Creep and rupture of an ods alloy with high stress rupture ductility
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I.
INTRODUCTION
A new oxide dispersion strengthened (ODS) alloy made by a powder metallurgical technique different from mechanical alloying has been developed.~ The alloy, like some other ODS alloys, 2'3 is characterized by 3/precipitation strengthening at low and intermediate temperatures and by oxide dispersion strengthening at all temperatures. One property that distinguishes this ODS alloy, YDNiCrA1, from others such as Inconel* alloys MA 754 and MA ~ 0 0 0 E TM is its *Inconel is a trademark of the INCO family of companies.
high stress rupture ductility. We have examined the unusual ductility along with the other creep and stress rupture properties. Two temperature regimes were chosen for study. One, around 760 ~ is below the T' solvus where T' is present, and one is above the solvus temperature around 1093 ~ where the oxide particles but not the T' precipitates contribute to the creep strength. The creep and stress rupture properties of the alloy at these two temperatures are compared to the creep and stress rupture properties of other ODS alloys and superalloys. In addition to the rupture ductility and fractographic observations, creep curves, the stress dependencies of the creep rates, the apparent activation energies at the different test temperatures, and crept microstructures are presented and discussed with respect to the resisting stress model 27'2s,29 of creep. II.
MATERIAL CHARACTERIZATION AND EXPERIMENTAL PROCEDURE
The chemical composition of YDNiCrA1 investigated in wt pet is 16Cr-4.2AI (as soluble aluminum)-0.19Fe-0.4700.015C-0.026N-balance Ni, with a total volume pet of dis-
persed oxides of 2.5 pet to 3 pet, of which approximately a third is yttria and the balance predominately alumina. 5 YDNiCrA1 is made by a powder process where yttria powder is mixed with elemental nickel powder in an attritor under flowing air. ~ The mixed powder is first hydrogen reduced before it is blended with a master alloy of premilled chromium and aluminum. This process is reported to be often faster and to provide greater powder yields than mechanical alloying because the flowing air prevents the powder particles from welding to either the steel balls in the attritor or the other powder particles. The powder mix was canned, evacuated, and consolidated by hot extrusion. Following the extrusion the material was further hot worked, with no dynamic recrystallization, by rolling parallel to the extrusion direction, and then a recrystallization heat treatment was given. The material was put into a furnace at 1204 ~ and as soon as the material temperature reached 1204 ~ the temperature was slowly raised to 1343 ~ over the span of one hour. Once the furnace temperature had reached 1343 ~ it was held there for one hour, after which the material was air cooled. This consolidation and thermomechanical treatment resulted in a preferred orientation of (100) along the extrusion axis. In order to examine the macrostructure of the alloy, polished metallographic specimens were electrolytically etched for 30 to 45 seconds at amb
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