On the origin of the stress decrease for nickel polycrystals with few grains across the thickness

The mechanical behaviour under uniaxial and loading/unloading tensile tests of high purity nickel with different number of grains across the thickness is studied experimentally. The specimens have a constant 500 mu m thickness and the mean number of grains across the thickness (i.e., thickness t to grain size d ratio) lies between 0.9 and 15. An extended microstructural study is operated and no change of the microstructure appears with a modification of t/d. The experimental results show that the t/d ratio affects the hardening stages, flow stress, intragranular and intergranular backstress of the samples. For specimens with few grains across the thickness, the flow stress is reduced due to a decrease in the intragranular backstress. The main explanation of these results is a delay of the generalization of cross-slip for the lowest t/d ratio specimens due to surface effects. (C) 2008 Elsevier B.V. All rights reserved.