooled copper mold; a copper ‘‘cap’’ is used to compress the alloy melt at 1 kN from the top surface immediately after casting. This cap is needed to prevent shrinkage cavity and to enhance the cooling speed of the alloy melt. Low oxygen level, less than 45 mass ppm in the starting zirconium metal, was required to obtain homogeneous Zr55Cu30Ni5Al10 glassy alloy rod. Otherwise, crystalline inclusions were observed in the glass matrix. It is also stated that ‘‘the formation of a single glassy phase in the half topside of master alloy’’ was confirmed for the as-arc melted button ingots, cooled naturally on the copper hearth. It is concluded that (1) the purity level of alloy component is an important factor to produce glassy alloy rods with large size and (2) the Zr55Cu30Ni5Al10 glassy alloy rods of up to 30 mm in diameter could be produced by means of the cap-cast technique; the critical size of glassy alloy rods is 20 mm by the conventional cast technique using copper molds.

Cap casting and enveloped casting techniques for Zr55Cu30Ni5Al10 glassy alloy rod with 32 mm in diameter

In order to produce centimetre-sized bulk glassy alloys (BMGs), various cast techniques have been developed. We succeed in the development of cap casting and enveloped casting technique to accomplish the fabrication of centimetre sized BMGs. The former has an advantage to increase cooling rate and the later has an advantage to joint another materials instead of welding. This paper presents the production of a glassy Zr55Cu30Ni5Al10 alloy rod with a diameter of 32 mm and joined glassy Zr55Cu30Ni5Al10 alloy parts with another materials for industrial applications.

Formation, Thermal Stability, and Mechanical Properties of Glassy Zr<SUB>60</SUB>Al<SUB>10</SUB>Ni<SUB>10</SUB>Cu<SUB>20</SUB> Alloy Rods with Diameters of 18 and 20 mm

Bulk glassy alloy rods of diameters 18 and 20mm were formed during the tiltcasting of Zr 60 Al 10 Ni 10 Cu 20 . A further increase in the rod diameter to 22 mm resulted in the formation of a mixed structure with approximately 60 vol% glassy phase and 40 vol% crystalline phase, which comprised the central region of the transverse cross-section of the alloy rod. The glass transition temperature (Tg), onset temperature of crystallization (T x ), and liquidus temperature (T 1 ) were measured to be 666 K, 768 K, and 1153K, respectively, and the resultant ΔT x (= T x - Tg), Tg/T 1 , and γ (= T x /(Tg + T 1 )) values were 102 K, 0.578, and 0.422, respectively. The high glass-forming ability of supercooled liquids is attributed to the high resistance of the liquids against crystallization due to the large ΔT x . and high Tg/T 1 and γ values. For the alloy rod of diameter 20 mm, the Young's modulus, yield strength, elastic strain, and plastic strain under a uniaxial compressive load are 80 GPa, 1820 MPa, 0.022, and 0.060, respectively; the plastic strain of the rod with diameter 20 mm is greater than that (0.04) of the glassy alloy rod of diameter 2 mm, although there exist no appreciable differences between their strength levels and fracture behaviors. The success of formation of the large-sized Zr-based bulk glassy alloy rod, which exhibits large plastic strains in conjunction with high strength levels, is promising with regard to further extension of the applications of such highly functional materials.

Reply to Comments by Tsuyoshi Kajitani on Our Paper Titled ‘Production of Zr<SUB>55</SUB>Cu<SUB>30</SUB>Ni<SUB>5</SUB>Al<SUB>10</SUB> Glassy Alloy Rod of 30 mm in Diameter by a Cap-Cast Technique (by Y. Yokoyama, <I>et al.</I>, Mater. Trans. <B>48</B> (2007) 3190–3192)’

Our paper (Mater. Trans. 48 (2007) 3190-3192) reports (1) the importance of controlling the impurity oxygen content during the manufacture of Zr 55 Cu 30 Ni 5 Al 10 bulk metallic glasses (BMGs) and (2) the successful fabrication of a BMG rod of 30 mm diameter rod by our newly developed 'cap casting' technique. In response to Kajitani's comments, where he questions the validity of the proposed method through his eight specific criticisms, we have made our assertion from an academic viewpoint. He has rightly pointed out a typographical error, where the TEM apparatus has been typed as JEOL 4000 FX instead of JEM-4000EX, for which we apologize. Nevertheless, our TEM, JEM-4000EX has indeed better resolution than that of 'FX' and, thus, this typographical error does not diminish the scientific significance of our results.

