The Plastic Deformation And Strengthening Mechanism Of Nano-structured Polycrystalline Materials

In recent years, with the rapid development of the manufacture technology, nano-structural or micro/nano-grained materials are extensively used in integrated circuit chip, coating materials, micro/nano sensors and Micro-Electro-Mechanical Systems (MEMS). More and more attention is being paid to the size effect of the nano-structural or nano-grained materials. Generally, more than one feature size, such as geometric dimension, grain size, layer thichness, twin spacing and so on, are included in those materials, resulting in complex size effects as well as different deformation mechanisms due to the coupling of these feature sizes. Although nano-grained polycrystalline materials always exhibit high strength, but their poor plasticity results in limited industrial applications. So how to simultaneously improve the strength and ductility of polycrystalline material is a big challenge to materials scientists. Motivated by these background, the molecular dynamics method which can effectively capture the atomistic scaled deformation is used to study the micro-mechanical properties of nano-structured polycrystalline materials, the underlying deformation mechanisms due to the coupling size effect, and possible way to improve the strength and ductility. Several detailed researches on nano-structural or nano-grained materials are performed in this thesis:(1) The coupling effect of the sample size and grain size on the strength and plastic deformation mechanisms of polycrystalline Al nanowires are investigated. With the decrease of the sample size, the 5nm grained nanowires show "smaller is stronger", the 10 run grained nanowires do not show significant size effect, while the 20nm grained nanowires show "smaller is weaker". Different size effects are induced by the competition between surface strengthening mechanism, grain boundary or dislocation mediated deformation mechanism and twinning deformation mechanism.(2) A new nano-structural model is proposed to solve the strength-ductility dilemma. By adding "chain-like" 10 or 20nm nano-grains into 100 nm grained polycrystalline Ni, ductility can be effectively enhanced without sacrificing strength. On the one hand, in those new structures, stress concentration is relieved by introducing nano-grians, which can delay crack nucleation. On the other hand, nano-grain rotation plays a key role in suppressing crack propagation. Those two mechanisms synergistically improve the ductility.(3) The strengths and deformation mechanisms of the nanolayered polycrystalline metallic multilayers are investigated, with special attentions to the coupling effect of grain size and layer thickness. The results indicate that the strength of multilayers does not always increase sensitively with the decrease of layer thickness or grain size. Due to the confine of GBs and phase interface to gilding dislocations, there are several possible deformation mechanisms which govern the strength of NPMMs, including confined partial dislocation slip, confined extended dislocation slip and confined grain boundary slip. With the increase or decrease of the feature size of multilayers (i.e. layer thickness or grain size), the dominant deformation mechanism will transform from one to another, resulting in very intricate size effects on the strength.(4) To study the assistance of growth twins in nano-layered polycrystalline metallic multilayers to their strength, a series of uniaxial tensile modeling of nano-twinned multilayers are performed with special attentions to the influence of the twin lamella thickness on the strengthening mechanism. The results indicate that the strength of nano-twinned multilayers could be significantly improved, showing strong twin thickness effect. There exists a critical twin lamella thickness above which the deformation mechanism of metallic multilayers is the hairpin-like partial dislocation gliding dominated and below which it becomes the necklace-like multiple jogged dislocation gliding dominated, however. The formation and transition of the above two mechanisms are discussed in detail and analyzed theoretically. The effects of the distinct deformation mechanisms on the strength of multilayers are also depicted quantitatively. In addition, the strengths and deformation mechanisms of multilayers with non-uniform twin lamellae distribution are also discussed. Although there is synergetic interaction between the above two deformation mechanisms, the strengths of multilayers with non-uniform nano-twin lamella thickness can be well predicted by the rule of mixture.