SIMULATION OF TENSILE FRACTURE BEHAVIOR OF GRADIENT NiTi ALLOY CONSIDERING GRAIN SIZE EFFECT

NiTi alloy is widely regarded as one of the most promising materials in the field of functional materials due to its unique shape memory properties and superelasticity. Implementing a gradient grain structure is an effective method for enhancing the strength and ductility of the material, and the performance of the NiTi alloy can be further improved by achieving a gradient distribution of grain sizes within the material. In this study, a gradient finite element model is constructed based on the grain distribution function, which incorporates the crystal plasticity theory model and cohesive elements. The mechanical behavior of both homocrystalline and gradient polycrystalline NiTi during uniaxial and compact tensile fracture is simulated. The effects of grain size, orientation, strain rate, and gradient structure on the strength and ductility of polycrystalline NiTi are discussed. The results indicate that grain size significantly influences the fracture behavior of NiTi polycrystalline materials. Specifically, smaller grain sizes enhance the polycrystalline resistance to crack initiation, while larger grain sizes improve resistance to crack propagation. The fracture behavior of NiTi polycrystals exhibits a clear orientation correlation, with the 110 texture demonstrating the highest fracture resistance among the three typical orientations. High strain rates increase yield strength but reduce ductility, whereas low strain rates promote uniform stress distribution and excellent plastic deformation capabilities. The graded polycrystalline structure serves a coordinating role between the strength and ductility of NiTi alloy materials. A fine crystalline structure is located at both ends of the polycrystal, enabling it to withstand high stress and effectively inhibit crack formation. In contrast, a coarse crystalline structure is situated in the middle of the polycrystal, preventing crack propagation by providing a zigzag grain boundary morphology.