Eiji Fukushima1 and Akira Goto1
1Metals and Ceramics Laboratory, Research and Development Center, Tokyo-shibaura Electric Co., Ltd., Kawasaki
Observations on the changes in microstructures and electrical resistivities with the serrations, and other morphological observations on the serrations were made to make clear the type, the mechanism and the characteristics of the serrations that were observed in various metals and alloys at cryogenic temperatures. Conclusions obtained are as follows:
(1) Serrations observed at cryogenic temperatures are classified morphologically into three types: (a) one accompanying martensitic transformation, (b) one accompanying deformation twinning, and (c) one accompanying coarse slip.
(2) Serrations with martensitic transformation are observed, for instance, in 18-8 stainless steel. In this case, martensitic transformation plays an important role in load drops at the serrations.
(3) Serrations with deformation twinning are observed, for instance, in copper and titanium. In these cases, about 1×105∼8×106 of twins are formed locally with a serration.
(4) Serrations with coarse slips are observed for instance, in aluminum and duralumin. In these cases, serrations occur as a result of slip in the regions of comparatively small strain-hardening rate. They are regarded as the repetition of Johnston-type yielding.
(5) Serrations observed in aluminum and duralumin are considered to be caused instantaneously by large deformations occurring near the places where dislocations are by chance released from short-range obstacles, on which dislocations are apt to be locked at cryogenic temperatures because of the going down of thermal fluctuation.
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