Influence of interface microstructure on the strength of the transition joint between Ti-6Al-4V and stainless steel

Solid-state diffusion bonding of Ti-6Al-4V and type 304 SS was investigated in the temperature range of 750 °C to 950 °C, under a uniaxial load for 5.4 ks in vacuum. The diffusion bonds were characterized using light and scanning electron microscopy. The scanning electron microscopic images in backscattered mode show the existence of different reaction layers in the diffusion zone. The composition of these layers was determined by energy-dispersive X-ray spectroscopy (EDS) to contain the α-Fe, χ, λ, FeTi, β-Ti, and Fe₂Ti₄O phases. The presence of these intermetallics was confirmed by X-ray diffraction. The bond strength was evaluated, and the maximum tensile strength of ∼342 MPa and the maximum shear strength of ∼237 MPa were obtained for the diffusion couple processed at 800 °C due to the finer width of the brittle intermetallic layers. With a rise in joining temperature, the bond strength drops owing to an increase in the width of the reaction layers. The activation energy and growth constant were calculated in the temperature range of 750 °C to 950 °C for the reaction products. The χ phase showed the fastest growth rate. A fracture-surface observation in a scanning electron microscope (SEM) using EDS demonstrates that failure takes place mainly through the β-Ti phase for the diffusion couples processed in the aforementioned temperature range.     

Discussion of “Microstructure and Phase Constituents in the Interface Zone of Mg/Al Diffusion Bonding”<Superscript>⋆</Superscript>

In a recent article, Liu et al. described the formation of phases during the diffusion bonding of Al to Mg at temperatures in the range 450 °C to 490 °C and pressures in the range 0.08 to 0.10 MPa. They present micrographs of the phases in the weld zone resulting from welding at 480 °C and 0.08 MPa for 60 minutes and a concentration-distance plot resulting from welding at 470 °C and 0.08 MPa for 60 minutes. However, the mechanism they propose for the formation of the phases in the weld zone is incorrect. It is the purpose of the present discussion to provide a correct analysis of the formation of the phases in the weld zone. ^⋆PENG LIU, YAJIANG LI, HAORAN GENG, JUAN WANG, HAIJUN MA, and GUOLIN GUO: Metall. Mater. Trans. B, 2006, vol. 37B, pp. 649–54. Discussion submitted September 21, 2006.

---

Oxidation behavior of niobium aluminide intermetallics protected by aluminide and silicide diffusion coatings

The isothermal and cyclic oxidation behavior of a new class of damage-tolerant niobium aluminide (Nb₃Al-xTi-yCr) intermetallics is studied between 650 °C and 850 °C. Protective diffusion coatings were deposited by pack cementation to achieve the siliciding or aluminizing of substrates with or without intervening Mo or Ni layers, respectively. The compositions and microstructures of the resulting coatings and oxidized surfaces were characterized. The isothermal and cyclic oxidation kinetics indicate that uncoated Nb-40Ti-15Al-based intermetallics may be used up to ∼750 °C. Alloying with Cr improves the isothermal oxidation resistance between 650 °C and 850 °C. The most significant improvement in oxidation resistance is achieved by the aluminization of electroplated Ni interlayers. The results suggest that the high-temperature limit of niobium aluminide-based alloys may be increased to 800 °C to 850 °C by aluminide-based diffusion coatings on ductile Ni interlayers. Indentation fracture experiments also indicate that the ductile nickel interlayers are resistant to crack propagation in multilayered aluminide-based coatings.