Effect of ramp-treatment on the cyclic stress-strain curve of “small grained” copper

Abstract

Various cyclic mechanical tests have been used in order to elucidate the effects of ramp-loading, as mechanical pretreatment, on the cyclic response of “small-grained” copper. The grain size of this copper is not small in an absolute sense, but small in relation to that of other copper samples we have studied. This “small” grain size is associated with a particular texture which is significant here. Two parameters definitive of ramp-loading—peak stress and number of cycles during the ramp—have been used as the main experimental variables. The results have been analyzed in relation to transmission electron microscopy (TEM) reports of the structures published elsewhere. In studying the cyclic stress-strain curve (CSSC) of ramp-treated small-grained copper it is found that the well-defined matrix structure inherited from the efficient cyclic hardening characteristic of ramp-treatment promotes higher stress and lower cyclic hardening values in the CSSC of ramp-loaded specimens, at low strain amplitudes, than those obtained in the CSSC of conventionally tested samples. At intermediate strain amplitudes, pronounced strain localization is promoted in ramp-treated copper for a very uniform and homogeneous evolution, from grain to grain, of the developed matrix into primary slip dislocation structure, i.e. persistent slip band (PSB) and wall structure. However, plateau-like behavior was not observed. The observation of early PSB nucleation in some grains, in association with a weak but noticeable 〈111〉-〈100〉 hard texture in the ramp-treated specimens, showed that the variations in PSB nucleation stress from grain to grain were not totally suppressed, and therefore plateau-like behavior in the CSSC was inhibited. Comparison of the results presented here with those of Z. Wang and C. Laird, Mater. Sci. Eng., 100 (1988) 57, whod did report a plateau, suggests that the observation of a plateau in the CSSC of polycrystals may be very sensitive to small differences in material structure and experimental procedure.