Nucleation of recrystallization in a Co- Cr- Mo alloy
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I.
INTRODUCTION
THE Co-Cr-Mo alloy system is widely used in the fabrication of surgical implants. Previous work on this alloy system has described some of the microstructural characteristics of this alloy, especially in relation to its mechanical properties. 1,2.3 In an earlier paper 4 (which we will refer to as Part I in the rest of this paper), we had described some of the basic features of the phase transformations occurring in this alloy. In Part I it was shown that a recrystallization type process occurs during isothermal aging. This recrystallization was shown to be the second of a two-stage fcc ~ hcp transformation. The initial fcc ~ hcp transition (producing hcp0 was described as a typical martensitic transformation as observed in pure cobalt, for example. We analyzed the second stage of the fcc ~ hcp transformation (producing hcp2) in terms of the initiation and growth of discontinuous precipitation due to the presence of carbides. However, we did not assign a specific origin to the formation of hcp2. In this paper we examine the origins of this recrystallized hcp quite independently of precipitation effects. Using transmission electron microscopy, we present here direct evidence of the nucleation of hcp2. By directly observing the early stages of growth of recrystallization, the mechanism of initiation can be better understood. II.
E X P E R I M E N T A L DETAILS
A Co-26.7 pct Cr-5.5 pct Mo-0.15 pct C alloy was used in this study. The material was press forged and hot rolled at 1150 ~ given a solution treatment at 1230 ~ for four hours, and gas quenched. Following this, the alloy was given an aging treatment for 15 hours at 750 ~ This would correspond to the early stages of the fcc ~ hcp transformation as described in Part I.
III.
shaped boundaries. This is in contrast to the hcp platelets in the form of long straight bands which are consistent with the conventional martensitic transformation. This is similar to the one described in Part I, although as will be shown here, in the early stages of growth at least, there is a specific crystallographic relationship between the recrystallized region and the matrix. A. Twin-Twin Intersection
Figure 1 shows a region of intersection between two twins. Marked is a triangle-shaped region ' R ' which is a recrystallized grain. In Figure 2 each of the twin variants is illuminated. The recrystallized grain alone is shown in the dark-field mode in Figure 3. However, we see in Figure 4 that one of the twin planes is common to the recrystallized grain. In Figure 4 we used a diffraction spot one-third the distance between the 111 and the 022 matrix reflections (Figure 5). This would, of course, be consistent with a {111} type fcc twin. Based on our earlier work, the recrystallized region should be hcp. Given the small volume of material, the hcp reflections (if they exist) are very weak. However, the recrystallized region, if it is in fact hcp, would be clearly illuminated in the dark-field mode if imaged using a region in reciprocal space where an hcp spot would be
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