临界低温时效诱发多驱动NiTi合金四程形状记忆效应的内应力场研究

高功能密度的形状记忆合金因能大幅降低驱动结构体积并增加其自由度而成为极具潜力的智能驱动材料，同时低温时效NiTi合金反常双程形状记忆效应和临界低温时效NiTi合金四程形状记忆效应的相继发现使得利用单一约束时效工艺获取多驱动NiTi合金成为可能，但尚不清楚其诱发机制。于是，本项目拟基于几何相位分析和X射线衍射技术分别量化共格应力（应变）场和残余应力场，并结合虚拟暗场成像、定量热分析及宏观形变行为研究临界低温时效NiTi合金的内应力场演变规律、拉-压不对称性和分布不均匀性，从而阐明内应力场演变与形状记忆效应类型变化的关系、明确约束应力对四程形状记忆变形的影响程度以及探明微观内应力场导致宏观变形方向和变形次数多样性的根源。本研究能够揭示纳米Ni4Ti3相周围残余应力和共格应力共存的内应力场诱发异常形状记忆效应的机理，并为发展多驱动NiTi合金的约束时效工艺奠定理论基础。

The shape memory alloy with high functional density has become promising candidates of smart actuation materials due to its outstanding effect in reducing the volume and increasing the freedom of actuation structure. Meanwhile, the abnormal two-way shape memory effect induced by low-temperature aging and the four-way shape memory effect induced by critical low-temperature aging have been successively discovered, making it possible to obtain multi-drive NiTi alloy through constraint-aging treatment alone. However, the inducing mechanism of the abnormal phenomenon has not been clarified yet. In this project, the coherent stress (strain) field and residual stress field will be quantified basing on geometric phase analysis and X-ray diffraction. Also, the virtual dark field imaging, quantitative thermal analysis and macroscopic deformation characterization will be performed on the critical low-temperature aged NiTi alloy, so as to study the evolvement of internal stress field, the tension-compression asymmetry and its inhomogeneity of distribution. Through this way, the internal stress field evolution dominated shape memory effect variation will be elucidated; the effect of constraint stress on the four-way shape memory performance will be determined; and the reason for the diversity of macroscopic deformation directions and times related to microscopic internal stress field will be ascertained. Ultimately, it is expected this project will reveal the mechanism of abnormal shape memory effect induced by the coexisted coherent stress field and residual stress field around nano Ni4Ti3 precipitates, laying a theoretical foundation for the preparation of multi-drive NiTi alloy through constraint-aging process.