Effect of directional solidification process on microstructure and stress rupture property of a hot corrosion resistant single crystal superalloy

Abstract

The influences of different directional solidification processes, i.e., the high rate solidification (HRS) and liquid metal cooling (LMC), on microstructure and stress rupture property of DD488 alloy were investigated. The DD488 alloy was directional solidified by both HRS and LMC processes. The microstructure and stress rupture properties at 980 oC/250 MPa were investigated by using optical microscopy (OM), scanning electron microscopy (SEM), electron microprobe analyzer (EPMA), transmission electron microscopy (TEM) and stress rupture testing. The results indicated that the LMC process refined the primary dendrite arm and decreased the microporosity volume fraction and solidification segregations of Cr and Co in as-cast DD488 alloy. After standard heat treatment of 1,260 oC/4 h, AC (air cooling) + 1,080 oC/4 h, AC + 870 oC/24 h, AC, the γ′ morphology in LMC alloy was more cuboidal than that in HRS alloy, and the γ′ volume fraction of LMC alloy was higher than that of HRS alloy. The stress rupture life at 980 oC/250 MPa of HRS alloy was 76.8 h, and it increased to 110.0 h in LMC alloy. The LMC process increased the stress rupture life due to the higher γ′ volume fraction, more perfect rafting structure and finer interfacial dislocation networks.