高体积分数纳米片层NiTi-NbTi记忆合金复合材料研究

Recently proposed Nb nanowire reinforced NiTi shape memory alloy composites have become a hot spot in materials researches, as the embedded nanowire could exhibit an ultra-large elastic strain comparable to that of a freestanding nanowire (4%–7%). The work broke the stereotype that the remarkable properties of the nano reinforced phase cannot be exploited successfully in bulk composites, and proposed that the friendly interface of NiTi is helpful for the nanoreinforcement to exhibit its intrinsic high strength. However, the Nb nanowire fraction in these proposed composites is not more than 25%. It seems not possible to synthesis Nb nanowire composite by further increasing the content of Nb in the as-cast NiTiNb alloy. In this project, a high Nb content NiTiNb alloy was prepared by vacuum induction melting, and a nano lamellae NbTi–NiTi composite was obtained by hot-forging and wire-drawing of the ingot, with ~70% volume fraction of NbTi nano lamellae. Imagine that the Nb nano wires can exhibit an elastic strain of 4%-7% corresponding to a stress up to 4-7GPa, and therefore a bulk composite could exhibit a similar remarkable property due to the significantly high fraction of nano wire. As a result, the aim of this project is to obtain exceptional mechanical property nano composite, with NiTi matrix which could provide a friendly interface to benefit the exhibition of intrinsic high strength in nano reinforced phase, and with high fraction nanoreinforcement which could further improve the translation of high strength from the nano reinforced phase into bulk strength.

NiTi记忆合金基纳米复合材料有利于纳米增强相本征高性能的体现，因而在近年来得到广泛关注。通过塑性变形NiTiNb合金可获得Nb纳米线增强NiTi记忆合金复合材料，然而以往研究中Nb纳米线体积分数较低，进一步增加Nb含量会粗化NiTiNb合金中Nb相的尺寸，不利于纳米线的形成。若能够显著增加纳米增强相的体积分数，宏观复合材料的整体性能就会更接近所复合的单根纳米线的性能。本课题制备高Nb含量的NiTiNb合金，通过锻造拔丝，获得高体积分数纳米片层NiTi-NbTi记忆合金复合材料。用记忆合金作为基体有利于纳米结构相的本征高性能的体现，而高体积分数的纳米结构相则有利于其超常性能在宏观复合材料中体现。通过本课题的研究，有望获得超强性能的宏观材料，这必然有重要的科学意义和广阔的应用前景。

NiTi记忆合金基纳米复合材料有利于纳米增强相本征高性能的体现，因而在近年来得到广泛关注。然而以往报道的NiTi记忆合金/Nb纳米线复合材料，Nb纳米线体积分数较低，进一步增加Nb含量不利于纳米线的形成。本研究显著增加了纳米增强相的体积分数，使宏观复合材料的整体性能更接近所复合的单根纳米增强相的性能。课题制备高Nb (~70 vol.%) 含量的NiTiNb合金，通过锻造拔丝，获得NiTi与NbTi纳米片层交替分布的纳米结构材料。用记忆合金作为基体有利于纳米增强相本征高性能的体现，而高体积分数的纳米结构相则有利于其超常性能在宏观复合材料中体现。该复合材料经历较高温度（~650℃）退火，容易发生吕德斯带型非均匀变形和非均匀相变，屈服平台应力超过1 GPa。而经历较低温度退火（~450℃），则易发生均匀变形和均匀相变，屈服平台应力高达~1.6 GPa。此外，研究发现这种复合材料具有类似于TRIP钢的相变增塑功能特性。本研究另针对中、低体积分数（10-25 vol.%）Nb纳米线增强NiTi复合材料循环变形力学性能的演变进行研究，揭示了NiTi基复合材料吕德斯带型非均匀变形的发生机制，以及循环导致的功能特性发生偏移变化的原理，并解释了适当预变形所产生的耦合内应力如何使材料展现超常准线性超弹特性。项目还针对其他体系记忆合金复合材料（NiTi-W）的变形和相变进行了初步研究。