The Effect of a Grain Boundary Pinning B2 Phase on Polycrystalline Co-Based Superalloys with Reduced Density
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The Effect of a Grain Boundary Pinning B2 Phase on Polycrystalline Co-Based Superalloys with Reduced Density LISA P. FREUND, ANDREAS STARK, ANDREAS KIRCHMAYER, NORBERT SCHELL, FLORIAN PYCZAK, MATHIAS GO¨KEN, and STEFFEN NEUMEIER The Al/W ratio was increased in a new polycrystalline c/c¢ Co-based superalloy in order to introduce a grain boundary pinning phase while at the same time reducing the density and increasing the oxidation resistance. The grain boundary pinning phase was identified as b-phase (B2-(Ni,Co)Al) by high-energy X-ray diffraction. The grain boundary pinning effect was confirmed to be effective up to temperatures of 1050 C by synchrotron radiation in-situ heating experiments. The precipitation of the b phase led to a reduced c¢ volume fraction thereby decreasing the high-temperature strength. The density was lowered to 8.52 g/cm3, so the specific strength was equivalent to the strength of the reference alloy up to 750 C. Due to the low c¢ volume fraction and the low W content, the creep resistance was also reduced compared with the reference alloy. The oxidation resistance, however, was improved by the formation of a thin, continuous Al-oxide layer during oxidation at 900 C for 50 hours. https://doi.org/10.1007/s11661-018-4757-2 The Minerals, Metals & Materials Society and ASM International 2018
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INTRODUCTION
TERNARY and quaternary alloys based on the Co-A-W system show good high-temperature strength and a pronounced flow stress anomaly.[1] Especially their high melting point, and, at the same time, fairly low c¢ solvus temperature make them interesting alloys that can be produced by casting and forging. Therefore, polycrystalline c/c¢ Co-based superalloys are regarded as potential candidates that could replace Ni-based superalloys in some high-strength applications at high temperatures. New polycrystalline c¢-hardened Co-based superalloys have been developed, which can compete with the commercially available Ni-based wrought superalloys Alloy 720Li and Waspaloy in terms of mechanical properties and oxidation resistance.[2] Especially, the creep properties were found to be superior to those of these two Ni-based superalloys. To further improve the
LISA P. FREUND, ANDREAS KIRCHMAYER, MATHIAS GO¨KEN, and STEFFEN NEUMEIER are with the Materials Science t Erlangen-Nu¨rnberg, Martensstraße 5, 91058 Erlangen, Germany. Contact e-mail: steff[email protected] ANDREAS STARK, NORBERT SCHELL, and FLORIAN PYCZAK are with the Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502 Geesthacht, Germany. Manuscript submitted March 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
new alloys, special emphasis was put on processing and inhibition of grain growth. Commercially available wrought Ni-based superalloys use either primary c¢ precipitates (Alloy720Li) or additional intermetallic phases, such as d phase (Alloy718) or g phase (A718Plus) for grain boundary pinning. Therefore, it was intended to also introduce an additional intermetallic phase for grain boundary pinnin
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