W纳米线/NiTi记忆合金复合材料的形变及相变行为研究

It is known that the nanowires have an ultra-large elastic strain limit due to their dislocation starvation. Among all the metal nanowires, tungsten (W) nanowire could be the one that possess the highest yield strength for its largest elastic modulus, and therefore become the best reinforcement of the metallic matrix composite. However, up to now, there is no report about the W nanowire reinforced metallic matrix composite probably due to its highest melting point about 3420 °C and reaction with most of the metals at high temperature. Recently, we have found that there is no reaction and dissolution occurred between the W and NiTi at high temperature, and thus obtained the nanowire W/NiTi composite by vacuum induction melting Ni4W master alloy, Ti and Ni elements and subsequent forging and drawing. During the deformation of the composite, it is preliminary found that the W nanowire with an average diameter of 150 nm shows an elastic strain up to 3.4% (yield strength of 13.6 GPa). Also, there is a unique load transfer between the W nanowire and the NiTi matrix characterized by one increasing and the other one decreasing. This project aims to clarify the size effect of the W nanowire showing large elastic strain, uncover the mechanism of the unique load transfer and the interface impeding dislocation slip between the W nanowire and the NiTi matrix, and illuminate the martensitic transformation characteristic of the NiTi nanowire under the constraint of W nanowire.

纳米线因位错匮乏而具有超大弹性应变极限，W的弹性模量为金属材料之最（400GPa），因此，其纳米线可望具有超高屈服强度（弹性模量×弹性应变），而成为优异的复合材料强化组元。然而，W在高温下与多数金属反应，且熔点达3420℃，迄今尚难获得其纳米线复合材料。最近，我们发现，W与NiTi在高温下不发生反应与溶解，并利用低熔点Ni4W与Ti、Ni进行感应熔炼、锻造、拔丝获得了W纳米线/NiTi基体复合材料。初步研究表明：在复合材料拉伸过程中，平均直径150nm的W纳米线展现的弹性应变高达3.4%（屈服强度13.6GPa），且其与NiTi间表现出“此消彼长”的独特承担载荷特征。本项目拟澄清W纳米线大弹性应变的尺寸效应特征与机制，揭示其与NiTi的独特承担载荷的特征与机制，搞清位错滑移组元/相变组元界面对位错滑移的软阻碍作用与机制，阐明NiTi在W约束下的马氏体相变特征与机制，可望获得高水平学术成果。

纳米线因位错匮乏而具有超大弹性应变极限，W的弹性模量为金属材料之最，因此，其纳米线可望具有超高屈服强度，而成为优异的复合材料强化组元。然而，W在高温下与多数金属反应，且熔点达3420℃，迄今尚难获得其纳米线复合材料。由于W与NiTi在高温下不发生反应，我们采用熔炼、锻造、拔丝等手段获得了W纳米线/NiTi基体复合材料。原位同步辐射研究发现，在拉伸加卸载过程中，W纳米带在基体发生相变过程中展现的弹性应变高达3%，承担应力达11.6 GPa。W纳米带在点阵切变基体中的大弹性应变极限具有明显尺寸效应。原位TEM拉伸表明复合材料中W纳米线弹性应变可达4.59%。复合材料断裂后，断口附近出现大量位错，同时W纳米线发生劲缩，表明W纳米线在室温下具有很大的塑性变形能力。采用真空热压及锻造制备的W/TiNi复合材料在变形过程中展现出两个屈服平台，其压缩强度是纯W的3倍，塑形是纯W的5倍。原位同步辐射研究表明两段屈服与TiNi的马氏体相变有关，该相变有助于抑制W中的裂纹扩展，提高复合材料塑形。采用熔渗法制备的W/TiNi复合材料，由于NiTi在马氏体相变过程中发生应变均匀的点阵切变变形，使金属钨获得均匀应力传递，从而使金属钨免于脆性断裂，使复合材料呈现高强度。采用叠轧及随后拔丝获得的纳米片W/Nb/TiNi复合材料屈服强度可达800MPa，断裂强度可达1200MPa，复合材料中垂直于拉伸轴方向的纳米片Nb（110）晶面的晶格应变可达2.3%，W（110）晶面的晶格应变可达1.2%。获得的多层Nb纳米片/TiNi记忆合金复合材料平台应力可达2000 MPa以上，Nb-(110)晶面的晶格应变可达4.61%。通过磁控溅射在TiNi记忆合金表面制备出不同厚度的W薄膜，采用自行设计的拉伸装置研究了W薄膜的变形行为，发现W薄膜沿拉伸轴方向的弹性应变最大不超过0.5%，且大小与膜厚无关，在卸载后可受到约0.8%的压应变。探索制备了Cu/Nb异构叠层复合材料。. 本项目发表论文4篇，申请发明专利5项，获得发明专利授权3件，授权实用新型专利1项，在国际会议做学术报告2次，培养博士研究生2名，硕士研究生4名。