Effect of grain refinement on superplasticity in micrograined materials

Abstract

Under superplasticity conditions, micrograined materials (grain sizes in the range 1 µm — 10 µm) exhibit extensive neck-free elongations during tensile deformation at elevated temperatures (T>0.5Tm, where Tm is the melting point). The deformation mechanism responsible for such behavior is based on grain boundary sliding accommodated by the generation and movement of lattice dislocations in the grains blocking sliding. Such a mechanism is characterized by a stress exponent of 2, an activation energy that is close to that for boundary diffusion, and a grain size sensitivity of 2. When the grain size of the material is refined to the ultrafine-grained range (300 nm-900 nm), there is no change either in the characteristics of superplasticity or in the details of its deformation mechanism. By contrast, when the grain size of the material is refined to the nanoscale range (grain size ≤100 nm), superplasticity is lost due to the emergence of a new deformation mechanism. This new deformation mechanism is also based on grain boundary sliding accommodated by lattice dislocations. It is characterized by a stress exponent that is high and variable, an activation energy that is close to the activation energy for boundary diffusion but decreases with increasing applied stress, and a grain size sensitivity of 3.

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