Common Mechanical Properties of Diffusion Bonded Joints and Their Corresponding Microstructure Features

The mechanical properties of diffusion bonded joints embodied from 29 different kinds of alloys obtained from experiments (37 specimens) or literature (194 specimens) were analyzed and compared with their corresponding base alloys. The results indicated that toughness was most sensitive to the bonding quality as only 7.8% of the joints had a relative (ratio between the joint value and base alloy value) impact toughness of higher than 0.9, where the percentage of joints with a relative elongation (area reduction) and a tensile strength of higher than 0.9 was approximately only 21.9% and 48.4%, respectively. The fatigue and high-temperature mechanical properties of joints were significantly lower than those of the base alloys. Microstructure analysis revealed that this characteristic of "high strength-low toughness" resulted from defects of the joint, which included not only the widely accepted interfacial void defects but also the defects of the bond line and the crystallographic mismatch. Among these defects, the bond line primarily dispersed the plasticity and toughness of the joint. If the bond line was removed by post-bonding heat treatments or by insertion of an interlayer, the joint mechanical properties, particularly toughness, could potentially reach the value shown by the base alloy that had undergone the same heating process.