Influence of Substituting W for Nb or Hf on Solidification Behavior of a Typical Co–Ni–Al–W Based Superalloy

Abstract

Understanding the effects of various elements on solidification behavior is crucial for designing the composition of γ’-strengthened Co-based superalloys and is fundamental for controlling the as-cast structure and formulating subsequent heat treatment processes. This research investigated the effects of replacing 1 at.% W with 1 at.% Nb or Hf elements on the solidification behavior of Co–Ni–Al–W-based superalloys. The findings revealed that substituting W with Nb and Hf resulted in a notable decrease in both the solidus temperature (T_S) and liquidus temperature (T_L). Specifically, the substitution of W with Nb lowered T_S from 1353 °C to 1332 °C and T_L from 1383 °C to 1368 °C, while replacing W with Hf decreased T_S from 1353 °C to 1330 °C and T_L from 1383 °C to 1366 °C. Moreover, both Nb and Hf element are positive segregation element, while Nb decreases and Hf increases W segregation, respectively. During the final solidification stage, the substitution of W with Nb resulted in the formation of eutectic (γ + γ’), Co₃Ta, and a small amount of μ-Co₇Nb₆ phase, while replacing W with Hf resulted in the formation of the Laves phase and β-CoAl phase. The solidification paths of the three alloys were confirmed based on the result of differential scanning calorimetry, isothermal solidification experiment and Thermo-calc simulation. These results offer a theoretical basis for the composition design and optimization of heat treatment processes for Co–Ni–Al–W-based superalloys.