Effects of Boron Addition on the Microstructure and Mechanical Properties of γ′-Strengthened Directionally Solidified CoNi-Base Superalloy

The effects of B addition on microstructure and mechanical properties of a γ′-strengthened CoNi-base superalloy are investigated. The addition of B leads to a substantial increase in the volume fraction of both the eutectic structure and borides. The CoNi-base alloy shows a high solubility limit for the element B. Borides become noticeable in the area surrounding the eutectic structure after the B level exceeds 0.46 at.%. It is found that the compression property and stress rupture life of the 4W2Ta alloys exhibit an initial rise followed by a subsequent drop as the B content gradually increases from 0.08 to 0.78 at.%. The 4W2Ta0.46B alloy demonstrates the most excellent high-temperature strength and stress rupture life, revealing that a moderate amount of B in the alloy noticeably enhances its mechanical properties by enhancing the grain boundary cohesion.