双相变组元互增强高超弹特性复合材料研究

This project aims to design and create high-performance shape memory materials capable of exhibiting superelasticity at cryogenic temperatures, over wide temperature windows and with high work output (high superelastic stress) that surpass the limitations of existing shape memory alloys..Existing shape memory alloys have severe limitations in meeting the challenges of some extreme working conditions, for example in astronomy and space exploration, nuclear technology and deep sea engineering. The main limitation is the low strength of the alloys. To overcome this challenge, we propose to design a dual-phase composite between a phase transforming component and an ultrahigh-strength component. This is based on the innovative concept of using the uniform lattice strain of the martensitic transformation of the former to load the latter at the lattice level to achieve ultra-high strength, and the macroscopic level, utilize the post-transformation strain hardening of the former to protect the latter from premature brittle or ductile failure, thus to achieve ultrahigh-strength superelasticity. .Based on this concept, a dual phase in-situ shape memory composite was created by conventional metal production techniques. This composite exhibits an ultrahigh superelasticity within a wide temperature range down to the cryogenic temperatures, surpassing the property record of existing superelastic alloys. Early analysis revealed clear reciprocal enhancement between the two components. The martensitic transformation in the former successfully suppressed the brittleness of the latter and induced an elastic strain of 3.4%, which is 1.4 times of that achievable in its free-standing state and 2 times of the elastic strain limit of nanowires in composites. This study aims to reveal the mechanism of this dual phase synergy and to establish the theoretical foundation and the technology for creating high performance phase transforming composites.

现有形状记忆合金难以满足我国空间探测器等对低温、宽温域、高应力超弹性的需求，而其关键为母相强度低。对此，从提高强度入手，选择高应力相变组元/超高应力相变组元相复合，利用前者马氏体相变的均匀点阵切变，将外界对后者宏观拉伸转化为原子尺度均匀拉伸，此均匀拉伸阻止后者变形局域化；同时，利用前者相变后显著应变强化，抑制后者早期断裂或塑性失稳，从而在微观与宏观上均可使后者具有超高强度而呈现超高超弹应力。基于此全新设计思想，采用常规冶金及热成型加工方法制备了双相变组元复合材料，其在低温、宽温域呈现了高应力超弹性，突破了现有超弹合金的最高纪录。初步实验证实，上述两组元具有互增强作用；尤其，前者的马氏体相变，使后者克服了单体态脆性，呈现高达3.40%的弹性应变极限，是其单体态的1.4倍，是复合材料中纳米线弹性应变极限的近2倍。本项目拟揭示双相变组元互增强机制，建立高性能相变组元复合材料的基本理论与制造方法。

近年来，深空探测器等要求形状记忆合金（SMAs）在低温、宽温域（-190℃至50℃）呈现高超弹应力的超弹性，然而，现有SMAs不能满足要求，其机理性障碍是：（1）母相屈服强度低，（2）超弹应力温度系数高。. 针对母相屈服强度低的障碍，本项目提出“原子尺度均匀施载增强”概念。基于大块金属材料在拉伸过程中发生应变局域化，提前失效而不能展现本征屈服强度，提出利用马氏体相变的集体原子同步切变，对周围区域进行原子尺度均匀施载，抑制应变局域化而延迟失效，使大块金属材料展现高屈服强度。其次，针对超弹应力温度系数高的障碍，提出“亚稳母相降低超弹应力温度系数”新思路，基于稳态母相相变受C-C方程限制，导致超弹应力温度系数高，提出亚稳母相的马氏体相变主要受储存弹性能限制，而降低超弹应力温度系数。. 本项目首先发现少量Nb元素掺杂NiTi记忆合金，造成NiTiNb合金纳米晶内部存在化学成分起伏，而在降温过程中形成亚稳与稳态母相的双相变组元。NiTiNb合金屈服强度是纳米晶NiTi的1.5倍，超弹应力温度系数降低10倍，证实上述概念与思路的有效性。这使其在低温、宽温域（-196℃至60℃）呈现的平均超弹应力高达1.6GPa，是近期文献（如Science，2020）报道的3倍，制作的深空探测器关键部件，已通过地面试验。. 实验证实，本项目提出的“原子尺度均匀施载增强”概念，适于提高非晶相与亚微米晶大块金属材料的屈服强度；提出的“亚稳母相降低超弹应力温度系数”新思路，普适于系列Nb元素掺杂NiTi及NiTiCo（Fe）记忆合金。研究成果为研发高超弹性的SMAs与高屈服强度的金属结构材料提供新概念与新思路，研制的系列NiTiNb记忆合金可望在深空探测等领域获得重要应用。 . 本项目在Mater. Today、Adv. Mater.、Acta Mater.等国际期刊发表论文46篇，获得专利授权9件，举办国内会议2次。