Formation, Thermal Stability and Mechanical Properties of Bulk Glassy Alloys with a Diameter of 20 mm in Zr-(Ti,Nb)-Al-Ni-Cu System

Bulk glassy alloy rods with a diameter of 20 mm were produced for Zr61Ti2Nb2Al7:5Ni10Cu17:5 and Zr60Ti2Nb2Al7:5Ni10Cu18:5 by a tilt casting method. The replacement of Zr by a small amount of Ti and Nb caused a distinct increase in the maximum diameter from 16 mm for Zr65Al7:5Ni10Cu17:5 to 20 mm, accompanying the decrease in liquidus temperature and the increase in reduced glass transition temperature. The primary precipitation phase from supercooled liquid also shows a distinct change, i.e., from coexistent Zr2Cu, Zr2Ni and Zr6NiAl2 phases for the 65%Zr alloy to an icosahedral phase for the 61%Zr and 60%Zr alloys. These results allow us to presume that the enhancement of the glassforming ability is due to an increase in the stability of supercooled liquid against crystallization caused by the development of icosahedral shortrange ordered atomic configurations. The 60%Zr specimens taken from the central and near-surface regions in the transverse cross section at the site which is 15 mm away from the bottom surface of the cast glassy rod with a diameter of 20 mm exhibit good mechanical properties under a compressive deformation mode, i.e., Young’s modulus of 81 GPa, large elastic strain of 0.02, high yield strength of 1610 MPa and distinct plastic strain of 0.012. Besides, a number of shear bands are observed along the maximum shear stress plane on the peripheral surface near the final fracture site. The finding of producing the large scale Zr-based bulk glassy alloys exhibiting reliable mechanical properties is encouraging for future advancement of bulk glassy alloys as a new type of functional material. [doi:10.2320/matertrans.MER2008179]

Corrosion behavior of Fe-based ferromagnetic (Fe, Ni)–B–Si–Nb bulk glassy alloys in aqueous electrolytes

Fe-based ferromagnetic [(Fe 1− x Ni x ) 0.75B 0.2Si 0.05] 96Nb 4 ( x = 0, 0.2 and 0.4) bulk glassy alloys (BGAs) with a diameter of 2 mm were prepared by copper mold casting. The corrosion behavior of glassy alloy rods obtained was investigated in 0.5 M NaCl, 0.5 M NaOH and 0.5 M H 2SO 4 aqueous electrolytes, respectively, using weight loss and electrochemical polarization measurements. It was found that the corrosion rates significantly decrease with an increase in Ni content in all examined solutions. The Ni-containing BGAs are spontaneously passivated with wide passive regions and low passive current densities in NaCl and NaOH solutions, but exhibit the active–passive–transpassive behavior in H 2SO 4 solution. The partial substitution of Ni for Fe results in a considerable improvement on the corrosion resistance of [(Fe 1− x Ni x ) 0.75B 0.2Si 0.05] 96Nb 4 BGAs, because of the structural and chemical homogeneousness of the amorphous phase and the effect of Ni on promoting the formation of a passive film. Besides their high glass-forming ability (GFA), excellent soft-magnetic properties and good mechanical performance, which have been reported before, these FeNi-based BGAs also feature rather high corrosion resistance.

