Collaborative Research: Size Effects on Magneto-Mechanics of Ni-Mn-Ga Fibers

技术总结：该项目为一类新的活性材料奠定了基础——具有定制几何形状、微观结构和磁机械性能的磁性形状记忆纤维——将被用作微型器件的换能器，以及复合材料或细胞结构的基石。磁场诱导孪晶是单晶Ni-Mn-Ga获得高磁塑性应变的原因。相比之下，多晶Ni-Mn-Ga没有表现出磁塑性，因为相邻取向错误晶粒之间产生的内部不相容应力抑制了孪晶的形成。pi最近发现，由于孔隙度降低了内应力，因此可以在多晶Ni-Mn-Ga泡沫中发生孪晶，从而导致泡沫支柱中的磁塑性应变。将这一概念应用于单个纤维，我们的假设是定制晶粒尺寸（相对于纤维尺寸）和晶粒取向将允许将磁塑性应变从多晶（0%）调整为单晶（~10%）行为。在这项基础研究中，我们将对纤维几何形状和晶粒微观结构如何使多晶Ni-Mn-Ga纤维中的磁场诱导应变产生基本理解，从而建立实验验证的模型，该模型可以定量预测给定纤维结构的磁塑性应变大小。将进行基本的实验和理论研究，探讨光纤中磁塑性的机制。首先，通过使用两种通用的制造方法（泰勒拉丝和熔体提取），纤维几何形状将在横截面形状和直径方面有所变化。然后，对纤维的粒度和质地进行定制：颗粒与纤维直径的比值从1（多晶纤维）到~1（竹结构），并与单晶纤维进行比较；晶粒取向将从随机到纤维纹理变化。第三，纤维的磁力学性能将在两个长度尺度上进行表征和数值模拟：(i)在较短的长度尺度上，基于孪晶位错和位错-界面相互作用的模型将预测自由表面对Ni-Mn-Ga小体积本构行为的影响；（ii）在更大的长度尺度上，基于本构行为的有限元模型将预测纤维内竹颗粒组合的磁力学行为。合作者将在聚合物基体中嵌入纤维来制造复合材料，以研究其磁机械性能，或者制造纤维束来研究其磁热学性能。非技术概述：本项目是对磁性形状记忆纤维（一种新型材料）的磁力学进行实验-理论耦合研究。它的重点是识别、量化和预测纤维几何形状和晶粒微观结构对内应力降低和磁塑性应变增强的影响，这是pi最近在泡沫支柱中证明的一种现象。所得结果具有通用性，因此不仅适用于Ni-Mn-Ga，而且适用于整个磁性形状记忆合金类别。具有定制晶粒结构的Ni-Mn-Ga纤维有望显示出比含有战略稀土元素的磁致伸缩材料高得多的大磁塑性应变（即它们在暴露于可变磁场时变形）。这些Ni-Mn-Ga纤维可以在智能执行器中无需进一步处理即可实现，因此在工业上的重要性可能会迅速增长，从而对各种传感器和执行器技术产生变革性影响，包括生物医疗泵、喷墨打印机阀、发电传感器和触觉设备。除了传感器和执行器应用之外，纤维和纤维结构还可以实现新的应用，例如由于其大的特定区域而具有高传热率的高效磁冷却装置。该项目将培养两名研究生和几名本科生，其招聘将强调女性和少数民族。除了研究之外，学生们还将利用形状记忆材料参加各种外展活动，向年轻女性、少数民族和小学（K-12）学生介绍材料科学和技术。该项目将利用与四个国际合作伙伴（欧洲和亚洲）的合作，从而产生高知名度和影响力。ppi最近的结果在科学界引起了强烈的共鸣，并在国家媒体上得到了突出报道。将利用这些联系来传播拟议项目的成果。这些私人制药公司已经提交了两项专利，并正在进行一个分拆项目，以将该领域转化为美国高科技产业。

TECHNICAL SUMMARY:This project lays the foundation for a new class of active materials - magnetic shape-memory fibers with tailored geometry, microstructure and magneto-mechanical properties - to be used as transducers for micro-devices and as building blocks for composites or cellular structures. Magnetic-field-induced twinning is responsible for the high magnetoplastic strains achievable in monocrystalline Ni-Mn-Ga. By contrast, polycrystalline Ni-Mn-Ga shows no magnetoplasticity because twinning is inhibited by internal incompatibility stresses developed between adjacent, misoriented grains. The PIs recently discovered that porosity, because it reduces internal stresses, allows twinning to occur in polycrystalline Ni-Mn-Ga foams, resulting in magnetoplastic strains in the foam struts. Applying this concept to individual fibers, our hypothesis is that tailored grain size (with respect to fiber size) and grain orientations will allow tuning the magnetoplastic strain from polycrystalline (0%) to monocrystalline (~10%) behavior.In this basic study, we will develop a fundamental understanding of how fiber geometry and grain microstructure enable magnetic-field-induced strains in polycrystalline Ni-Mn-Ga fibers, leading to experimentally-validated models that can quantitatively predict the magnitude of magnetoplastic strain for a given fiber structure. Fundamental experimental and theoretical studies probing the mechanisms responsible for magnetoplasticity in the fibers will be carried out. First, the fiber geometry will be varied, in terms of cross-sectional shape and diameter, by using two versatile manufacturing methods (Taylor wire drawing and melt extraction). Then, the fiber grain size and texture will be