Dresden 2003 – scientific programme
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M: Metallphysik
M 21: Mechanische Eigenschaften II
M 21.2: Talk
Tuesday, March 25, 2003, 12:00–12:15, IFW B
The influence of temperature and strain rate on the flow stress of ECAP nickel — •Lutz Hollang1, Ellen Thiele1, Carl Holste1, and Dieter Brunner2 — 1Institut für Metallkunde, Technische Universität Dresden, 01062 Dresden — 2Max-Planck-Institut für Metallforschung, Heisenbergstr.3, 70569 Stuttgart
The plasticity of sub-microcrystalline nickel produced by equal channel angular pressing (ECAP, grain size (gs) 0.5 to 0.8 µm) and fine-crystalline (FC, gs 35µm) Ni were studied between 4K and 320K in tension tests which were combined with stress-relaxation experiments (SRE) performed along the stress strain curves. Depending on grain size and deformation temperature two types of stress-strain curves have been observed: i) curves showing nearly constant hardening rates and ii) curves with short work-hardening intervals followed after a maximum stress by work softening. ECAP Ni exhibits Stage III hardening at 77K and 4K. Above a critical temperature the deformation becomes unstable due to local lowering of the average dislocation density after changing the strain path from ECAP to uniaxial deformation. For both types of Ni samples the strain-rate sensitivity λ determined from SRE follows the Cottrell-Stokes law meaning a linear relationship between λ and the flow stress σ. Due to the smaller grain size, for ECAP Ni the straight λ-σ lines meet the σ axis at a characteristic athermal stress component, σδ. Above 180K an additional thermally activated process seems to occur. These results are discussed by means of the thermal activation analysis. (in German)