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Ni–Mn–Ga Heusler alloys are multifunctional materials that demonstrate macroscopic strain under an externally applied magnetic field through the motion of martensite twin boundaries within the microstructure. This study sought to comprehensively characterize the microstructural, mechanical, thermal, and magnetic properties near the solidus in binder-jet 3D printed 14M Ni50Mn30Ga20. Neutron diffraction data were analyzed to identify the martensite modulation and observe the grain size evolution in samples sintered at temperatures of 1080 °C and 1090 °C. Large clusters of high neutron-count pixels in samples sintered at 1090 °C were identified, suggesting Bragg diffraction of large grains (near doubling in size) compared to 1080 °C sintered samples. The grain size was confirmed through quantitative stereology of polished surfaces for differently sintered and heat-treated samples. Nanoindentation testing revealed a greater resistance to plasticity and a larger elastic modulus in 1090 °C sintered samples (relative density ~95%) compared to the samples sintered at 1080 °C (relative density ~80%). Martensitic transformation temperatures were lower for samples sintered at 1090 °C than 1080 °C, though a further heat treatment step could be added to tailor the transformation temperature. Microstructurally, twin variants ≤10 μm in width were observed and the presence of magnetic anisotropy was confirmed through magnetic force microscopy. This study indicates that a 10 °C sintering temperature difference can largely affect the microstructure and mechanical properties (including elastic modulus and hardness) while still allowing for the presence of magnetic twin variants in the resulting modulated martensite.
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Traveling surface undulation on a Ni-Mn-Ga single crystal elemen
Active materials couple a stimulus (electrical, magnetic, thermal) with a mechanical response. Typical materials such as piezoelectrics strain as bulk materials to the stimuli. Here we consider an undulation created by heterogeneous deformation within a magnetic shape memory alloy (MSM) transducer. We study the mechanical response of an MSM element vs. two surface treatments: a polished state with minimal surface stresses, and a micropeened state with compressive surface stress. The polished element had a sharp-featured, faceted trough shape. The micropeened element had a smooth trough shape and an additional crest. The undulation was created by a rotating localized magnetic field, which caused heterogeneous variation of the twin-microstructure. For the polished and micropeened elements, the twin-microstructures were coarse and fine, respectively. For the polished element, the undulation moved by the nucleation of a few twin boundaries, which traveled along the entire element. For the micropeened sample, the twin boundaries moved back and forth over a short distance, thereby creating a dense twin lamellar, which formed the trough. The motion of the lamellar approximated the single thick twin while allowing additional degrees of freedom due to increased mobile interface density and different initial conditions of domain volume fraction. The dense twin microstructure also smoothed the magnetic flux pattern. The undulation amplitude was about 40 μm for the sample in both treatments.
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- Award ID(s):
- 1710640
- PAR ID:
- 10272996
- Date Published:
- Journal Name:
- Smart materials and structures
- Volume:
- 30
- ISSN:
- 1361-665X
- Page Range / eLocation ID:
- 085001
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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