Collaborative Research: Enabling Magnetoplasticity in Polycrystalline Ni-Mn-Ga by Reducing Internal Constraints Through Porosity

技术：磁场诱导孪生是单晶Ni-Mn-Ga产生高磁塑性应变的原因。而多晶Ni-Mn-Ga则没有表现出磁塑性，这是因为相邻晶粒之间的内部不相容应力抑制了孪生。PIS最近发现，由于孔隙率降低了内部应力，使得多晶Ni-Mn-Ga泡沫中可以发生有限的孪生，从而导致磁塑性应变。然后，设计泡沫结构和颗粒微结构将允许在多晶(~0%)和单晶(~10%)之间连续调节这些泡沫的磁塑性应变。在这项基础研究中，PI将对泡沫结构和颗粒微结构如何在多晶磁性形状记忆合金中实现磁场诱导应变产生基本的理解，从而产生实验验证的模型，该模型可以定量地预测给定泡沫结构的磁塑性应变的大小。为了实现这一目标，将对单个泡沫塑料支柱的磁塑性机理进行基本的实验和理论研究。通过使用两种泡沫制造方法(铸造和粉末冶金)，泡沫结构将在节点和支柱体积分数以及支柱尺寸和高宽比方面有所不同。泡沫颗粒的大小和质地将被量身定做：颗粒与支柱直径的比例将从远小于单位(多晶微结构)到与单位(竹子微结构)相当，质地将从随机质地变化到强烈的纤维质地。最后，将在两个长度尺度上对所得到的泡沫的磁力学性能进行表征和数值模拟：在较短的尺度上，将开发基于位错-位错和位错-界面相互作用的模型来预测自由表面对小体积Ni-Mn-Ga的本构行为的影响；在较大的尺度上，将创建有限元模型来基于本构行为来预测整体泡沫的磁力学行为。非技术性：PIS在初步研究中生产的新型磁性形状记忆泡沫，其应变和响应时间可与最好的商用磁致伸缩材料Terfenol D相媲美，并有望在这些基础研究的基础上显示出进一步的改进。与Terfenol D相比，Ni-Mn-Ga泡沫密度更低，含有更便宜的金属，因此可能会在工业上迅速增长，从而对各种传感器和致动器技术产生革命性的影响。此外，虽然目前的研究将集中在Ni-Mn-Ga上，但所研究的机理本质上是一般性的，因此将适用于所有其他磁性形状记忆合金。除了传感器和执行器的应用，开放的泡沫孔隙率还可以实现新的应用，例如(I)没有移动部件的微型泵，其中流体通过磁性变形的气孔进行置换，或(Ii)由于泡沫的较大特定面积而具有高传热率的高效磁冷却设备。最后，该项目将培养两名研究生和几名本科生，他们的招生将侧重于妇女和少数民族。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。私人投资公司已经提交了一项临时专利，并打算继续进行工业申请，这是将该领域过渡到美国高科技行业的关键。

TECHNICAL: 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 grains. The PIs recently discovered that porosity, because it reduces internal stresses, allows limited twinning to occur in polycrystalline Ni-Mn-Ga foams, resulting in magnetoplastic strains. Then, designing the foam architecture and grain microstructure will allow tuning continuously the magnetoplastic strain of these foams between those of a polycrystal (~0%) and a single crystal (~10%). In this basic study, PIs will develop a fundamental understanding of how foam architecture and grain microstructure enable magnetic-field-induced strains in polycrystalline magnetic shape-memory alloys, leading to experimentally-validated models that can quantitatively predict the magnitude of magnetoplastic strain for a given foam structure. To achieve this goal, fundamental experimental and theoretical studies of the mechanisms responsible for magnetoplasticity in the individual struts of foams will be carried out. The foam architecture will be varied, in terms of node and strut volume fraction as well as strut size and aspect ratio, by using two foam manufacturing methods (casting and powder metallurgy). The foam grain size and texture will be tailored: the ratio of grain to strut diameter will be varied from much smaller than unity (polycrystalline microstructure) to comparable to unity (bamboo microstructure), and the texture will be varied from random to strong fiber texture. Finally, the magneto-mechanical properties of the resulting foams will be characterized and numerically modeled on two length scales: at a shorter length scale, models based on dislocation-dislocation and dislocation-interface interactions will be developed to predict the effect of free surfaces on the constitutive behavior of Ni-Mn-Ga in small volumes; at larger length scale, finite-element models (FEM) will be created to predict, based on the constitutive behavior, the overall foam magneto-mechanical behavior. NON-TECHNICAL: The novel magnetic shape-memory foams, produced by the PIs in preliminary research, exhibit strains and response times comparable to Terfenol D, the best commercial magnetostrictive material, and are expected to show further improvements based on these fundamental study. As compared to Terfenol D, Ni-Mn-Ga foams have lower density and contain less expensive metals, and may thus grow rapidly in industrial importance, thus having a transformative effect on various sensor and actuator technologies. Also, while the present research will focus on Ni-Mn-Ga, the mechanisms studied are general in nature, and will thus apply to all other magnetic shape-memory alloys. Beyond sensor and actuator applications, the open foam porosity may enable new applications such as (i) micropumps without moving parts where fluids are displaced by magnetically deforming pores, or (ii) efficient magnetic cooling devices with high heat-transfer rates due to the large specific areas of foams. Finally, 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. The PIs have submitted a provisional patent and intend to pursue industrial applications which is key for transitioning the field to the US high-technology industry.