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

技术概述：该项目为一类新型有源材料--具有定制的几何结构、微观结构和磁力特性的磁性形状记忆纤维--用作微型设备的换能器和复合材料或蜂窝结构的构建块奠定了基础。磁场诱导孪生是单晶Ni-Mn-Ga产生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，因为孪生被相邻的、取向错误的晶粒之间产生的内部不相容应力所抑制。PIS最近发现，由于孔隙率降低了内部应力，使得多晶Ni-Mn-Ga泡沫塑料中发生孪生，导致泡沫塑料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，定制的颗粒尺寸(相对于纤维尺寸)和颗粒取向将允许将磁塑性应变从多晶(0%)调节到单晶(~10%)行为。在这项基础研究中，我们将对纤维几何结构和颗粒微结构如何使多晶Ni-Mn-Ga纤维产生磁场感应应变有一个基本的理解，从而得到实验验证的模型，可以定量地预测给定纤维结构的磁塑性应变的大小。将进行基础性的实验和理论研究，探索纤维中磁塑性的机制。首先，通过使用两种通用的制造方法(泰勒拉丝和熔体提取)，纤维的几何形状将在横截面形状和直径方面有所不同。然后，对纤维的颗粒大小和纹理进行裁剪：颗粒与纤维直径的比例将从1(多晶纤维)到~1(竹子结构)，并与单晶纤维进行比较；颗粒取向将从随机变化到纤维纹理。第三，纤维的磁力学特性将在两个长度尺度上进行表征和数值模拟：(I)在较短的尺度上，基于孪生位错和位错界面相互作用的模型将预测自由表面对小体积Ni-Mn-Ga本构行为的影响；(Ii)在较大尺度上，有限元模型将基于本构行为来预测纤维中竹子颗粒的磁力学行为。合作者将在聚合物基质中嵌入纤维来创建复合材料来研究其磁力学性能，或者创建纤维束来研究其磁热性能。非技术概述：本项目是一种新型材料--磁性形状记忆纤维的磁力学实验-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颗粒微结构对降低内应力和由此导致的磁塑性应变增强的影响，这一现象最近通过PI在泡沫塑料支柱中得到证实。所得结果将具有一般性，因此不仅适用于Ni-Mn-Ga，而且适用于整个类别的磁性形状记忆合金。具有定制晶粒结构的Ni-Mn-Ga纤维有望表现出大的磁塑性应变(即在可变磁场下的变形)，远高于含有战略稀土元素的磁致伸缩材料。这些镍-锰-镓纤维无需在智能执行器中进一步处理即可实现，因此可能在工业上迅速发展，从而对各种传感器和执行器技术产生革命性影响，包括生物医用泵、喷墨打印机阀门、发电换能器和触觉设备。除了传感器和执行器应用，光纤和光纤结构还可以实现新的应用，如高效的磁致冷设备，由于其大的特定面积，具有高传热率。该项目将培养两名研究生和几名本科生，他们的招生重点是妇女和少数民族。除了研究，学生们还将参加各种外展活动，使用形状记忆材料向年轻女性、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伙伴(在欧洲和亚洲)的合作，从而产生很高的知名度和影响力。最近的绩效指标结果引起了科学界的强烈共鸣，并在国家媒体上得到了重点报道。将利用这些联系来传播拟议项目的成果。私人投资公司已经提交了两项专利，并寻求将该领域过渡到美国高科技行业的剥离项目。

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.