Effect of prior creep at 1365 K on the room temperature tensile properties of several oxide dispersion strengthened alloys |
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Authors: | J Daniel Whittenberger |
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Affiliation: | (1) NASA-Lewis Research Center, 44135 Cleveland, OH |
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Abstract: | A study was undertaken to determine if oxide dispersion strengthened (ODS) Ni-base alloys in wrought bar form are subject
to a loss of room temperature tensile properties after elevated temperature creep similar to that found in a thin gage ODS
alloy sheet. The bar products evaluated included ODS-Ni, ODS-NiCr, and advanced ODS-NiCrAl types. Tensile type test specimens
were creep exposed in air at various stress levels at 1365 K and then tensile tested at room temperature. Low residual tensile
properties, change in fracture mode, the appearance of dispersoid free bands, grain boundary cavitation, and/or internal oxidation
in the microstructure were interpreted as creep degradation effects. This work has shown that many ODS alloys are subject
to creep damage. Degradation of tensile properties occurs after very small amounts (≲0.2 pct) of creep strain; ductility being
the most sensitive property. The amount of degradation is dependent on the creep strain and is essentially independent of
the alloy system. All the ODS alloys which were creep damaged possessed a large grain size (>100 μm). Creep damage appears
to be due to diffusional creep which produces dispersoid free bands around boundaries acting as vacancy sources. Low angle
and, possibly, twin boundaries were found to act as vacancy sources. The residual tensile properties of two alloys were not
affected by prior creep parallel to the extrusion axis. One of these alloys, DS-NiCr(S), was single crystalline. The other
alloy, TD-Ni, possessed a small, elongated grain structure which minimized the thickness of the dispersoid free bands produced
by diffusional creep. |
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