Synthesis of ferromagnetic Fe-based bulk glassy alloys in the Fe–Si–B–P–C system

Multi-component Fe76P5(BxSiyCz)19 glassy alloys were fount to exhibit a distinct glass transition, followed by a supercooled liquid region before crystallization in a rather wide composition range. The largest value of the supercooled liquid region defined by the difference between the glass transition temperature (Tg) and crystallization temperature (Tx), ΔTx (=Tx − Tg) was 54 K for Fe76P5(Si0.3B0.5C0.2)19. Furthermore, the crystallization of the glassy alloy occurs through a single exothermic reaction, which means simultaneous precipitation of several kinds of crystallites leading to higher glassy forming ability (GFA) due to the necessity of the atomic rearrangement on a long range scale. By copper mold casting, bulk glassy alloy rods with diameters up to 3 mm were produced. The representative Fe76P5(B0.5Si0.3C0.2)19 alloy exhibits rather high saturation magnetization of 1.44 T with good soft-magnetic properties, i.e., low coercive force of 1.2 A/m, and high effective permeability of 17,600 at 1 kHz under a field of 1 A/m. The bulk glassy alloy also possesses superhigh fracture strength of 3700 MPa and Young's modulus of 185 GPa. This new ferromagnetic bulk glassy alloy simultaneously exhibiting high GFA, rather high saturation magnetization, excellent soft-magnetic properties and superhigh fracture strength is promising for future applications as not only functional but also structural material.

Structure, Thermal Stability and Mechanical Properties of Zr<SUB>65</SUB>Al<SUB>7.5</SUB>Ni<SUB>10</SUB>Cu<SUB>17.5</SUB> Glassy Alloy Rod with a Diameter of 16 mm Produced by Tilt Casting

Bulk glassy alloy rods with a diameter of 16 mm and a length of 40 to 45 mm were produced for Zr65Al7:5Ni10Cu17:5 alloy by a tilt casting method. The alloy specimens taken from the different sites which are away by about 10 mm, 15 mm and 30 mm from the bottom surface of the cast rod consist of a glassy phase and their thermal stability, mechanical properties and fracture mode are almost independent of the specimen sites. The glass transition temperature, temperature interval of supercooled liquid region before crystallization and heat of crystallization are about 643 K, 102 K and 55 J/g, respectively, for the specimens taken from the three different sites. Besides, Young’s modulus, yield strength, fracture strength, yield strain and plastic strain of the specimens are about 87 GPa, 1540 MPa, 1580 MPa, 0.018 and 0.005, respectively. The fracture mode consisting of shear plastic deformation along the maximum shear stress plane, followed by an instantaneous final rupture is also independent of the specimen sites. These data indicate that the cast glassy alloy rod with a diameter of 16 mm has nearly the same characteristics, though the cooling rate is significantly dependent on sample sites. The knowledge of producing the Zr-based bulk glassy alloy rod with nearly the same characteristics in the large diameter range up to 16 mm is encouraging for future applications of bulk glassy alloys as a new type of engineering material. [doi:10.2320/matertrans.MER2008127]

Influences of Temperature and Strain Rate on Mechanical Behavior of a Cu<SUB>45</SUB>Zr<SUB>45</SUB>Al<SUB>5</SUB>Ag<SUB>5</SUB> Bulk Glassy Alloy

Since the invention of bulk glassy alloys, a number of studies have been performed at ambient temperatures or above. However, little is known about mechanical properties of bulk glassy alloys at cryogenic temperatures. In this study, we investigated the effects of temperature and strain rate on the mechanical properties of a Cu45Zr45Al5Ag5 bulk glassy alloy fabricated by high pressure die casting methods. Compression tests were performed for the Cu45Zr45Al5Ag5 bulk glassy alloy rods with a diameter of 3 mm at temperatures of 298, 223, 173 and 77 K and at strain rates from 5 10 5 to 5 10 3 s 1 . It is found that the maximum compressive stress and plastic strain to failure increase monotonically with decreasing testing temperature. Multiple shear bands are observed on the side surface of the specimen deformed plastically at cryogenic temperatures. The maximum compressive stress and plastic strain are almost independent of strain rate. The reason for the changes in the maximum compressive stress and plastic strain with temperature and strain rate is discussed. [doi:10.2320/matertrans.MBW200734]

