Kovar Alloys Research Articles

Variation with Ni and Mn of the structural stability, magnetic properties and Kovar effect in Fe–Ni–Co based alloys

As a typical commercial Kovar alloy, the Fe–Ni–Co based alloy with nominal ∼29 wt% Ni, ∼17 wt% Co, and minor Mn and Si has found abroad applications in electronic and optoelectronics fields because of its analogous thermal expansion coefficient to borosilicate glass as well as high Curie temperature. Owing to the limited structural stability in cryogenic environments, the alloy is suggested to use above 193 K to avoid the failure by austenite to martensite transformation. In this work, the structural stability, magnetic properties, and Kovar effect of Fe54-xNi29+xCo17 (x = 0, 3, 6) and Fe54-xNi29Co17Mnx (x = 0, 1, 2, 4) alloys were systematically investigated, attempting to extend the application of Fe–Ni–Co based Kovar alloys in extremely low-temperature environments. The results achieved in the study indicated that the martensitic transformations could be completely suppressed in Fe54-xNi29+xCo17 (x = 3, 6) and Fe54-xNi29Co17Mnx (x = 2, 4) alloys at 77 K. The structural stability, Curie temperature, and CTEs increase with increasing Ni content in Fe54-xNi29+xCo17 (x = 0, 3, 6) alloys. The increase of Mn in Fe54-xNi29Co17Mnx (x = 0, 1, 2, 4) alloys gave rise to the enhancement of structural stability, reduction of Curie temperature, and sluggish increment of CTE with invariant CTE value in the temperature range from 550 K to 200 K. By comparison with Ni, the incorporation of Mn has a lesser effect on the CTE of Fe–Ni–Co based Kovar alloys, bringing benefit to the application in hermetic seals between metals and glass.

Impact of ZnO content in BAS glass–ceramics and pre‐oxidation temperature on Kovar alloys to their sealing strength

AbstractThis work studied the effect of Si/Zn ratio on the structure of B2O3–Al2O3–SiO2 (BAS) parent glass and the microstructure, properties of the resulting glass‐ceramics after heat treatment. Additionally, the effects of different pre‐oxidation temperature of Kovar alloys and sealing temperature on the shear strength of these sealed glass–ceramics‐to‐Kovar samples were studied. First, the parent glass precipitated three kinds of crystals Al0.52Zr0.48O1.74 (PDF#53‐0294), ZrO2 (PDF#37‐1484), and ZnAl2O4 (PDF#05‐0669) in BAS glass–ceramics. The coefficient of thermal expansion of glass–ceramics increased from 48.17 × 10−7 to 54.92 × 10−7°C−1 with the gradual increase of ZnO content. With an optimizing pre‐oxidized temperature of Kovar at 950°C, a champion sample with the highest sealing strength of 6.086 MPa was achieved when BAS glass–ceramics sealed to Kovar at 1000°C for 30 min. Moreover, the sealing interface was observed to consist of four distinct regions: the matrix alloy, the alloy iron‐deficiency area, the iron‐rich glass–ceramics, and the matrix glass–ceramics.

Competitive correlation between the interface layers determines the shear strength in Kovar/AgCuTi/Si3N4 joints

AbstractAs a promising high‐thermal‐conductivity ceramics, the direct bonding of Si3N4 with metals remains challenging. Here, we employed active metal brazing of Si3N4 ceramics and Kovar alloys using AgCuTi metal filler, and found that the shear strength in the prepared Kovar/AgCuTi/Si3N4 joints depends on the competitive correlation between the interface layers. The competitive consumption for active element Ti by Kovar/AgCuTi interfacial reactions reduced AgCuTi/Si3N4 interface layer thickness and deteriorated final shear strength, even when using a high‐Ti‐content filler. Surface analysis indicated that interfacial fracture between Ag‐based solid solution and Ti5Si3 was the primary cause of failure. Density functional theory (DFT) revealed that Ag (111)/Fe2Ti (001) interface had higher ideal work of adhesion (Wad) than Ag (111)/Ti5Si3 (001) interface, confirming the importance of AgCuTi/Si3N4 interface layer. Ultimately, by adjusting interface reaction layer thickness, the joints reached a high shear strength of 154.3 MPa.

