High temperature deformation of ultra-fine-grained oxide dispersion strengthened alloys
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I.
INTRODUCTION
THE invention of mechanical
alloying ~ made possible the development of a new class of oxide dispersion strengthened (ODS) alloys. These materials are distinguished by the presence of fine oxide particles distributed homogeneously throughout the matrix. Two commercial alloys of this type INCONEL* Alloys MA 754 and MA 6000, both of which *INCONEL is a trademark owned by the INCO family of companies.
were developed by INCO. Except for the oxide particles, the former alloy is primarily a single phase alloy whereas the latter is an ODS superalloy containing 50 to 55 vol pct 3,'. 2 The synthesis of ODS alloys begins with mechanical alloying of powders to produce the desired composition. Following consolidation of the powders into the form of a billet, the material is hot-extruded and hot-rolled; this processing produces an extremely fine grain size and a high dislocation density. Finally, the material is zone annealed at approximately nine-tenths of the melting temperature to produce an extremely coarse, elongated grain structure. ODS alloys produced in this way have excellent resistance to creep, 2'3'4 but limited tensile ductilities. Consequently they cannot be shaped by metal working methods. However, in the as-hot-worked condition, before the coarse grains are formed, the material naturally has much higher ductility. Indeed, Singer and Gessinger 5 have shown that MA 6000 in the as-hot-worked condition is superplastic under the right testing conditions. They also showed that under certain conditions MA 6000 deformed in compression retains the capacity to form coarse grains upon annealing. 6 Thus it may J. K. GREGORY is a Research Associate at the Technical University of Hamburg-Harburg, 2100 Hamburg 90, West Germany. J. C. GIBELING is Assistant Professor, Department of Mechanical Engineering, Division of Materials Science, University of California, Davis, CA 95616, W. D. NIX is Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted September 8, 1983. METALLURGICAL TRANSACTIONS A
be possible to form intricately shaped parts using ODS alloys in the fine-grained condition, and then to obtain coarse grains and high creep resistance by subsequent annealing. In part, the present study was undertaken to characterize the deformation of fine-grained MA 754 and MA 6000, to examine the microstructural changes which take place, and to understand the effects of these changes on the subsequent annealing behavior. During the course of the work it became apparent that these ultra-fine-grained ODS alloys provide an opportunity to examine certain aspects of superplasticity that have been outside the reach of most previous studies, in particular, because of the extraordinarily fine grain sizes involved, superplasticity occurs at very high strain rates, so that the flow behavior at strain rates below the superplastic regime can be studied easily. For most superplastic alloys it is difficult to measure strain rates many orders of magnitude below the sup
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