Ni-Mn-Ga合金非调制马氏体界面结构与变形机制研究

Ni-Mn-Ga铁磁性形状记忆合金具有大磁致应变和高响应频率，是理想的驱动材料。通过合金化方法改变马氏体正方度，该合金非调制马氏体可以获得大的磁致应变。磁致应变产生的前提是马氏体具有较低的孪生应力，它的大小主要取决于马氏体界面结构和变形方式。系统研究二者将为马氏体力-磁性能改善提供实验基础和理论依据，推动Ni-Mn-Ga合金工业化应用。无外力作用下非调制马氏体分布是一种纳米孪晶嵌套微米孪晶结构，研究这种多层次孪晶变形机制可以促进孪晶理论的深化、拓展。本项目拟研究不同正方度下Ni-Mn-Ga非调制马氏体的界面结构和变形机制，观察马氏体界面形态、位错分布和特征；计算马氏体界面能量、移动所需孪生应力；通过原位拉伸观察、总结马氏体变形方式，确定孪生应力，揭示马氏体变形机制；阐明正方度—马氏体界面—力-磁性能的内在联系；力图获得具有较低孪生应力的马氏体界面结构特征和正方度范围。

Ni-Mn-Ga ferromagnetic shape memory alloys are ideal materials for actuator application owing to their large strain output and quick response under moderate magnetic field. Through alloying method, the tetragonality of non-modulated martensite had been changed, and its magnetic-field-induced strain became quite large. For the occurrence of this strain, low twinning stress must be satisfied, which is greatly influenced by interface structure and deformation behavior of martensite. Systematic investigations on these two scopes are benefit for modification of the magneto-mechanical properties of martensite from experimental and theoretical foundations, and thus support improvement of industrial application of the Ni-Mn-Ga alloys. Without external stress the distribution of non-modulated martensite is a hierarchy of nanotwins forming within microtwins microstructure. Also, study on deformation of the hierarchically twinned martensite would benefit for deepening and expansion of the twinning theory. In this project, interface structure and deformation mechanism of the non-modulated martensite in Ni-Mn-Ga alloys with a range of tetragonality will be investigated. Morphology of interface and distribution and characteristic of interface dislocation will be observed. Interface energy and twinning stress resulting in the interface motion will be calculated. In-situ tensile experiment will be carried out at micro-scale to observe the deformation behavior, to determine the twinning stress and to summarize the micro-deformation mechanism. Inner relation of tetragonality–interface structure–magneto-mechanical properties of the twinned martensite will be clarified. Finally the interface structure and tetragonality range of martensite exhibiting relatively low twinning stress will be revealed.