Enhancing the strength and plasticity of Kovar alloy without sacrificing thermal expansion properties

Enhancing the strengths of most constant-expansion alloys, including Kovar alloys, typically results in reduced plasticities and increased coefficients of thermal expansion (CTEs). In this study, we systematically investigated the effects of carbon on the microstructure, mechanical properties, and CTE of a 4J29 Kovar alloy prepared via metal injection molding (MIM). When the carbon content was in the appropriate range, the strength and plasticity could be improved without compromising the CTE. MIM samples prepared by adding 1000 ppm graphite and oxalic acid for degreasing exhibited increases of 28.2 % and 17.3 % in tensile strength and elongation, respectively, compared to those of samples with low carbon contents of 46 ppm. The average CTE remained stable at 4.92 × 10⁻⁶ ℃–1 (25–400 ℃). The reduced number of pores at the grain boundaries was the main cause of the improvement in plasticity. The low CTE is attributed to the combined effect of the grain boundary volume fraction and porosity. This study provides significant theoretical and practical guidance for use in producing high-performance 4J29 Kovar alloys that may be used in complex components with constant-expansion properties.

Microstructures and properties of Cu/Fe/Kovar composites prepared with cu-coated Kovar alloy powders

To improve the thermal conductivities of Kovar alloys (4J50), Fe and Cu coatings were prepared on surfaces of one of Kovar alloys (4J50) powders by electroless plating and Cu/Fe/Kovar composites were hot-pressing sintered. The Fe interlayer is prepared to decrease the mutual diffusion between the Cu coating and the Kovar alloy matrix. Since the existence of Cu network in the composites, the composites present a better thermal conductivity than Kovar alloys. The Cu/Fe/Kovar composites containing 10 wt% and 20 wt% Cu have a thermal conductivity of 24.7 W/(m·K) and 25.8 W/(m·K), respectively, which is 48 % and 54 % higher than that of the Kovar alloy (16.7 W/(m·K)). Due to the presence of Cu network, the coefficients of thermal expansion of the composites is slightly higher than that of the Kovar alloy. The method reported in this study can be also applied to improve the thermal conductively of other Kovar alloys.

Microscopic Analysis of the Wetting Morphology and Interfacial Bonding Mechanism of Preoxidised Kovar Alloys with Borosilicate Glass

This paper investigates the wettability of Kovar alloys with high-borosilicate glass and microscopically analyses the mechanism of wettability and diffusion between Kovar and borosilicate glass. First, Kovar was oxidised at 800 °C for 5, 15, 25, 35, and 60 min to observe the oxide morphology of the Kovar surface layer and to analyse the composition of the oxide layer. To investigate the wetting pattern formations of Kovar and high-borosilicate glass under different wetting temperatures, times, and preoxidation conditions, Kovar and high-borosilicate glass obtained from different oxidation treatments were held at 1060 °C for 20 min for wetting experiments, and the glass–metal wetting interface morphology and elemental distribution were observed using SEM and EDS. The elemental diffusion at the wetting interface between the borosilicate glass and the Kovar with different preoxidation and at the glass spreading boundary was investigated. The longitudinal diffusion of the liquid glass in the metal oxide layer formed a new tight chemical bond of Fe2SiO4, and the lateral diffusion of the liquid glass in the Kovar surface layer formed a black halo.

OPTIMIZATION OF NANO FILLERS CONTENT TO FABRICATE ELECTRICALLY CONDUCTIVE CARBON FIBER REINFORCED POLYMER FOR SPACE USE

Carbon fiber reinforced polymer (CFRP) composites are used in aerospace sector due to their high specific stiffness and light weight, and are a better alternative for aluminum, Invar, and Kovar alloys. However, lower electrical and thermal conductivity of CFRP was the main restriction for extensive use in aerospace sector as compared to aluminum alloys. High aspect ratio, high mechanical stability, high electrical as well as thermal conductivity of carbon nanotubes (CNTs) makes them excellent filler material in CFRP to enhance its electrical, mechanical, and thermal properties. CNTs add synergetic effect to CFRP giving improved CNT-CFRP composites in terms of electrical and mechanical properties. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) are used as filler for fabricating conductive CFRP. Selection of type of CNT (SW/MW-CNT), percentage of CNTs to be used as filler, extent of dispersion in thermosetting resin and carbon fiber layers, governs the properties of CNT-CFRP composites. Objective of the current study was to find optimum percentage of SWCNT and MWCNT to get highly electrically conductive CNT-CFRP composites with ease of manufacturing.

