The influence of hot working on the subsequent recrystallization of a dispersion strengthened superalloy-MA 6000
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INTRODUCTION
O X I D E dispersion strengthened alloys attract much attention because of their good elevated temperature strength. The intermediate temperature strength, however, is inadequate compared with conventional superalloys. 1 Numerous attempts have been made to combine oxide dispersion strengthening for elevated temperatures and "/-precipitation strengthening for intermediate temperatures using the mechanical alloying process. 2-6 The alloy MA 6000, made by the International Nickel Company, is the first commercial alloy developed along these lines. The production of MA 6000 has been described elsewhere in detail. 7's'9 Briefly, MA 6000 is fabricated by milling the respective powders and the dispersoids in a high energy ball mill and consolidating the powders by hot extrusion, followed by hot rolling. The as-consolidated material contains fine equiaxed grains.6'1~Dispersion strengthened superalloys in this condition exhibit good hot workability." MA 6000 can even deform superplastically.12 In order to develop high temperature strength, the fine grains must be transformed into large grains. This is achieved by recrystallization, which occurs only at very high temperatures for materials with high -/'-volume fractions like MA 6000. 3'1~Hotzler and Glasgow I~ suggest that recrystallization is triggered by T' dissolution. The recrystallized grains in dispersion strengthened alloys are elongated with the large grain axis parallel to the working direction. The elongation can be further increased by heating in a thermal gradient rather than isothermally,6 which results in a further improvement of high temperature strength. 1 The mechanism of coarse grain formation is not completely understood. It is not clear whether the grains are formed via primary or secondary recrystallization. Cairns ~3 and Hotzler and Glasgow 1~employed electron microscopy to study as-consolidated material. Primary recrystallization was judged to be essentially completed. It was concluded, R. E SINGER, Research Scientist, and G. H. GESSINGER, Head, are both with the Department of Physical Metallurgy, Brown Boveri Research Center, CH-5405 Baden, Switzerland. Manuscript submitted December 15, 1981. METALLURGICALTRANSACTIONS A
therefore, that the large grains are formed via secondary recrystallization. On the other hand, Gessinger3'14 could rationalize the recrystallization behavior of mechanically alloyed IN 738 + Y203 on the basis of the assumption that the large grains are formed via primary recrystallization. The driving force for primary recrystallization mainly originates from the removal of the stored energy associated with the free dislocations, whereas the driving force for secondary recrystallization mainly originates from the reduction of grain boundary area. The TEM studies mentioned above l~ did not provide conclusive quantitative evidence about the free dislocation density in the as-consolidated material. The purpose of the present work was to clarify the mechanism of recrystallization by evaluation of the driving forces invol
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