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.

Project Report

Magnetically active "smart" Ni-Mn-Ga foams or microcomponents could be used as actuator or sensor in the field of mechanical engineering, as micro-pumps in the biomedical field and as efficient, miniature magnetic cooling systems. Ferromagnetic Ni-Mn-Ga shape memory alloys with large magnetic field-induced strains (MFIS) are promising candidates for such applications. Magnetic shape memory alloys display very large MFIS of up to 10%, as single crystals. Polycrystalline Ni-Mn-Ga is much easier to manufacture but display a near-zero MFIS because twinning of neighboring grains introduces strain incompatibility leading to high internal stresses. We discovered that pores reduce these incompatibilities between grains and thus increase the MFIS of polycrystalline cast Ni-Mn-Ga which after training (thermo-magneto-mechanical cycling) exhibits MFIS as high as 8.7%. The porosity, texture and thermo-magnetic training effect on the magnetic shape-memory effect (MSME) of cast replicated Ni-Mn-Ga foams were studied and it was found that: i. The MFIS of Ni-Mn-Ga foams increased with increasing porosity, demonstrating that removal of constraints by addition of porosity is responsible for the high MFIS in polycrystalline foams. ii. The MSME enhancement of cast replicated Ni-Mn-Ga foams after the texture introduction via directional solidification is consistent with a reduction of incompatibility stresses under magnetic field from the reduced crystallographic misorientation between neighboring grains. iii. Polycrystalline Ni-Mn-Ga foam displays ~1% MFIS after the thermo-magnetic training. To show this training effect in this highly textured sample, neutron diffraction texture measurements were conducted with a magnetic field applied at various orientations to the sample, demonstrating that selection of martensite variants takes place during cooling. Oligocrystalline Ni-Mn-Ga foams with an open porosity of ~64% were created by a sintering replication process (via powder metallurgy) using near-monocrystalline Ni-Mn-Ga powders and NaCl space-holders with the combined advantages of built-in oligocrystalline grain-structured struts and large space-holder solubility. Ni-Mn-Ga microwires have been drawn by a custom self-built Taylor machine. Ni-Mn-Ga wires with sub-millimeter diameter, either as individual wires or as part of a 2D/3D wire assemblies, are promising candidates for actuators, sensors, magnetic cooling systems and energy harvesting devices. These wires are under current study. Ni-Mn-Ga tubes can be used in magnetic actuators or magnetic refrigerators and are even better suited than foams and fiber constructs for this application (and also possibly for low weight actuators). However, fabrication of Ni-Mn-Ga tubes with sub-millimeter diameter by classical cold or hot drawing methods is nearly impossible due to the brittleness of the alloy. We invented a new process, where Ni-Mn-Ga tubes were synthesized by interdiffusion of Mn and Ga into drawn, ductile Ni tubes with 500 and 760 micrometers inner and outer diameters. Finally, one major manufacturing issue - the Mn and Ga evaporations occurring during grain growth and long-time chemical ordering - has been solved by developing a new cover atmosphere, which guarantees composition control during processing.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Diana Farkas
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Northwestern University at Chicago
United States
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