Effects of silver interlayer thickness on the microstructure and properties of electron beam welded joints of TC4 titanium and 4J29 Kovar alloys

Abstract Electron beam welding of TC4 titanium alloy and 4J29 Kovar alloy was performed by using different thicknesses of silver interlayers. The microstructure and the composition of welded joints were characterized by scanning electron microscopy, X-ray diffraction, and energy-dipersive spectrometry. The mechanical properties of welded joints were evaluated by tensile strength tests. The results indicated that Ag thickness has great effects on the weld appearance, microstructure, and mechanical properties of electron beam-welded joints. In case of 0.3 and 0.4 mm thickness of Ag interlayers, a considerable part of 4J29 and TC4 melts and a large amount of Fe, Ti, and Ni elements diffuse into the molten pool forming intermetallic compounds (IMCs) such as TiFe2, NiTi, and FeTi. In case of 0.6 mm thickness of Ag interlayer, the welded joint exhibits brazing characteristics with little IMCs near the fusion line of 4J29 Kovar alloy side. When the thickness of the Ag interlayer increases to 0.8 mm, the diffusion of Ti and Fe elements is completely inhibited by Ag. As the thickness of the Ag interlayer increases from 0.3 to 0.8 mm, the tensile strength of the welded sample shows a phenomenon that first rises and then falls, and the largest tensile strength is 243 MPa.

Effects of Brazing Technology on Hermeticity of Alumina Ceramic-Metal Joint Used in Nuclear Power Plants

Taking the brazing mechanism of alumina ceramics and kovar alloys as the main research object, based on the molybdenum–manganese metallization method, the influence of the direct and indirect brazing processes on the morphology of the final connected layer is explored. Combined with SEM, EDS, the microscopic morphology, and hermeticity affecting the final ceramic–metal composite component is discussed. Finally, through the indirect brazing process, various ceramic–metal composite joints with good airtightness satisfying the requirements were prepared.

Dry and Wet Wear Rate Determination of Steel and Kovar Alloys

Two types of alloy (Steel and Kovar) were prepared and machined as half ring. Wear testing system was used to determine the wear rate of specimens without any lubrication, and after modified wear testing system which was used to test the specimen under lubricant condition. The wear rate wasdetermined by using weighing method and then calculated. The results shows the values of wear rate of kovar specimens are higher than the values of wear rate for steel specimens in both two cases with and without lubrication. The result of wear rates wereplotted as function of sliding time at a fixed normal load.

Joining of Silicon Particle-Reinforced Aluminum Matrix Composites to Kovar Alloys Using Active Melt-Spun Ribbons in Vacuum Conditions

The vacuum brazing of dissimilar electronic packaging materials has been investigated. In this research, this applies silicon particle-reinforced aluminum matrix composites (Sip/Al MMCs) to Kovar alloys. Active melt-spun ribbons were employed as brazing filler metals under different joining temperatures and times. The results showed that the maximum joint shear strength of 96.62 MPa was achieved when the joint was made using Al-7.5Si-23.0Cu-2.0Ni-1.0Ti as the brazing filler metal at 580 °C for 30 min. X-ray diffraction (XRD) analysis of the joint indicated that the main phases were composed of Al, Si and intermetallics, including CuAl, TiFeSi, TiNiSi and Al3Ti. When the brazing temperature ranged from 570 °C to 590 °C, the leakage rate of joints remained at 10−8 Pa·m3/s or better. When the joint was made using Al-7.5Si-23.0Cu-2.0Ni-2.5Ti as the brazing filler metal at 580 °C for 30 min, the higher level of Ti content in the brazing filler metal resulted in the formation of a flake-like Ti(AlSi)3 intermetallic phase with an average size of 7 µm at the interface between the brazing seam and Sip/Al MMCs. The joint fracture was generally in the form of quasi-cleavage fracture, which primarily occurred at the interface between the filler metal and the Sip/Al MMCs. The micro-crack propagated not only Ti(AlSi)3, but also the Si particles in the substrate.

Analysis of the Technology for Ceramics Fabrication from Silicon Carbide by the Methods of Experimental Design

In the present work, using methods of experimental design, we considered the relationship between the process parameters and strength characteristics of SiC-based composite materials with metal additives, the compositions of which correspond to the 42Ni (NILO) and 29NiCo (kovar) alloys. Significant factors for the studied materials were revealed. It was shown that the design of experiment in preparation of ceramic from SiC enables reducing the number of full-scale experiments, especially at high temperatures, and forms the basis for the development of formulations of this ceramic and methods for its fabrication.

