Crack initiation and propagation during high-temperature fatigue of oxide dispersion-strengthened superalloys
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INTRODUCTION
ALLOYSstrengthened by a fine, homogeneous dispersion of oxide particles represent a relatively new class of materials intended for critical high-temperature applications, such as gas turbines. Among the important material properties required for such applications are excellent creep strength and corrosion resistance. The superior creep strength of dispersion-strengthened alloys at very high temperatures and relatively low stresses in comparison with dispersion-free alloys has motivated a great deal of study of the creep behavior of these materials./~,2'3] As a consequence, the mechanisms responsible for the improvement of creep resistance, obtained by introducing a fine dispersion, are now reasonably well understood, t41 Certain applications, such as gas turbine blades and vanes, require, in addition to creep strength, adequate resistance to high- and low-cycle fatigue (HCF and LCF, respectively). The fatigue behavior of dispersion-strengthened alloys is much less well documented, and in particular, the role of the dispersion in determining the high-temperature fatigue strength is only poorly understood. The current study of the hightemperature cyclic behavior of the oxide dispersionstrengthened (ODS) superalloys INCONEL* MA 6000 *INCONEL MA 6000 and MA 754 are trademarks of Inco Alloys International, Inc., Huntington, WV.
and MA 754 has been undertaken in order to determine the mechanisms leading to failure under these conditions
D.M. ELZEY, formerly with Max-Planck-lnstitut for Metallforschung, is Research Assistant, Department of Materials Science, University of Virginia, Charlottesville, VA 22901. E. ARZT, Professor of Materials Science, is with University of Stuttgart and Max-Planck-Institut fiir Metallforschung, D-7000 Stuttgart 1, Federal Republic of Germany. Manuscript submitted July 30, 1990. METALLURGICAL TRANSACTIONS A
and to compare the fatigue behavior of modern ODS superalloys with that of conventional high-strength turbine alloys. Oxide dispersion-strengthened superalloys such as MA 6000 and MA 754 are powder metallurgy materials produced by means of the mechanical alloying (MA) process, tSl Here, elemental and master powders are blended together with the fine dispersoid particles and subsequently milled in a high-energy steel ball attritor. The fully mixed compact is then extruded. A final recrystallization heat treatment is applied in order to obtain the coarse, directional grain structure needed for optimal high-temperature creep resistance. Prior to the advent of dispersion-strengthened superalloys made possible by the MA process, investigations of the potential of dispersion strengthening were practically limited to elemental metals (Pb, Cu, A1, or Ni), for which the difficulties of introducing the dispersed second phase were surmountable. Perhaps the earliest report of the fatigue behavior of a dispersion-strengthened metal is that due to Martin and Smith t6] for Cu-containing SiO2 and A1203 particles. The authors reported an improvement in the room-temperature fatigue strength
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