Effects of Ta/Nb/Ti addition on the microstructure and mechanical properties of novel CoNi-based superalloys fabricated by directed energy deposition

Abstract

The influence of alloying on the microstructure, particularly the phase precipitation behavior and the volume fraction of the γʹ phase, is critical for the development of precipitation-strengthed cobalt-based superalloys. In the context of additive manufacturing, it is also essential to consider how alloying elements affect the formability of these alloys. In this study, three developed CoNi-based superalloys, named as AlTa, AlTaNb and AlTaTi, were fabricated using laser directed energy deposition, and followed by homogenization and aging treatment. The effect of alloy composition on additive manufacturing formability, microstructure and mechanical properties was analyzed. The AlTa and AlTaNb specimens show good additive manufacturing formability, but the AlTaTi specimen appears several long cracks along deposition direction. The addition of Ti is not conducive to the additive manufacturing formability of CoNi-based superalloys. The volume fraction of the γ′ phase in the three heat-treated specimens is 65∼70 %, and the average size is about 200 nm. The AlTa specimen maintains its tensile strength without softening from room temperature to 900 °C, possessing an excellent yield strength of 471 MPa at 900 °C. The AlTaNb and AlTaTi specimens achieves higher tensile strength at room temperature due to the strengthening effect of Nb and Ti elements, but underwent premature intergranular fracture during high-temperature tensile testing due to the insufficient grain boundary strength. Futher grain boundary strengthening should be considered for them.