Influence of Welding Speed on the Microstructure and Mechanical Properties of Electron Beam-Welded Joints of TC4 and 4J29 Sheets using Cu/Nb Multi-Interlayers

Dissimilar metal joining between titanium and kovar alloys was conducted using electron beam welding. Metallurgical bonding of titanium alloys and kovar alloys was achieved by using a Cu/Nb multi-interlayer. The effects of welding speed on weld appearance, microstructure and mechanical properties of welded joints were investigated. The microstructure of welded joints was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). The mechanical properties of welded joints were investigated by tensile strength and micro-hardness tests. The results showed that welding speed had great effects on the weld appearance, microstructure, and mechanical properties of electron beam-welded joints. With an increase of welding speed, at the titanium alloy side, the amount of (Nb,Ti) solid solution was increased, while the formation of brittle FeTi was effectively suppressed. At the kovar alloy side, microstructure was mainly composed of soft Cu solid solution and some α-Fe + γ phases. In addition, higher welding speeds within a certain range was beneficial for eliminating the formation of cracks, and inhibiting the embrittlement of welded joints. Therefore, the tensile strength of welded joints was increased to about 120 MPa for a welding speed of 10 mm/s. Furthermore, the bonding mechanism of TC4/Nb/Cu/4J29 dissimilar welded joints had been investigated and detailed.

Microstructure and Mechanical Properties of Electron Beam‐Welded Joints of Titanium TC4 (Ti‐6Al‐4V) and Kovar (Fe‐29Ni‐17Co) Alloys with Cu/Nb Multi‐Interlayer

Electron beam welding of a titanium alloy (Ti‐6Al‐4V) and a kovar alloy (Fe‐29Ni‐17Co) was performed by using a Cu/Nb multi‐interlayer between them. Microstructure and composition of welded joints were analyzed by means of optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X‐ray diffraction. Mechanical properties of welded joints were evaluated by microhardness and tensile strength tests. Results indicated that in case of 0.22 mm thickness of Nb foil, microstructure of the titanium alloy side was mainly composed of Ti solid solution and some intermetallic compounds such as FeTi and CuTi2, whereas in case of 0.40 mm thickness of Nb foil, the appearance of weld was more uniform and hardness of the weld zone decreased sharply. However, tensile strength of welded joints was increased from 88.1 MPa for the 0.22 mm Nb foil to 150 MPa for the 0.40 mm Nb foil. It was found that thicker Nb foil could inhibit diffusion of Fe atoms towards the titanium alloy side, thus promoting the formation of Ti solid solution and a small amount of CuTi2 and eliminating FeTi. In addition, in both cases, Cu0.5Fe0.5Ti was found in the fusion zone of the titanium alloy side, which had an adverse effect on mechanical properties of welded joints.

Investigation of alloying effects on XRF parameters of 3d transition metals in Permendur49, Kovar and Ti50Co50 alloys

The alloying effects on X-ray fluorescence (XRF) parameters such as K shell fluorescence cross-sections (σKα, σKβ and σK), K shell average fluorescence yields (ωK) and K to L shell vacancy transfer probabilities (ηKL) of V, Ti, Fe, Co and Ni 3d transition metals in Permendur49 (Fe49Co49V2), Kovar (Fe54Ni29Co17) and Ti50Co50 alloys have been carried out by X-ray fluorescence studies. K X-ray intensity ratios of these 3d transition metals in both pure form and present alloys have been measured following excitation by 22.69 keV X-rays from a 10 mCi 109Cd radioactive point source. The characteristic K-X-ray spectra from samples were detected by a high-resolution Si(Li) solid-state detector. The investigated fluorescence parameters of present 3d transition elements in alloys indicate significant differences with respect to the pure metals. Consequently, the observed change of the investigated fluorescence parameters can be attributed to the delocalization and charge transfer phenomena between the 3d elements in alloys.