tailored: the ratio of grain to fiber diameter will be varied from 1 (polycrystalline fiber) to ~1 (bamboo structure) and compared to single-crystal fibers; grain orientation will be varied from random to fiber texture. Third, the magneto-mechanical properties of the fibers will be characterized and numerically modeled on two length scales: (i) at a shorter length scale, models based on the mutual interaction of twinning dislocations and dislocation-interface interactions will predict the effect of free surfaces on the constitutive behavior of Ni-Mn-Ga in small volumes; (ii) at larger length scale, finite-element models will predict, based on the constitutive behavior, the magneto-mechanical behavior of an assembly of bamboo grains within a fiber. Collaborators will embed fibers in polymer matrix to create composites to study their magneto-mechanical properties, or create fiber bundles to study their magneto-caloric properties.NON-TECHNICAL SUMMARY:The present project is a coupled experimental-theoretical study of the magneto-mechanics of magnetic shape-memory fibers, a novel class of materials. It focuses on identifying, quantifying and predicting the effects of fiber geometry and grain microstructure upon reduction of internal stresses and the resulting enhancement in magnetoplastic strain, a phenomenon recently demonstrated in struts of foams by the PIs. The results obtained will be general in nature and thus applicable not only to Ni-Mn-Ga but also to the whole class of magnetic shape-memory alloys.Ni-Mn-Ga fibers with tailored grain structures are expected to show large magnetoplastic strain (i.e. they deform when exposed to a variable magnetic field) which are much higher than magnetostrictive material containing strategic rare-earth elements. These Ni-Mn-Ga fibers may be implemented without further processing in smart actuators and may thus grow rapidly in industrial importance, resulting in a transformative effect on various sensor and actuator technologies including bio-medical pumps, ink-jet printer valves, power-generation transducers, and haptics devices. Beyond sensor and actuator applications, fibers and fiber constructs may enable new applications such as efficient magnetic cooling devices with high heat-transfer rates due to their large specific areas. This project will educate two graduate students and several undergraduate students, whose recruitment will emphasize women and minorities. Beside research, the students will participate in various outreach activities using the shape-memory materials to introduce materials science and technology to young women, minorities, and grade school (K-12) students. This project will leverage collaboration with four international partners (in Europe and Asia) thereby generating high visibility and impact. The recent results of the PIs resonated strongly with the scientific community and were highlighted in national media. These contacts will be leveraged for disseminating results of the proposed project. The PIs have submitted two patents and pursue a spin-off project for transitioning the field to the US high-technology industry.