Effects of tantalum on microstructure and mechanical properties of cast IN617 alloy

Effects of Ta on microstructure and mechanical properties of cast IN617 alloy were investigated. The results showed that the addition of Ta increased size and area fraction of MC carbides, which was in agreement with the prediction of thermodynamic calculation, rather than influencing as-cast microstructure features such as grain size, secondary dendrite arm spacing, and carbide distribution. Ta-doped cast microstructure showed that Ti (C, N) nitride-carbide was substituted by MC carbide and at the same time, M6C and MC tended to form a eutectic structure. The lattice constant of the matrix in a heat treatment condition increased from 3.593 to 3.601 nm and the high temperature tensile strength also improved, when Ta content are from 0 to 2.0 wt%. It was attributed to the solution of Ta with solid solution strengthening in the matrix. However, when the Ta level exceeded 1.0 wt%, the ductility and fracture toughness significantly reduced. It was believed that remained Ta-rich MC carbides in the supersaturated matrix accelerated the initiation and propagation of cracks. Therefore, the optimized Ta content in cast IN617 alloy is suggested to below 1.0 wt% for balanced properties.