In this research, the microstructure of the joint of nickel-based superalloy X-750 to 304 austenitic stainless steel with BNi2 interlayer by transient liquid phase method has been investigated. For this purpose, Bonded specimens were cut in the dimensions of 10×10×5 mm by wirecut. In order to joining, Samples were taken after being placed in the fixture in a vacuum furnace under vacuum 10-2 and a heating rate of 10 °C / min. The bonding was performed at two temperatures of 1050 and 1100 °C in five times of 5, 15, 30, 45 and 60 minutes in a vacuum atmosphere and then cooled in furnace and removed from the furnace. For microstructural study, the samples were cross-section and the samples were sanded, polished and etched for metallography. In order to study the microstructure of the scanning electron microscope and the chemical composition of the phases formed in different areas of the bonding, a scanning electron microscope equipped with energy separation analysis was used. In order to eva‎luate the mechanical properties of the joint, the shear and microhardness test of the joint was used. The bonding area of samples in which isothermal solidification is not complete was showed three areas of isothermal solidification zone, athermal solidification zone and the diffusion affected zone. The results of microstructural studies of the samples were showed that isothermal solidification at 1050 °C and 45 minutes and at 1100 °C in 30 minutes. At any temperature, at time less than the stated time, athermal solidification occurred due to insufficient bonding time. The chemical composition of the isothermal solidification zone is a nickel solid solution, while in samples with athermal solidification, its chemical composition indicates the formation of nickel and chromium rich borides. Also, diffusion affected zone is formed only on of 304 austenitic stainless steel base metal side, the chemical composition of this zone shows the formation of boride compounds. Mechanical tesets showed that the shear strength of the joint increases with increasing bonding time. On the other hand, the shear strength at the bonding temperature of 1100 °C at all times was higher than the shear strength of the bonded specimens at 1050 °C. Fractography images of specimens showed ductile fracture characteristics in the specimens where the isothermal solidification was complete. The results of microhardness show that the highest amount of microhardness is related to the samples that have an athermal solidification zone in the bonding zone.

مسعود عطاپور، مرتضي شمعانيان اصفهاني

علي شفيعي، عباس بهرامي