Improvement for creep strength of a second-generation single crystal superalloy by design of heat treatments

Design of heat treatments is related to the key technology for development of nickel-based single crystal superalloys (Ni-SXs). Based on the full understanding of the solidification characteristics, this work applies optimization design of heat treatments for a second-generation Ni-SX. Microstructure evolution and creep properties are compared in the material under conventional/standard (Std.) and optimized (Opt.) treatments. For the Std. sample, strong dendritic segregations determine inconsistent microstructure evolution in the dendritic (D) and interdendritic region (ID), while the latter serves as weak area to have the prior microcrack initiation, damaging overall performance of the alloy. The Opt. treatment applies higher homogenization temperature, leading to overall reduced segregations, while not inducing incipient melting. A lower temperature of first-step ageing is used to lower the size of γ′ particles. These help to form the more uniform microstructure in dendritic and interdendritic region and relieve the inconsistent microstructure evolution. The balanced local strength makes ID no longer as the weak area, thus restricting microcrack initiation. Great improvement of high temperature and low stress property is obtained by this progress, leading to the pronounced increase of creep rupture life under 1100 °C /140 MPa.