Ultrafine-grained metal materials exhibit strong mechanical properties, but often have poor plasticity. Since the volume fraction of the triple junctions of the grain boundaries in the material is very high, the dislocation at triple-junction of grain boundaries can play a key role in excellent performance of the material, such as high strength and good plasticity in many cases. In this paper, based on one-time decomposition mode of the dislocation pile-up at the triple junction of grain boundaries proposed by Fedorov et al. [Acta Materialia, 51 (2003) 887-898], a new energy model is proposed for the two-time decomposition mode of the dislocation pile-up at the triple junction. With this model, we explore the effect of the critical conditions on the decomposition mode of the second dislocation and their transformation between two modes and get the criterion for the transformation of two modes, such as the magnitude of stress, the angle of triple junction and the distance of the decomposed dislocations moving. The transformation of the different decomposition modes of the second dislocation is related to the displacement distance of the dislocations decomposed from the first dislocation of the dislocation pile-up. The transformation criterion of the different decomposition mode of the second dislocation, the strengthening and weakening of the triple junction of the grain boundaries and the dislocation mechanism of superplasticity in plastic deformation processes under loading can be used to analyze semi-quantitatively.
