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I.

INTRODUCTION

O X I D E dispersion-strengthened (ODS) materials produced by mechanical alloying (MA) can be recrystallized to give highly anisotropic columnar grain microstructures that are ideal for high-temperature applications. Ill The alloys are available commercially and have had a few applications in the aerospace and energy conversion industries, t2~ It is possible that wider applications will follow with a better understanding of the factors governing the recrystallized grain structure and the role of dispersed particles. Because MA is a relatively difficult process, most commercial materials are inevitably heterogeneous, and this will probably always be the case. It is therefore necessary to develop heat treatments that will give reproducible and uniform microstructures in spite of the heterogeneities and to seek systematic methods for controlling the recrystallization grain structures. The work presented here deals with the detailed microstructural characterization of INCONEL* MA760, a nickel-base superalloy produced

studies. [4-7] However, as will be seen later, the characterization is incomplete for the process of process modeling. In particular, we require information about the stored energy prior to recrystallization, the features of the microstructure responsible for this stored energy, and details about the variety of precipitate and oxide phases.

II.

EXPERIMENTAL P R O C E D U R E

Table I has the composition of the MA760. The alloy was supplied in the form of a flat bar (--~25 x 55 x 500 mm). In the as-received condition, the mechanically alloyed powder has been canned, extruded, decanned, extruded, and finally cut. Extrusion was carded out at 950 ~ The as-received alloy had not been otherwise heat-treated. Experimental details can be found in previous articles, tS,9A~

III.

RESULTS AND DISCUSSION

*INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.

A. Microstructural Characterization

by MA, and contains a variety of dispersed phases. The work is a part of a wider program on the process modeling of nickel- and iron-base mechanical alloys. MA760 is a relatively new variant of commercial mechanical alloys. Like conventional nickel-base superalloys, it contains ---45 pct of 3" but in addition has Y203 dispersion strengthening. It is designed for the purpose of achieving a balance of high-temperature strength, longterm microstructural stability, and oxidation resistance. [31 This alloy has been the subject of a few recent

Transmission electron microscopy (TEM) images of longitudinally sectioned samples clearly revealed an equiaxed fine grain structure with a mean linear intercept of 0.14 --- 0.09/xm (Figure 1), where the error refers to a standard deviation in the random intercept data. There is an apparently low dislocation density within the grains, and the samples contain many undistorted annealing twins, as shown in Figure 1. It was felt necessary to verify that these ultrafine grains do not represent a recovery substructure within coarser grains as is typical in