高圧水素を用いたポーラスバルク金属ガラスの創製

Porous Pd42.5Cu30Ni7.5P20 bulk glassy alloy rods with porosities of up to 70% were successfully prepared by high pressure hydrogen of 15 MPa. The melt of Pd42.5Cu30Ni7.5P20 alloy kept under high pressure hydrogen absorbs hydrogen and subsequent water quenching of the melt causes the homogeneous dispersion of hydrogen bubbles, which was resulted from the decrease of hydrogen solubility with decrease of pressure. Annealing the hydrogen bubble containing sample at a supercooled liquid state under vacuum, the bubbles are allowed to expand due to the decrease of viscosity of metallic glass matrix. Pores expansion continues until glassy matrix crystallizes or the equilibration among pressure of the pores, pressure of the atmosphere and surface tension is achieved. By utilizing these phenomena, pores up to 80 m in diameters are homogeneously distributed over the whole cross-sectional area of a fully glassy matrix. Under compressive deformation, the porous alloys with porosities exceeding 40% did not show macroscopic fracture in a wide compressive strain range up to 0.6 whereas the non-porous alloy fractures instantly after elastic limit of about 0.02. Porous bulk glassy alloys exhibit higher plateau stress, lower Young′s modulus and higher energy absorption capacity compared with the conventional crystalline metal foams.

Production of Zr<SUB>55</SUB>Cu<SUB>30</SUB>Ni<SUB>5</SUB>Al<SUB>10</SUB> Glassy Alloy Rod of 30 mm in Diameter by a Cap-Cast Technique

used for the preparation of the master alloy. The purity of these metals; Zr(+Hf), Cu, Ni and Al were 99.99 mass%, 99.999 mass%, 99.99 mass% and 99.999 mass%, respectively. The alloy ingots were completely remelted, and cast into cylindrical rod samples with diameters from � 16 mm to � 30 mm by a modified cap-casting method, as shown in Fig. 1. The cap-cast technique was designed to obtain high cooling rate even at the upper part of cast specimen. The cast structure was examined by scanning electron microscopy (SEM), and the compositions of the quenched samples were determined with an electron probe microanalyzer (EPMA). The phase identification of the cast samples was performed by X-ray diffractometry. Oxygen concentration of the bulk glassy alloys was measured by a helium carrier fusioninfrared absorption. Microscopic cast structures were examined by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) using a JEOL 4000 FX. TEM samples were prepared by an electrolytic polishing machine using nitric methanol (30 vol% HNO3) at about 250 K.

Effects of Al and Ti additions on the thermal stability, glass-forming ability and mechanical properties of Ni 60Nb 20Zr 20 glassy alloy

The glass-forming ability and thermal stability was investigated in a family of Ni-based alloys. The alloys were made by the addition of Al and Ti into Ni 60Zr 20Nb 20. Upon Al addition, the supercooled liquid region (Δ T x ), the reduced glass transition temperatures ( T g/ T l) and γ = T x /( T g + T l) gradually increase with the increasing Al content. The maximum values of Δ T x = 54 K, T g/ T l = 0.615, and γ = 0.405 are reached at the Ni 60Zr 20Nb 15Al 5 composition, which exhibits the largest GFA, and can be made into BMG rods of at least 3 mm in diameter. A further substitution of Nb by Ti can extend Δ T x from 54 to 70 K, and glassy alloy rods with 2 mm in diameter can be obtained for the alloys with Ti contents less than 10 at.%. Under uniaxial compression at room temperature, these new Ni-based BMGs exhibit high fracture strengths of 2775–2900 MPa and Young's modulus of 145–152 GPa. The Ni 60Zr 20Nb 15Al 5 BMG also exhibits a plastic elongation of 2.5%.

