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Size effects make traditional forming theories infeasible in analyzing the micro-forming process,so it is necessary to develop an accurate material model to describe the material flow behavior with consideration of size effects.By studying the size effects of the flow behavior of H80 foils experimentally,it is found that the foil flow stress and strain hardening ability reduce significantly with the decrease of foil thickness.The reduction of the proportion of intal grains which own complete grain boundaries is the main cause of size effects of foil flow behavior.Moreover,grain refinement can reduce the size effects on material flow behavior.On these bases,a phenomenological material model has been developed to mathematically describe the material flow behavior with consideration of the effects of geometry size,grain size and strain hardening behavior.The reasonability and accuracy of this new model are verified by comparing the calculation values with experimental results in metal foil tensile and micro-bulk upsetting experiments.These experimental results and the proposed model lay a solid foundation for understanding and further exploring the material flow behavior in the micro-forming process.