Substructure formation and nonuniformity in strain during high temperature creep of copper single crystals
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HASEGAWA,
SEIICHI
KARASHIMA,
AND
RYUZO
HASEGAWA
The change in substructure during high temperature creep of copper single crystals was examined to make clear the relation between the substructure and the creep rate. The structural observations by means of etch-pit technique were made in the same area of the specimen surface after successive straining by creep. Different types os substructures were formed in different localities at an early stage of transient creep. Small subgrains elongating in the direction of the deformation band were found in some regions (region A). Large subgrains containing cells within them were observed in other regions (region B). With the progress of creep, the substructures in both regions underwent gradual changes to become alike in their appearance with each other. The creep strains in regions A and B were examined at various creep stages. The ratio of creep strain to the average creep strain throughout the specimen, ~/Eav , was a few times larger in region B than in region A at an early stage of transient creep. With the structural changes mentioned above, the difference between the ratios in the two regions came to disappear, both becoming almost unity at the later stage of creep. These results suggest the close relation between the substructure and the creep rate.
A number of studies have recently been reported on substructure developed during high-temperature creep I 3 4 8 by means of X-ray technique, - etch-pit technique, " and transmission electron microscopy.~'2'5's-15 At present, it is widely accepted that well-defined substructure is formed in metals and alloys during creep. Only a few systematic works, however, have so far been reported with regard to the relation between the substructure formation and the creep rate. In previous experiments7 by the present authors, each copper single crystal specimen was deformed by creep up to a variant amount of strain and the substructure due to creep was examined by means of etchpit technique on the primary and critical slip planes prepared by cutting the specimen electrochemically. It was found that at an early stage of transient creep there existed two kinds of regions in which the appearance of substructure was different from each other; small subgrains elongating in the direction of the deformation band were found in s o m e regions (region A), while cells were observed in other regions (region B). With the progress of creep, the substructures in both regions underwent gradual changes to b e c o m e similar in appearance. In order to confirm these results obtained on separate specimens, 7 structural changes during creep were examined successively in the same area of specimen surface of a copper single crystal. Creep strains in the two regions A and B were measured at various creep stages to make clear their contribution to the total creep strain of the specimen. TADASHI HASEGAWA, SEIICHI KARASHIMA, and RYUZO HASEGAWA are Research Fellow, Professor, and Graduate Student, respectively, Department of Materials Science, FaculW of
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