Microstructure and mechanical properties of Ti-Ti2AlNb interface

Diffusion bonding of Ti2AlNb alloy using pure titanium (Ti) foil as an interlayer was carried out on superplastic forming and diffusion bonding special equipment by gas pressure loading method. The microstructure of Ti-Ti2AlNb interface was observed using scanning electron microscope and energy-dispersive spectrometer while the mechanical properties of the joints were evaluated by shear test. The results show that the thickness of Ti foil interlayer has a great influence on the microstructure and shear strength of the interface diffusion region. When the thickness of the intermediate layer is thin (25 µm), Ti, aluminum (Al), and niobium (Nb) elements are fully diffused with uniform element distribution through the diffusion region. The diffusion layer region presents uniform, fine, and disordered lamellar α-Ti + β-Ti dual-phase structure with high shear strength. When the thickness of Ti foil interlayer is thick (50 µm), the distribution of Al elements is relatively uniform through the diffusion region due to its smaller radius and faster diffusion speed, and Ti and Nb elements present gradient distribution from the middle to both sides. The diffusion layer region presents a coarse and long strip shape α-Ti + β-Ti dual-phase structure in the middle part and a fine needle-like or irregular α-Ti + β-Ti dual-phase structure in both side parts, with slightly lower shear strength. Temperature has a great influence on the microstructure and mechanical properties of the diffusion bonding joints. The diffusion region presents a black α-Ti strip area in the middle part with the width of about 10 µm at lower temperature (910°C) with poorer property, due to the grain growth of the parent metal, the property is slightly poorer when the temperature is too high (960°C), and the optimal temperature is 930°C with a higher shear strength.

Effect of Holding Time on Microstructure and Mechanical Properties of Diffusion-Bonded Mg1/Pure Ag Foil/1060Al Joints

Mg1 and 1060 Al were diffusion-bonded by using pure silver foil under different holding times (10 min~120 min). The interface of the joint consists of Mg-Ag diffusion zone, Ag foil interlayer and Ag-Al diffusion zone. The distributions of Mg, Ag and Al show ladder-like distributions at the interface of the joints. When the holding time is below 90 min, silver foil has impeded the inter-diffusion of Mg and Al. When the holding time is beyond 90 min, the brittle eutectic Mg-Al intermetallic compounds (IMCs) cannot be avoided. Mg3Ag and MgAg intermetallic compounds formed on Mg base side. Ag2Al intermetallic compound grew on Al base side. The thicknesses of Mg3Ag, MgAg and Ag2Al increased linearly with the increasing holding time, which is contrary to that of the silver foil. The growth rate relationship of the formed IMCs is MgAg > Ag2Al > Mg3Ag according the slope values of the fitted lines. The hardness sharply increased at the interface because of the formation of IMCs. The maximum hardness values of three IMCs Mg3Ag, MgAg and Ag2Al are 287.5 HV, 196.5 HV and 175.7 HV respectively. The hardness of each IMC layer increased with the extension of holding time. The shear strength of the joints decreased from 10.5 MPa to 4.6 MPa with the rising holding time.