High Temperature Characteristics of Coating Layer on Alloy 617 after Al-pack Cementation

Abstract

Alloy 617 are well known high temperature materials used in turbine blades, nuclear reactors, and high temperature gas-cooled reactors (HTGRs) because of their excellent strength, creep-rupture strength, stability, and oxidation resistance at high temperatures. Among the alloying elements in Alloy 617(Ni-22.6Cr- 9.53Mo-13.1 Co-1.07 AI-0.4Ti-0.09C,wt.%)， chromium (Cr) and aluminum (Al) can form an oxide surface layer that protects against degradation at high temperature. The protective oxide layer can be distinguished as two regions; the extemal oxide scale on the surface and internal oxide on the grain boundaries. The extemal oxide scale and intemal oxide consists of Cr203 and Al203, respectively. However, extemal oxide layer was thinned of scale by Cr03 evaporation and delamination. In this work, the oxidation behavior of Alloy 617 after Al-pack cementation was investigated by exposure to air for 480 h at 950oC. Al-pack cementation was carried out at 900oC for 1 h in Ar using an AI2O3 crucible containing the specimen and a powder mixture of Al: AI2O3: NH4Cl = 15:83:2 (wt.%). Thickness of coating layer was formed approximately 35 um. The phase of coating layer was analyzed to be NiAl and Ni2Al3 by XRD and EPMA. After aging, thickness of coating layer was increased from 35 um to 80 um. The phase of coating layer was transformed from NiAl and Ni2Al3 to fully NiAl. The Coating layer consisted of NiAl and inter-diffusion zone. Analysis of second phase in coating layer was carried out by SEM and EPMA. The hardness of coating layer was measured by nanoindentation. The oxidation resistance during aging at 950oC has improved.