Effect of Holding Time on Microstructure and Mechanical Properties of Dissimilar'-Strengthened Superalloys TLP Joint

To achieve efficient connectivity of dissimilar gamma'-strengthened superalloy, transient liquid-phase (TLP) diffusion bonding of 718Plus and Ni3Al-based superalloys is carried out using BNi-2 interlayer at a constant temperature of 1100 degrees C with holding times of 3, 15, 30, and 60 min. It is shown in the results that when the holding time is insufficient, isothermal solidification transforms into athermal solidification. Due to the enrichment of B and Cr elements in liquid phase, ternary eutectic consisting of gamma, Ni3B, and CrB is formed at the joint center. Ternary eutectic with higher hardness (approximate to 600 Hv) is poorly bonded to the matrix, which can easily become crack initiation location and significantly deteriorated the tensile properties. In addition, microhardness of the joint becomes more uniform and tensile strength gradually increases due to the reduction of eutectic with holding time increasing. Isothermal solidification completes and a joint without eutectic structure is formed at 1100 degrees C for 60 min. Diffusion of Al atoms leads to the formation of gamma' with gradient size and tensile strength is comparable to that of base metal which is about 690.93 MPa.