Interfacial microstructure and mechanical properties of diffusion-bonded joints of titanium TC4 (Ti-6Al-4V) and Kovar (Fe-29Ni-17Co) alloys

AbstractDiffusion bonding is a near net shape forming process that can join dissimilar materials through atomic diffusion under a high pressure at a high temperature. Titanium alloy TC4 (Ti-6Al-4V) and 4J29 Kovar alloy (Fe-29Ni-17Co) were diffusely bonded by a vacuum hot-press sintering process in the temperature range of 700–850 °C and bonding time of 120 min, under a pressure of 34. 66 MPa. Interfacial microstructures and intermetallic compounds of the diffusion-bonded joints were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The elemental diffusion across the interface was revealed by electron probe microanalysis. Mechanical properties of joints were investigated by micro Vickers hardness and tensile strength. Results of EDS and XRD indicated that (Fe, Co, Ni)-Ti, TiNi, Ti2Ni, TiNi2, Fe2Ti, Ti17Mn3 and Al6 Ti19 were formed at the interface. When the bonding temperature was raised from 700 to 850 °C, the voids of interface were reduced and intermetallic layers were widened. Maximum tensile strength of joints at 53. 5 MPa was recorded by the sintering process at 850 °C for 120 min. Fracture surface of the joint indicated brittle nature, and failure took place through interface of intermetallic compounds. Based on the mechanical properties and microstructure of the diffusion-bonded joints, diffusion mechanisms between Ti-6Al-4V titanium and Fe-29Ni-17Co Kovar alloys were analyzed in terms of elemental diffusion, nucleation and growth of grains, plastic deformation and formation of intermetallic compounds near the interface.

Delocalization and charge transfer studies of PERMENDUR49, KOVAR and Ti50Co50 alloys from relative K X-ray intensity ratios

Kβ-to-Kα X-ray intensity ratios of Fe, Ni, Co, Ti, V in pure form and PERMENDUR49 (Fe49Co49V2), KOVAR (Fe54Ni29Co17) and Ti50Co50 alloys were calculated in order to determine valence-electron structure of the samples. The obtained experimental Kβ-to-Kα X-ray intensity ratios were found to be 0.1252 ± 0.0025, 0.1256 ± 0.0027, 0.1298 ± 0.0026, 0.1327 ± 0.0028, 0.1346 ± 0.0027 for pure Ti, V, Fe, Co and Ni, respectively. These values of present pure metals have increased with the increment in atomic number of these pure metals. Also, although the obtained these values for metal atoms in PERMENDUR49, Ti50Co50 and for Fe, Ni metal atoms in KOVAR approximately compatible with respect to results same pure metals, the obtained experimental Kβ-to-Kα X-ray intensity ratios for Co in KOVAR are significantly different from the appropriate values for pure Co metal. Then, the valence-electron populations of these alloys were calculated in order to answer as to why the characteristic properties of the studied alloys are distinct from each other. The valence-electron structures of these metals in pure form and alloys were determined by comparison of the calculated Kβ-to-Kα X-ray intensity ratios with the results of multi-configurations Dirac-Fock (MCDF) calculations to understand the charge-transfer and/or delocalization phenomena of these alloys. The experimental n3d values were found to be 3.2694, 4.7905, 7.7941, 7.9618, 8.500 for pure Ti, V, Fe, Co and Ni, respectively. The calculated 3d and 4s, 4p electron population values for Ti, V and Co in pure form have shown very little deviation from the appropriate values of Ti in Ti50Co50 and Co, V in PERMENDUR49. Also, the deviation for these values is greater for Fe, Ni, in KOVAR, Co in Ti50Co50, Fe in PERMENDUR49 with respect to the corresponding values of pure metals and the largest deviation for these values is observed for Co metal atoms in KOVAR alloy. We have also calculated the weighted average numbers of 3d and 4s, 4p electrons per one atom as well as the superposition of 3d and 4s, 4p electrons, as obtained from pure metal values. While the obtained superposition 3d electron numbers of present alloys exhibit a decreasing tendency as S3d(Ti50Co50) < S3d(PERMENDUR49) < S3d(KOVAR), the obtained weighted average 3d electron numbers of present alloys has opposite tendency (A3d(KOVAR) < A3d(PERMENDUR49) < A3d(Ti50Co50)). The analysis show that valance-electron population of elements in these alloys indicated differences with respect to the pure metal and each other. These differences can be related to type of alloy elements, concentration of alloy elements, charge transfer and/or delocalization phenomena between of individual metal atoms in alloys. Also, the obtained results show that there are alloying effect on valence-electron population due to charge transfer and/or delocalization phenomena.

