Abstract
A Load-elongation curve of 18-8 stainless steel was obtained at −269°C with an extensometer specifically made for cryogenic use. The role of martensitic transformation in the serrations was discussed from the results of strain and hardness distribution measurements, X-ray analysis and optical microscopic observation of surface relieves. The change in electrical resistivity with the increase of tensile strain was also measured at −269°C with the specimen of different shape prepared from the same material, and the cause of the overshoots observed in the electrical resistivity change was also discussed. The results are summarized as follows:
(1) The serrations observed were large in general except for those at the very early stage of the deformation. The plastic deformation component, i.e. a slight deviation from a straight line, was observed at the upper part of the load increasing portions of the serrations.
(2) The serrations in the yielding and uniform elongation regions occurred with localized plastic deformation of the specimen. The deformation was accompanied by martensitic transformation.
(3) The over-shoots were observed in the electrical resistivity changes, which took place discontinuously with load drops at the serrations. The main cause for these overshoots was considered due to heat generation accompanied by martensitic transformation (γ→α′).
