Effect of Solution Heat Treatment on Dendritic Segregation and Creep Strength of Ni-Base Single Crystal Superalloy TMS-238

To reduce the cost of solution heat treatment process Ni-base single crystal superalloy TMS-238 containing Re and Ru, quantitative analysis of dendrite-interdendrite segregation of alloying elements under various solution heat treatment conditions were conducted, and influence on high-temperature creep strength were investigated. In this study, we defined the solution rate Rsol (= (1 − Vf,e) × 100%, where Vf,e is volume fraction of eutectic γ′ that precipitates in the final solidification zone during casting) as a parameter to reveal the microstructure homogeneity. The Rsol values were 71%, 97%, 99%, 100% for solutioning at 1250°C/20 h, 1320°C/5 h, 1320°C/20 h and 1335°C/20 h, respectively. Furthermore, it was confirmed that Re and W segregated in the dendrite core area and γ′ formers whereas Ta and Al segregated in the interdendrite. The magnitude of these segregations decreased as the solution temperature and time increased, and eventually the structure became almost homogeneous for solutioning at 1335°C for 20 hours. Additionally, creep test results indicate that Larson-Miller parameter (LMP) at 800°C–735 MPa, 900°C–392 MPa and 1000°C–245 MPa creep conditions show the same values for Rsol ≥ 97%. On the other hand, under 1100°C–137 MPa creep condition, LMP decreased as the Rsol decreased. A factor analysis of creep rupture properties suggested that the degradation of LMP under 1100°C–137 MPa was caused by the decrease of Re content and γ/γ′ lattice misfit in the interdendritic region.