Effect of diffusion heat treatment on microstructure and mechanical properties of a boron-containing nickel-based superalloy

There is serious component segregation during the solidification of boron-containing nickel-based superalloys. The composition of the finally solidified eutectic structure and its surrounding areas is greatly different from that of the matrix, especially for some elements such as B, C, Cr and Mo, which leads to the formation of boride eutectic (BE). The BE has a negative effect on the mechanical properties and reduces the service life of the alloys and decreases the safety and reliability of aero engines. In this study, the morphology and component characteristics of BE structure in a nickel-based superalloy are determined by scanning electron microscopy and Energy Diffraction X-ray analysis. The main components of BE structure are B, C, Cr and Mo, and the initial melting temperature is between 1110 and 1120 degrees C. After diffusion treatment at 1110 degrees C for 10 h, the serious segregation elements are dissolved into the matrix, and the BE structure disappears. Tensile tests show that the BE structure is the weak link in the as-cast alloy and is the crack initiation source. After diffusion treatment, the BE structure dissolves into matrix, cracks initiate at carbides, and the intergranular fracture is transformed into dimple fracture, which improves the tensile strength and plasticity of the alloy.