Nanocomposites and nanomaterials

Stress relaxation in ultrafine–grained copper at low temperatures
Hryhorova T. V., Isaev N. V.

B.Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 47 Nauky Ave., Kharkiv- 61103, Ukraine
E-mail: grigorova@ilt.kharkov.ua

The purpose of the study is the thermal activation analysis of the plastic deformation using data from the stress relaxation experiments in oxygen-free copper (Cu-OF) polycrystals at temperature range of 77-295 K.
The ultrafine-grained (UFG) microstructure of polycrystals was obtained by the method of angular channel hydroextrusion. The grain size was 500 nm [1]. The samples were deformed by tension with a constant strain rate of 10–4 s–1. The temperature dependences of the flow stress σ(T) at a fixed strain and the activation volume Va(T), calculated from stress relaxation data, were used to discuss the mechanisms controlling the rate of plastic deformation UFG Cu-OF at the test temperatures.
The analysis shows that the experimental dependence of Va(T) has a maximum at T200 K. At T below 200 K, the growth of Va with temperature corresponds to the mechanism of thermally activated intersection of forest dislocations. Above 200 K, a decrease of the Va with temperature contradicts this mechanism. Such an anomaly may indicate the presence of an additional mechanism of plastic deformation. 
Accounting for hardening in the calculation slightly changes the obtained temperature dependence of the activation volume. Moreover the absence of hardening in UFG copper at elevated temperatures can indicate that during plastic deformation the dynamic recovery process is dominant ones. This is mainly due to the high density of dislocations in UFG polycrystals and high temperatures. Note, that accounting for recovery processes, leads to an increase in the magnitude of the activation volume at temperatures above 200 K, especially at room temperature. Thus the increased role of the recovery, stimulated by a high density of dislocations in the UFG copper, leads to a significant underestimation of the activation volume, calculated from the stress relaxation at elevated temperatures.

1. Isaev N. V., Grigorova T. V., at all. Plastic deformation mechanisms of ultrafine-grained copper in the temperature range of 4.2–300 K// Low Temperature Physics.- 2016.-42.-P. 825-835.