Effects of additional Ag on the thermal stability and glass-forming ability of La–Al–Cu bulk glassy alloys

The addition of Ag to La–Al–Cu ternary glassy alloys greatly increases the stabilization of supercooled liquid and glass-forming ability (GFA). The large supercooled liquid region (Δ T x ) exceeding 60 K was obtained in a wide composition range of 10–22.5 at.% Al, 15–25 at.% Cu, and 2–12.5 at.% Ag. The largest Δ T x of 83 K and the reduced glass transition temperature ( T g/ T l) were obtained for La 62.5Al 12.5Ag 5Cu 20 alloy, leading to formation of the glassy alloy sample with a diameter of 6.0 mm. The addition of small amounts of Fe or Co to La–Al–Ag–Cu alloy increases T g/ T l though the decrease in T l, resulting in an increase of GFA. As a result, the glassy alloy rods with diameters of over 7.0 mm were fabricated by addition of 2.5–5 at.% Fe or Co. The new La-based bulk glassy alloys exhibited high compressive fracture strength of 589–858 MPa with distinct plastic strain of 0.2–1.0%.

Enhanced soft-magnetic and corrosion properties of Fe-based bulk glassy alloys with improved plasticity through the addition of Cr

Abstract The influence of Cr addition on glass-forming ability (GFA), corrosion resistance, and magnetic and mechanical properties of the Fe–Co–B–Si–Nb bulk glassy alloy (BGA) has been investigated. Except for a slight decrease in its GFA, the Cr addition caused an increase of corrosion resistance as well as an improvement of soft-magnetic properties and mechanical behaviors for this alloy within the composition range studied. BGA rods with diameters up to 4 mm were produced by copper mold casting technique. These glassy alloys exhibit a rather high saturation magnetization of 0.811–1.040 T, low coercive force of 0.6–1.7 A/m, high effective permeability of (2.37–3.41) × 10 4 at 1 kHz under a field of 1 A/m, and extremely low saturation magnetostriction of (0.43–4.17) × 10 −6 . These BGAs also possess ultrahigh fracture strength of 4010–4274 MPa, Young's modulus of 200–215 GPa, elastic strain of about 2% and plastic strain reaching 0.7%. Moreover, the corrosion rate and corrosion current density of the glassy alloy rods in simulated industrial environments contaminated with SO 2 decreased significantly from 3.47 × 10 −2 to 9.2 × 10 −3 mm/year and 1.5 × 10 −5 to 6.7 × 10 −7 A/cm 2 , respectively, with increasing Cr content from 0 to 4 at.%, illustrating that the addition of Cr is effective in increasing the corrosion resistance. Furthermore, the mechanisms for the high GFA, distinct plasticity strain, good corrosion resistance, and excellent soft-magnetic properties of the Fe-based BGAs are discussed.

A cluster line approach to finding new Fe-B-Y-Nb-Zr bulk metallic glasses

The glass forming ability of the [(Fe 12/13Y 1/13) 100- x B x ] 96Nb 2Zr 2 ( x=9–26) system was investigated using a series of cluster lines. Three types of clusters, an icosahedron (Fe 12Y), a capped Archimedes anti-prism (Fe 8B 3) and a capped trigonal prism (Fe 9B), as well as a binary eutectic (Fe 83B 17) were considered. Bulk glassy alloy rods of 3 mm in diameter were synthesized using a copper mold suction-casting method. The glass transition temperature was observed for all samples in the boron range of 15.9at%-25.7at%, with the alloy at 15.9at% of boron having the best thermal properties. The ferrous-based bulk metallic glasses (BMG) obtained have high reduced glass transition temperatures with the maximum reaching 0.63 and large supercooled liquid regions with the maximum reaching 111 K. Magnetic testing revealed a large value of coercive force and remanent magnetization, being 11 kA/m and 0.1 T, respectively.

