High-temperature low-cycle fatigue of an iron-base oxide-dispersion strengthened alloy: Grain structure effects and life
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I.
INTRODUCTION
IN recent years, the demand for high-temperature materials by different industrial sectors has increased. I~] Ironbase oxide-dispersion strengthened (ODS) alloys are of increasing interest for technical applications because of their capability of retaining strength at very high temperature. The successful use of such alloys permits higher operating temperatures and (~onsequently enhanced thermal efficiencies of energy conversion processes. Monotonic mechanical properties of MA* 956, in shortterm tension and compression and in creep, have been published by several authors. 12-5] Detailed investigations with the recently developed ODS alloy PM** 2000 *MA is a trademark of INCO Alloys International, Inc., Huntington, WV. **PM is a trademark of PM Hochtemperatur-Metall GmbH, Lechbruck, Germany.
showed that it possesses high strength, even at 1200 ~ combined with acceptable ductility) 6,7'8] PM 2000 also has an outstanding resistance to oxidation due to a high aluminum content, which forms a dense layer of A I 2 0 3 at the exposed surface.l~.81 In contrast to ~/-strengthened nickel-base ODS superalloys, 19"~~ little work has been done to characterize the fatigue behavior of iron-base ODS alloys. While highcycle fatigue crack growth of MA 956 has been investigated at 1000 ~ its low-cycle fatigue (LCF) performance is known only for o n e strain amplitude at relatively high frequency under fully reversed bending. 114i Test temperatures were less than 980 ~ which is well below the envisaged operating temperatures of above 1000 ~ The purpose of this research was, therefore, to thoroughly investigate the high-temperature LCF and creep-fatigue performance of PM 2000 at a typical operating temperature. The thermomechanical processing of this material can V. BANHARDT and M. NADER, Ph.D Students, are with MaxPlanck-lnstitut fuer Metallforschung, and E. ARZT, Professor, is with the Max-Planck-lnstitut fuer Metallforschung and the University of Stuttgart, D-70174 Stuttgart, Germany. Manuscript submitted February 18, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
be controlled to result in different coarse-grained microstructures. IjS] It is possible to achieve grains with average lengths between less than 30 ~m and several centimeters. This is in marked contrast to conventional superalloys, for which LCF endurance data have consequently been reported only for grain size variations within one order of magnitude. [~6-~91The objective of this investigation was to correlate the thermomechanical treatment, which leads to these different microstructures, with the high-temperature fatigue performance and to identify the governing mechanisms of failure.
II.
MATERIAL
The iron-base ODS alloy PM 2000 is produced by PM Hochtemperatur-Metall GmbH in Lechbruck, Germany. It has the following chemical composition (in weight percent): 19Cr, 5.5A1, 0.5Ti, 0 . 5 Y 2 0 3 , and balance Fe. The microstructure consists of a ferritic body-centered cubic matrix with chromium and aluminum in solid solution, additionally strengthened
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