EFFECTS OF Hf ON HIGH TEMPERATURE LOW STRESS RUPTURE PROPERTIES OF A SECOND GENERATION Ni-BASED SINGLE CRYSTAL SUPERALLOY DD11

The effect of Hf on the as-cast, heat-treated microstructures and stress rupture properties under 1100 degrees C and 140 MPa was investigated in four second generation Ni-based single crystal superalloys DD11 with various levels of Hf (0 similar to 0.80%, mass fraction) additions. The results indicate that increasing Hf addition resulted in decreasing the solidus and liquidus temperatures, while it enhanced the volume fraction of (gamma+gamma') eutectic and MC carbide as well as solidification segregation. The number of micropores reduced significantly and the volume fraction of residual (gamma+gamma') eutectic and MC carbide increased after heat treatment as Hf content increased. Compared to the Hf-free alloy, the stress rupture life was observed to increase in the alloys with 0.40%Hf, but dropped in the alloy containing 0.80%Hf. Hf addition increased the elemental partitioning ratio of Re, Mo, Cr, resulting in increasing gamma/gamma' misfit and decreasing the spacing of gamma/gamma' interfacial dislocation networks. The solution strengthing effect was also improved with the enhanced concentration of Re, Mo and Cr in gamma phase in Hf-modified alloys. However, when the Hf content was 0.80% in DD11 alloy, the stress rupture properties was decreased obviously due to high volume fraction of residual (gamma+gamma') eutectic and MC carbide in heat-treated microstructures.