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Joining of stainless steel (AISI 304L) to hot isostatically pressed alumina (HIP-Al2O3) using the brazing alloy 72Ag-28Cu was investigated. The microstructural characterization at various stages of joining, including metallization, annealing of overlaid Ni coating, and brazing, was comprehensively evaluated. The interface structure and the growth of phases were analyzed with optical microscope, scanning electron microscope, and electron probe microanalyzer (EPMA). Additionally, the leak tightness of these joints was assessed using a He-leak detector. Experimental results indicated the development of the manganese aluminate spinel (MnAl2O4) layer at the metallizing stage, which penetrated into HIP-Al2O3. The Ni overlaid coating further resulted in the formation of the Ni(Mo) solid solution layer followed by the Mo-rich phase. During the solid-state reaction and subsequent brazing cycle, the growth of the spinel layer close to HIP-Al2O3 was not adversely affected. The microstructure of the brazed joint was complex. It showed a eutectic structure within the brazed zone and a thin layer of Mo-rich, Ni-rich phases close to HIP-Al2O3. Increasing the brazing time resulted in the excessive growth of the thin layer that seriously affected the leak tightness of the joint. [ABSTRACT FROM AUTHOR]

Diffusion bonding was carried out to produce transition joints between commercially pure titanium and 304 stainless steel at a temperature of 800°C for different times ranging from 30 to 180 min. in steps of 30 min under load in vacuum. The diffusion couples thus produced were studied using optical microscopy, scanning electron microscopy, and electron probe microanalysis to characterise the reaction layers formed in the diffusion zone. The chemical compositions of these layers indicate that intermetallics like σ phase, Fe2 Ti, Cr2 Ti, χ phase, FeTi, β-Ti, and Fe2 Ti4 O are formed in the reaction zone. The presence of these intermetallic compounds was also confirmed by the X-ray diffraction technique. Maximum bond strength of ~242 MPa was obtained for diffusion welded joints processed for 120 min. At this joining time, the plastic collapse of the surface asperities reaches near completion, favouring the interdiffusion of chemical species. Reduction in the bond strength of the transition joint processed for 180 min is due to the formation of a large volume fraction of voids in the reaction zone. Under tensile loading, failure takes place through α-Fe + χ phase mixture for transition joints processed in the time range of 30 – 90 min and through β titanium for joining times greater than 120 min. [ABSTRACT FROM AUTHOR]

Diffusion bonding between porous nickel mesh and stainless steel rings has been carried out in the temperature range of 700–900°C under uniaxial pressure. The specimens have been joined directly and also using Ni as an interlayer. The microstructure of the diffusion-bonded joints has been characterised by optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The concentration profiles are smooth and do not show the formation of any intermetallic compound. The Boltzmann–Matano method has been used to evaluate the effective interdiffusion coefficients. The interdiffusion coefficients are of the order of 10-15 m2s-1 and these values increase with increase in Ni concentration. The microhardness values near the diffusion zone vary in the range of 185–265 HV(50 g). The specimens bonded at optimised conditions, indicate that the shear strength is superior to the parent porous nickel. [ABSTRACT FROM AUTHOR]

Solid-state diffusion reaction between Zr and Al was studied in the temperature range 838–898 K employing bulk diffusion couples. The intermetallic phase ZrAl3 forms in the diffusion zone in all the diffusion couples in planar morphology. A second layer of Zr2Al3 forms in addition, at and above 873 K in the diffusion zone. The formation of ZrAl3 as the first phase was rationalised on the basis of the Modified Effective Heat of Formation (MEHF) model. The driving forces for formation of Zr2Al3 and ZrAl2 at the Zr/ZrAl3 interface were evaluated. The appearance of Zr2Al3 in preference to ZrAl2 at higher temperatures is attributed to the higher driving force for its formation at the Zr/ZrAl3 interface. The temperature dependence of growth of ZrAl3 phase layer was determined. The activation energy (Qk) for growth of ZrAl3 is 188.2±2.5 kJ/mol and the pre-exponential factor (k0) is 2.9–0.35+0.55x104ms12. [Copyright &y& Elsevier]

Our previous work revealed that significant enhancement in sp3-carbon content of amorphous carbon films could be achieved when grown on nanocrystalline palladium interlayer as compared to those grown on bare silicon substrates. To find out why, the nature of interface formed in both the cases has been investigated using Electron Probe Micro Analysis (EPMA) technique. It has been found that a reactive interface in the form of silicon carbide and/silicon oxy-carbide is formed at the interface of silicon/amorphous-carbon films, while palladium remains primarily in its native form at the interface of nanocrystalline palladium/amorphous-carbon films. However, there can be traces of dissolved oxygen within the metallic layer as well. The study has been corroborated further from X-ray photoelectron spectroscopic studies.