Thermal expansion of Fe-TM-B (TM=transition metal) bulk amorphous forming alloys

In order to clarify the potential of bulk amorphous alloys as a new class of structural materials with high dimensional stability, the thermal expansion and magnetic properties of amorphous Fe56Co7Ni7Zr10B20 doped with Cr have been investigated. Both the magnetic moment and the Curie point of amorphous Fe56Co7Ni7Zr10B20 were suppressed by Cr addition. The effect of Cr on the magnetic moment was well described by assuming a reduction of 4μB∕Cr atom. It was found that the Curie temperature can be controlled by Cr content over a wide range of 200K in the vicinity of room temperature. Although Invar characteristics were absent from the thermal expansion curves, amorphous Fe56Co7Ni7Zr10B20 doped with 1at.% Cr was found to exhibit a small coefficient of thermal expansion comparable to Kovar alloys, indicating that bulk amorphous alloys may have a potential for commercial low expansion materials.

Degradation behaviour of borosilicate glass: some studies

PurposeTo study the degradation behaviour of borosilicate glass, which is suitable for hermetic sealing with Molybdenum and Kovar (Fe/Co/Ni) alloys, as a function of concentration and temperature in both acidic and alkaline media for long durations, up to 160 h.Design/methodology/approachThe degradation (weight loss in mg/cm2 of the glass sample) was determined by immersing the glass sample in HCl and NaOH solutions at different temperatures for different periods extending up to 300 h. The damage to the glass surface was seen under an optical microscope and the chemical species on the corroded surface were identified by electron spectroscopy for chemical analysis.FindingsThe borosilicate glass, having the nominal composition 0.70 SiO2, 0.039 Na2O, 0.028 K2O, 0.21 B2O3, 0.01 Al2O3 was synthesized by melt and quench techniques. Degradation (corrosion) behaviour of this glass was investigated by immersing glass samples in 5 and 10 per cent HCl and 5 per cent NaOH solutions at different temperatures up to 90°C, for different periods and measuring dissolution rate (weight loss in mg/cm2 of the sample). Dissolution rates were found to be 5.47 mg/cm2 and 46.77 mg/cm2 in 5 per cent NaOH at 60 and 90°C, respectively, whereas they were comparatively low (2.59 and 5.80 mg/cm2 at 60 and 90°C, respectively, in 5 per cent HCl medium) after 160 h of total immersion period. The plot of dissolution rates against the temperatures showed the nonlinear behaviour at higher temperatures, probably due to the change in mechanism of corrosion. XPS studies exhibited the chemical species on the corroded surfaces. The optical microscopy of the corroded surface revealed that the corrosion mechanisms were different in acid and alkali media.Research limitations/implicationsThe degradation behaviour of borosilicate glass having a specific composition has been investigated as a function of concentration and temperature in both acid and alkaline media. The mixed alkali effect on the degradation behaviour may be studied by varying relative amount of Na2O and K2O in the glass composition.Practical implicationsThe glass composition under the present study has been used for fabrication of matched type glass‐to‐metal (GM) seals with kovar alloy. In this respect the present study is significant in deciding the environmental conditions for its use.Originality/valueThe degradation behaviour of borosilicate glass having alkali and alkaline earth metal oxides has been investigated as a function of concentration and temperature in both acid and alkali media. The findings in this paper have the potential implications in deciding the environmental conditions for use of GM seals fabricated using this glass.

Interfacial reaction in anodically bonded joints during receiving reverse voltage

A voltage in the opposite direction to that of anodic bonding was applied to anodically bonded joints of Fe and Kovar alloys to borosilicate glass at a temperature at which thermal diffusion of the alkali ions in the glass was activated. Disjunction of the bond interfaces occurred in the Kovar/glass joints. On the other hand, the Fe/glass joints retained cohesion of their bond interface after a long application of the reverse voltage. In an as-bonded Kovar alloy/glass joint, a reaction layer of a crystalline Fe oxide was found at the bond interface. The content of Fe in Kovar alloy adjacent to the bond interface decreased. The oxide layer was formed by the reaction between Fe from the Kovar alloy and O from the alkali ion depletion layer in the glass. The reaction layer in an Fe/glass joint was of amorphous Fe–Si oxide. This layer was much thicker than the reaction layer in the Kovar/glass joint. After application of the reverse voltage, Fe in the amorphous oxide layer in Fe/glass was substituted by alkali ions. It was suggested that this process mitigated concentration of alkali ions at the bond interface and prevented disjunction of Fe/glass joints by the reverse voltage.