【作者】 刘庆华；

【导师】 李建国；

【作者基本信息】 上海交通大学 ， 材料科学与工程， 2013， 硕士

【摘要】 Ni-Fe-Ga-Co合金是近年来新发展的铁磁形状记忆合金，因其良好的塑性，较大的磁各向异性和较高的转变温度而备受关注。基于磁-机械-热的能量相互转换，该合金同时展现了磁形状记忆效应，巨磁致伸缩效应和磁热效应，因而在智能磁驱动器和磁致冷方面具有广阔的应用前景。本文以Ni₅₂Fe₁₇Ga₂₇Co₄合金为研究对象，采用区域熔炼液态金属冷却（ZMLMC）定向凝固方法制备该合金，系统研究了该合金的定向凝固组织和择优取向。此外，本文报道了通过ZMLMC法制备的大体积马氏体单变体区（single-variant set）组织，并采用EBSD手段分析了该合金的马氏体组织，揭示了磁畴和马氏体间的耦合作用。ZMLMC法是制备具有高度择优取向的定向凝固柱状晶的有效方法。理论分析结果表明，高温度梯度和低凝固速度有利于定向凝固的稳定进行，并获得成分稳定且择优取向的粗大柱状晶组织。在低温度梯度和高凝固速度下，Ni₅₂Fe₁₇Ga₂₇Co₄合金凝固过程不稳定，成分起伏严重，出现了初生γ相，高温β相沿<100>方向生长；在提高温度梯度和降低凝固速度后，组织稳定，无初生γ相，高温β相沿<110>方向生长，室温时为沿（222）面的单相马氏体组织。Ni₅₂Fe₁₇Ga₂₇Co₄合金的马氏体为具有2层循环结构的L10相。在位向差较小的晶界上，马氏体可实现穿晶界生长。在高温度梯度和低凝固速度下，获得了大体积的稳定马氏体单变体区。该组织在大体积范围内具有取向一致的马氏体孪晶片层，对于获得巨磁致应变效应具有重要的意义。EBSD分析显示，单变体区内部完整，无微孪晶。此外，马氏体孪生面为（110）和（110）。然而，局部内应力促使局部马氏体板条中形成了微孪晶。EBSD结果显示，这些微孪晶通过分层孪生（hierarchical twinning）过程形成，亦即“孪晶内的孪晶”。马氏体中的内应力可能来自于热效应、晶界及马氏体块边界的晶格畸变能和磁畴与马氏体间的相互作用能等。在无外磁场的条件下即可通过光学显微镜观察到和马氏体板条交叉的磁畴。这些磁畴和马氏体具有相近的形态，而EBSD分析可以将二者区分开来。磁畴和马氏体之间的相互作用产生组织内应力，并产生表面浮凸现象。EBSD结果显示，孪晶界为80-85°的大角度界面，而畴臂为1-8°的小角度界面。孪晶界的位向差取决于孪生关系，而畴臂处的位向差是由表面浮凸现象造成的。更多还原

【Abstract】 Ni-Fe-Ga-Co is a new ferromagnetic shape memory alloy system. Because of its enhanced ductility,high magnetocrystalline anisotropy energy and wide transformation temperature range, this alloy systemattracts much attention. Based on the interplay of magnetic-mechanical-thermal variables, this alloyexhibits magnetic shape memory, magnetostriction and magnetocaloric effect. Therefore, it is promising formagnetically controlled actuators and refrigerators.In this work, directionally solidified Ni₅₂Fe₁₇Ga₂₇Co₄alloys were focused. They were prepared byzone melting liquid metal cooling （ZMLMC） method. The preferred crystal orientation and microstructurewere systemically studied. Meaningfully, the martensitic single-variant set was reported. In addition, themartensitic microstructure was analyzed by the EBSD method. The coupling of the magnetic domains andmartensitic lamellas were revealed.ZMLMC method is an effective way to prepare highly preferred columnar crystals. It is deduced thathigh temperature gradient and low crystal growth velocity are beneficial for stable solidification. Under thiscondition, coarse columnar crystals with uniform composition and highly preferred orientation are obtained.Under low temperature and high growth velocity, the solidified crystals grew along <100> direction andprimary γ phase was formed during the unstable growth; when increasing temperature gradient anddecreasing growth velocities, the coarse columnar crystals with uniform sizes grow along <110> directionand well-developed preferred orientation, i.e. along （222） planes of the martensite.The martensite of Ni₅₂Fe₁₇Ga₂₇Co₄alloy has2-layered L10structure. The martensitic lamelas cangrow transcrystalline on the crystal boundary with small misorientation. Giant martensitic single-variant setis obtained under high temperature gradient and low growth velocity. Because of its well-alignedmartensitic lamellas, it is meaningful for large magnetic-field-induced strain. EBSD result reveals that thesingle-variant set has integral inner structure and no microtwin. Moreover, the twinning planes are （110）and （110）.However, microtwins are formed in local area of the martensitic lamellas by internal stress. It isrevealed by EBSD result that the microtwins are “twins within twins”, which are formed by hierarchicaltwinning. The internal stress results from the thermal effect, the lattice distortion energy on the boundariesand the interaction of the magnetic domains and the martensitic lamellas.The magnetic domains intersecting with the martentic lamellas are directly observed by opticalmicroscope without polarization. These magnetic domains have similar morphology with the martensite lamellas but can be verified by EBSD method. The interaction of the two structures can be revealed by thesurface relief induced by internal stress. It is revealed by EBSD that the twin boundaries have highmisorientation of80-85°while the domain walls are small angle interface with misorientation of1-8°. Themisorientation of the twin boundaries is due to the twin relationship, while the misorientation of the domainwalls is caused by surface relief.更多还原