Micro-mechamical Investigation On Nano-/Micro-crystalline Composite Toughening

It attracts a great deal of researchers' attention to improve material's strengths and ductility by obtaining ultrafine grains and some development has been received, especially in nanocrystalline materials. Significant increase in hardness and strength has been documented for nano-crystalline materials-most dramatic for nano-structure materials, but ductility is disappointingly low and it is not satisfied in nano-crystalline materials toughening. Ductility enhancements were achieved in nano-structured metals through the incorporation of a bimodal grain size distribution, with micrometer-sized grains embedded inside a matrix of nano-crystalline grains, and at the same time the strength deceases so little that the nano-crystalline material is toughening well. This paper focuses in micro-mechanical investigation on the behavior performance of this nano-crystalline material with "bimodal structure" and main results are as the following:1. This nano-crystalline material with "bimodal " structure was viewed as a kind of composite and the idea of nano-/micro-crystalline composite toughening was put forward. By reviewing experimental results and TEM images made by other researchers, a micro-mechanical model of nano-/micro composite toughening was built by several defined steps including building unit cell, confirming basic hypothesis, boundary conditions, and terminating conditions.2. The static tensile test of unit cells was simulated by finite element method. The theoretical results were compared with the practically experimental results and the rationality and feasibility of the micro-mechanical model was checked. And more, the performance diversity of this nano-/micro-crystalline composite was studied that was influenced by loading direction of unit cells, volume; percentage and shape of toughening phase3. The reason of nano/micro-crystalline composite toughening was discussed. It was studied whether the micro-crystalline phase had to be completely closed by the nano-crystalline phase, so called full constrains, when it improved the ductility of nano-crystalline maters.4. To improve the calculating accuracy, it was suggested to chang the ideal connection conditions between micro-crystalline phase and nano-crystalline phase and a midst zone should be built between them. And then the results calculated by the last step was viewed as the performance parameters of this zone, and the real connection conditions were closed.