Cu<SUB>45</SUB>Zr<SUB>45</SUB>Al<SUB>5</SUB>Ag<SUB>5</SUB> Bulk Glassy Alloy with Enhanced Compressive Strength and Plasticity at Cryogenic Temperature

Compressive tests were made for the as-cast Cu 45 Zr 45 Al 5 Ag 5 bulk glassy alloy rods with a diameter of 3 mm at 298 and 77 K, at an initial strain rate of 5 × 10 -4 s -1 . It was found that at 77 K, the compressive yield strength, maximum strength and plastic strain to fracture of the glassy alloy rod are higher than those at 298 K. The maximum strength measured at 77 K is about 13% larger than that measured at 298 K. In particular, its plasticity at 77 K is significantly enhanced. SEM observations reveal that more shear bands appear near the fracture plane at 77 K. The higher strength and ductility at a cryogenic temperature (77 K) are probably due to the lower mobility of the squeezed free volume.

Fabrication of New Cu<SUB>34</SUB>Pd<SUB>2</SUB>Zr<SUB>48</SUB>Ag<SUB>8</SUB>Al<SUB>8</SUB> Bulk Glassy Alloy with a Diameter of 30 mm

Glass-forming ability (GFA) of Cu 36-x Pd x Zr 48 Ag 8 Al 8 alloys was examined with the aim of developing a bulk glassy alloy with large critical diameter. It was found that the addition of 2 at% Pd significantly improved the GFA of the Cu 36 Zr 48 Ag 8 Al 8 alloy. The fully glassy alloy rod with a diameter of 30 mm was successfully fabricated for the Cu 34 Pd 2 Zr 48 Ag 8 Al 8 alloy using a copper mold casting method. The further increase of Pd content causes the decrease in the GFA. For the Cu 36-x Pd x Zr 48 Ag 8 Al 8 alloys, the Tg and T x increase monotonously with increasing Pd content, while T 1 shows a minimum value at 2 at% Pd, resulting in a maximum y (= T x /(T g + T 1 )) value at 2 at% Pd. The increase of y parameter may be the reason for the significant improvement of the GFA of the quinary alloy.

Fabrication of ZrCuAlNi Metallic Glassy Matrix Composite Containing ZrO<SUB>2</SUB> Particles by Spark Plasma Sintering Process

Using a mixed powder of argon gas-atomized Zr 55 Cu 30 Alj o N1 5 metallic glassy alloy powder blended with 10vol% ZrO 2 powder, Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glassy matrix composite was fabricated by a spark plasma sintering process. The structure, thermal stability and mechanical properties of the sintered bulk metallic glassy matrix composite were investigated. The ZrO 2 particles were homogeneously dispersed in the Zr 55 Cu 30 Al 10 Ni 5 glassy matrix. No crystallization of the glassy matrix occurred during the spark plasma sintering process. The crystallization behavior of the sintered composite was similar to that of the metallic glassy powder. The sintered composite specimen exhibited larger plastic ductility than that of the as-cast Zr 55 Cu 30 Al 10 Ni 5 glassy alloy rod specimen. The increase in the plastic ductility is proposed to originate from the structural inhomogeneity caused by the ZrO 2 ceramic particles.

Production of Zr<SUB>55</SUB>Cu<SUB>30</SUB>Ni<SUB>5</SUB>Al<SUB>10</SUB> Glassy Alloy Rod of 30 mm in Diameter by a Cap-Cast Technique

used for the preparation of the master alloy. The purity of these metals; Zr(+Hf), Cu, Ni and Al were 99.99 mass%, 99.999 mass%, 99.99 mass% and 99.999 mass%, respectively. The alloy ingots were completely remelted, and cast into cylindrical rod samples with diameters from � 16 mm to � 30 mm by a modified cap-casting method, as shown in Fig. 1. The cap-cast technique was designed to obtain high cooling rate even at the upper part of cast specimen. The cast structure was examined by scanning electron microscopy (SEM), and the compositions of the quenched samples were determined with an electron probe microanalyzer (EPMA). The phase identification of the cast samples was performed by X-ray diffractometry. Oxygen concentration of the bulk glassy alloys was measured by a helium carrier fusioninfrared absorption. Microscopic cast structures were examined by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) using a JEOL 4000 FX. TEM samples were prepared by an electrolytic polishing machine using nitric methanol (30 vol% HNO3) at about 250 K.