Deformation mechanism of ultrafine-grained copper polycrystals

The deformation mechanism of ultrafine-grained copper produced by electro-forming is explored. The grain structure of this material is characterized by using optical and transmission microscopy. Grain foliation has been observed as columnar grain in the growth direction and growth twins in the planar view. Thermal response (DSC) shows it has lower heat capacity than conventional copper. General mechanical behavior exhibits an enhanced yielding strength and a lower work hardening exponent (0.10) compared with the conventional copper polycrystal. Mechanical response is discussed in terms of Bauschinger hysteresis behavior. Microstructures before and after plastic deformation are investigated. Results show twin boundary deformation activities accompanied with dislocation activities even in grains finer than 100 nm. The improvement of yielding point, the low hardening exponent, and, the unique deformation mechanism are related to interaction of long-range internal stresses.