Diffusion Bonding of Microduplex Stainless Steel and Ti Alloy with and without Interlayer: Interface Microstructure and Strength Properties

The interface microstructure and strength properties of solid state diffusion bonding of microduplex stainless steel (MDSS) to Ti alloy (TiA) with and without a Ni alloy (NiA) intermediate material were investigated at 1173 K (900 °C) for 0.9 to 5.4 ks in steps of 0.9 ks in vacuum. The effects of bonding time on the microstructure of the bonded joint have been analyzed by light optical microscopy and scanning electron microscopy in the backscattered mode. In the direct bonded joints of MDSS and TiA, the layer-wise σ phase and the λ + FeTi phase mixture were observed at the bond interface when the joint was processed for 2.7 ks and above holding times. However, when NiA was used as an intermediate material, the results indicated that TiNi₃, TiNi, and Ti₂Ni are formed at the NiA-TiA interface, and the irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ have been observed within the TiNi₃ intermetallic layer. The stainless steel-NiA interface is free from intermetallics and the layer of austenitic phase was observed at the stainless steel side. A maximum tensile strength of ~520 MPa, shear strength of ~405 MPa, and impact toughness of ~18 J were obtained for the directly bonded joint when processed for 2.7 ks. However, when nickel base alloy was used as an intermediate material in the same materials, the bond tensile and shear strengths increase to ~640 and ~479 MPa, respectively, and the impact toughness to ~21 J when bonding was processed for 4.5 ks. Fracture surface observations in scanning electron microscopy using energy dispersive spectroscopy demonstrate that in MDSS-TiA joints, failure takes place through the FeTi + λ phase when bonding was processed for 2.7 ks; however, failure takes place through σ phase for the diffusion joints processed for 3.6 ks and above processing times. However, in MDSS-NiA-TiA joints, the fracture takes place through NiTi₂ layer at the NiA-TiA interface for all bonding times.