纳米尺度成分非均匀NiTi基合金的热/力诱发演变行为研究

The parent phase of certain alloys, such as NiTi-based ones, exhibits abnormal evolution of electrical resistivity, modulus and electron diffraction pattern with decreasing temperature. These anomalies are called pre-martensitic transformation or strain glass transition. In the past 40 years, debates on the origin of these anomalies kept rising. The major theories are that they originate from the shear transformation of nanodomains or the variation of electronic states. We have recently found that, the parent phase of nanocrystalline NiTi (average grain size 10 nm) exhibits similar anomalies when the temperature is below the chemical equilibrium temperature T0 (determined by the chemical composition). We thereby hypothesize that there exist nanoscale areas where the martensitic shear is prohibited in the previously reported alloys. Based on the hypothesis, we have designed two types of NiTiNb(Fe) alloys and reproduced the alike anomalies. Furthermore, we have employed 3D atomic probe tomography (3D-APT) to reveal that the alloys contain chemically inhomogeneous areas (average size 6 nm) formed by spinodal decomposition, and have confirmed the existence of the anticipated nanoscale areas. In particular, the two alloys possess superior superelastic and mechanical properties to the alloys in the literature. We suggest that the mechanisms of the anomalies and unusual properties may be associated with the evolution of the nanoscale areas (shear transformation prohibited), which is however challenging to interpret using existing models. In this sense, this project will generate high-impact academic achievements based on novel theories.

NiTi基合金等在降温过程中，母相的电阻、模量及电子衍射斑点等呈现的异常现象，被称为马氏体预相变或应变玻璃转变。近40年来，许多学者争议其来源，目前，主要观点为：纳米尺度切变型相变或电子状态演变。最近，我们发现，在降温过程中（低于热力学温度T0），纳米晶（约10nm）NiTi合金的马氏体相变被抑制，也呈现类似异常现象。故猜测，以往报道的合金组织中，可能存在马氏体相变被抑制的纳米尺度区域。为求证，我们制备了NiTiNb（Fe）两种合金，均呈现了以往报道的异常现象；并采用原子探针首次证实，在两合金因调幅分解形成的成分非均匀组织中，存在马氏体相变被抑制的纳米尺度（约6nm）区域。特别地，两合金还分别呈现优于以往报道的超弹特性及高强度高塑性。初步推测，上述异常物理现象与优异性能，与马氏体相变被抑制的纳米尺度区域的演变有关，难以用现有原理解释，其研究可望形成新理论，获得高质量学术成果。

含“过冷母相”的NiTi基合金在热致、力马氏体正逆相变中呈现奇异性，并呈现优异的功能、结构特性，其物理来源难以用现有理论解释。在本项目资助下，项目组在近等原子比NiTi、NiTiNb、NiTiFe、NiTiNbFe等一系列合金中系统探索了相变规律及“过冷母相”演变（畴转变）特征，利用热分析、电学分析、原位同步辐射X射线衍射、动态力学测试技术、透射电子显微与衍射技术、分子动力学模拟等手段，理解认知了“过冷母相”的本质及其衍生的热、力致相变奇异性。基本明晰了上述现象与性能源于成分非均匀组织中亚稳母相的形核，系统认识了“过冷母相”间交互作用所带来的动力学效应，订正了学界部分认知，此外还初步探明了成分非均匀NiTi基合金的强韧化机制。基于对“过冷母相”的理解，设计制备了在77-333K温域内最高临界相变应力达1.8 GPa、平均达1.5GPa的高超弹力、宽温域形状记忆合金，性能远优于已有文献报道（包括近年来三篇Science期刊文献）。项目初步建立起含“过冷母相”金属材料的马氏体相变理论基础框架，对拓宽热弹性马氏体物理的边界有重要推动作用，所获高性能材料在航空航天等领域有潜在重要应用价值。. 项目期间，在Acta Materialia、Materials Research Letters、Scripta Materialia等国际著名期刊发表SCI论文 13篇（全部标注基金号，其中以第一或通讯作者身份发表10篇），在国际、国内学术会议宣读论文4次（均为特邀报告），获中国发明专利授权1项，培养博士生5名、硕士生9名（均已获学位）。