点阵切变基体中纳米线超大弹性变形的特征与机制

Freestanding nanowires often have ultrahigh elastic strain limits(4-7%), but exploiting their intrinsic mechanical properties in bulk composites has proven to be difficult. That the exceptional properties not going from nano to macro has commonly been dubbed the "Valley of Death". In our preliminary study, we have realised the ultra-large elastic strain (4-7%) of Nb nanowires in a bulk composite, and crossed the "Valley of Death". This breakthrough is accomplished by selecting a NiTi shape memory alloy as matrix instead of the conventional metals which is commonly used. The deformation mechanisms are totally different between these two matrix with the former deforms primarily by lattice shear and the latter by dislocation slip after an initial elastic deformation. Meanwhile, we found that the nanowires show the behaviors of instantaneous and local change of strain and inhomogeneious distribution of strain during their ultra-large elastic deformation. Also, the ultra-large elastic behaviors of nanowires are different when the shear strain of matrix is larger or smaller than the elastic strain of nanowires. By a pre-treatment, the large elastic strain of nanowires in the composite can be retained even after moving the external load. The project intends to reveal the characteristics and mechanisms of ultra-large elastic strain of the nanowires in a NiTi matrix, illustrate the relationship between the ultra-large elastic deformation of nanowires and the lattice shear deformation of matrix, and clarify the characteristics and mechanisms of retaining large elastic strain of the nanowires in the bulk composite without external load. This will lay a theoretical foundation for the development of high-performance composite reinforced by nanowires.

单体态纳米线具有超大弹性应变（4-7%）/超高屈服强度，然而，其在大块复合材料中却失去此性能，此"超常力学性能未能从纳米走向宏观" 现象被喻为"死亡之谷"。在前期研究中，我们一改以往文献中选择的常规金属基体（位错滑移塑性变形机制），选择以点阵切变为变形机制的NiTi记忆合金为基体，实现了宏观复合材料中纳米线呈现超大弹性应变（4-7%），跨越了"死亡之谷"。并初步发现，此基体中纳米线的超大弹性变形呈现瞬时性、局域性及应变分布不均匀性等特征；并当基体切变应变"过"与"欠"匹配于纳米线弹性应变时，纳米线呈现不同的超大弹性变形特征；且通过预拉伸处理可使自由态（无外载）复合材料中纳米线保持大弹性应变。本项目拟揭示纳米线呈现超大弹性应变的特征与机制，阐明其超大弹性变形与基体点阵切变变形之间的内在联系，澄清自由态复合材料中纳米线保持大弹性应变的特征与机制，为高性能纳米线复合材料的设计与应用奠定理论基础。

单体态纳米线具有超大弹性应变（4-7%）/超高屈服强度，然而，其在大块复合材料中却失去此性能，此“超常力学性能未能从纳米走向宏观” 现象被喻为“死亡之谷”。在本项目前期，我们一改以往文献中选择的常规金属基体（位错滑移塑性变形机制），选择以点阵切变为变形机制的NiTi记忆合金为基体，实现了宏观复合材料中Nb纳米线呈现超大弹性应变（4-7%），跨越了“死亡之谷”（刊于Science期刊，2013）。然而，点阵切变变形（马氏体相变或去孪晶）基体中Nb纳米线的变形特征与机制尚不清楚。. 我们知道，单体态纳米线在受外载发生弹性变形过程中，其弹性应变分布均匀，且变形速率受外载控制。本项目研究发现，相变基体中纳米线超大弹性变形与单体态不同，其受基体的马氏体相变变形控制。应力诱发马氏体相变属一级相变，其相变的点阵切变变形具有瞬时性与局域性，Nb纳米线与点阵切变基体在变形过程中，两组元的变形相匹配，Nb纳米线的超大弹性变形受控于基体相变点阵切变变形，呈现瞬时性与局域性。此外，还首次提出，纳米线（纳米棒、纳米带）呈现大弹性应变的本质，与基体的位错滑移塑性变形无关，而取决于基体产生大晶格应变（大弹性应变）；并采用实验证实，在发生应变强化的NiTiFe位错滑移基体中的Ti3Sn纳米棒（或纳米线），也呈现了大弹性应变。还发现NiTi记忆合金基体中金属W纳米片呈现大弹性应变。. 本研究对复合材料中纳米线（带或棒）呈现大弹性应变的特征与机制的揭示，指导研发了诸多高性能复合材料，如：高强高塑Ti3Sn纳米棒/NiTi、微纳米片W/NiTi、纳米叠层铜/碳钢复合材料。. 本项目在Nano Letters，Acta Materialia等国际有影响学术期刊发表论文24篇，其中，SCI收录21篇。已获得发明专利授权4件，在国际会议做学术报告20余次，培养博士研究生4名，硕士研究生8名。