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

技术：磁场诱导孪晶是在单晶Ni-Mn-Ga中实现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泡沫中产生有限的孪晶，从而导致磁塑性应变。然后，设计泡沫结构和晶粒微观结构，使这些泡沫的磁塑性应变在多晶（~0%）和单晶（~10%）之间连续调整。在这项基础研究中，pi将对泡沫结构和晶粒微观结构如何使多晶磁性形状记忆合金中的磁场诱发应变有一个基本的理解，从而导致实验验证的模型，可以定量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，将对泡沫材料各支柱的磁塑性机制进行基础实验和理论研究。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长径比方面有所不同。泡沫的晶粒尺寸和质地将进行定制：颗粒与支撑直径的比例将从远小于统一（多晶微观结构）到与统一（竹微观结构）相当，纹理将从随机到强纤维纹理变化。最后，所得到的泡沫的磁力学性能将在两个长度尺度上进行表征和数值模拟：在较短的长度尺度上，基于位错-位错和位错-界面相互作用的模型将被开发出来，以预测自由表面对Ni-Mn-Ga小体积本构行为的影响；在更大的长度尺度，将建立有限元模型（FEM）来预测，基于本构行为，整体泡沫的磁-力学行为。非技术：pi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业磁致伸缩材料Terfenol D相媲美，并有望在这些基础研究的基础上进一步改进。与Terfenol D相比，Ni-Mn-Ga泡沫具有较低的密度和较少的昂贵金属，因此在工业上的重要性可能迅速增长，从而对各种传感器和执行器技术产生变革性影响。此外，虽然目前的研究将集中在Ni-Mn-Ga上，但所研究的机制本质上是通用的，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应用之外，开放式泡沫孔隙率还可以实现新的应用，例如(i)没有运动部件的微型泵，其中流体被磁变形孔所取代，或（ii）由于泡沫的大特定区域而具有高传热率的高效磁冷却装置。最后，这个项目将培养两名研究生和几名本科生，他们的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。pi已经提交了一项临时专利，并打算寻求工业应用，这是将该领域过渡到美国高科技产业的关键。

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.