Nonuniform recrystallization in a mechanically alloyed nickel-base superalloy
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I.
INTRODUCTION
INCONEL* MA6000 is a nickel-base superalloy de*INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
signed for aerospace applications, strengthened with both 7' precipitates and yttria particles, u] It is manufactured by a powder metallurgical process called "mechanical alloying",t2] in w h i c h elemental powders o r compounds are heavily deformed while in intimate contact to such large strains that the mechanical mixture changes into a solid solution, although the yttrium oxide particles do not alloy but form a dispersion w h i c h adds to the elevated temperature strength. The alloyed powder is then canned and hot-rolled a n d / o r hot-extruded; the microstructure at this stage consists o f incredibly fine (-~0.4 /zm) equiaxed grains resulting from dynamic recrystallization during the hot deformation331 Subsequent annealing in a temperature gradient yields highly anisotropic grains by a process o f secondary recrystallization, giving a grain structure reminiscent o f directional solidification. This directionally recrystallized microstructure produced by solid-state processing is eminently suited for applications where creep resistance is o f paramount importance. Both the mechanical alloying process and the subsequent hot-deformation processes can lead to inhomogeneities, especially since the strength levels involved even at high temperatures are large. It is common knowledge that such materials tend to show batch-to-batch variations and variations within single extruded samples. The former can be minimized by careful control o f the manufacturing process. The purpose o f the present work is to examine the secondary recrystallization behavior K. MURAKAMI, Researcher, and H. HARADA, Senior Researcher, are with the Research Development Corporation of Japan, Tokyo, Japan. K. M I N O , formerly Senior Researcher, Research Development Corporation of Japan, is now Senior Researcher, Ishikawazima-Harima Heavy Industries Co,, Ltd., T o k y o , Japan. H.K.D.H. BHADESHIA, Lecturer, is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted February 7 , 1992. METALLURGICAL TRANSACTIONS A
across the cross section o f extruded bar in o r d e r to understand better the factors responsible f o r variations in grain structure across the cross section. It is unlikely that such variations can b e eliminated completely, given the difficulties in processing. Nevertheless, a better understanding o f the inhomogeneities that might exist can perhaps suggest heat-treatment procedures which give uniform microstructures in spite o f any heterogeneities. II.
EXPERIMENTAL PROCEDURE
A . Alloy Composition and Processing The alloy (Table I) was prepared at Inco Alloys (Hereford, England) by the mechanical-alloying technique. This involves the ball-milling o f a mixture o f nickel p o w d e r (particle size 4 to 7 /xm) and powdered chromium, molybdenum, tungsten, tantalum, and a master nickel-base alloy p o w d e r co
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