Masayuki Ohashi1, Takao Endo2 and Taku Sakai3
1The University of Electro-Communications, Tokyo. Present address: Keihin-Hatsujo Co., Ltd, Yokosuka
Initial grain size effects on dynamic recrystallization (DRX) of pure nickel were studied by means of tensile testing and microstructural observation over the range of strain rates from 10-5 s-1 to 10-1 s-1 and at temperatures between 923 K and 1123 K. The flow curves were categorized in terms of three types of shapes; (i) single peak type at high Z (= \dotε exp(Q/RT)), (ii) multiple peak type at medium Z, and (iii) the third type which showed a sharp stress dip following a stress peak and then strain hardening with serrations in stress at lower Z. The grain structural changes by DRX as well as the flow curves were affected sensitively by initial grain size Do. The coarse grained sample was refined continuously during single peak flow until the stable size Ds was attained. The fine grained one underwent some grain refinement at the first peak strain, even though it eventually coarsened up to the same size Ds during multiple peak flow. The critical value of Zc (or the first peak stress σpc at Zc), at which the flow curve shape changed from multiple to single peak type, depended on Do. The relationship between σpc and Do approximately overlapped that of σp vs. 2Ds, and thus the critical condition in question was expressed by the relation of Do = 2Ds. These results were analyzed by using the relative grain size model for DRX.
hot deformation, dynamic recrystallization, nickel, initial grain size effect, single peak and multiple perk type of flow
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