Nucleation and growth of hierarchical martensite in epitaxial shape memory films

Niemann, Robert; Backen, Anja; Kauffmann-Weiss, Sandra; Behler, Christian; Rößler, U.; Seiner, Hanuš; Heczko, Oleg; Nielsch, Kornelius; Schultz, L.; Fähler, S.

doi:10.1016/j.actamat.2017.04.032

Cited by 47 publications

(55 citation statements)

References 43 publications

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“…This indicates that more nuclei compared to bulk single crystals are required to transform a complete film. This is in agreement with the very fine martensitic microstructure observed in films . In polycrystalline Ni–Mn–Sn alloys, a nearly linear behavior for cooling (as observed in our work) but also in heating minor loops was obtained .…”

Section: Resultssupporting

confidence: 92%

“…Following Ref. , the diamond‐shaped nuclei consist of eight differently oriented martensitic variants (less when close to a surface), all connected by high‐order twin boundaries and are enclosed by habit planes. Due to the high amount of interface energy at the internal and external interfaces of the nucleus, a high nucleation energy barrier has to be overcome by undercooling several Kelvin.…”

Section: Nucleation and Growthmentioning

confidence: 99%

“…Regions of austenite, which remain untransformed, are mostly filled by new martensitic nuclei. 5) Below the martensitic finish‐temperature ( T < M F ) there is a hierarchical twins‐within‐twins microstructure which is typical for Ni–Mn–X(‐Co) films . However, at the disturbed interfaces between differently oriented, macroscopic twinned regions, a certain amount of residual austenite (marked by a green ellipsoid) remains untransformed.…”

Section: Nucleation and Growthmentioning

confidence: 99%

“…, they can be described as mesoscopic boundaries which are incompatible and thus can act as nucleation sites. Residual austenite is present for T < M F since the martensitic nuclei cannot fill infinitesimal small areas …”

Section: Nucleation and Growthmentioning

confidence: 99%

“…The high surface‐to‐volume‐ratio of films enables a fast heat exchange with the surrounding medium, allowing for high cycling frequencies and thus high specific cooling power. Due to their well‐defined crystallographic orientations, they are suitable as model system to analyze the influence of interfaces on phase transformation and to study nucleation and growth processes of martensite . For studying intrinsic properties, which influence hysteresis in materials with first‐order transitions, epitaxial films are an alternative that is easy to manufacture in comparison to single crystals …”

Section: Introductionmentioning

confidence: 99%

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Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

et al. 2018

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Magnetocaloric materials relying on first‐order phase transitions give the highest entropy changes compared to second‐order transitions. However, they have the drawback of a transformation hysteresis, which reduces the efficiency of a cooling cycle. Here an approach to reduce the thermal hysteresis losses by performing incomplete transformation cycles, so called minor loops, is presented. A freestanding, epitaxially grown Ni–Mn–Ga–Co film is used as a model system to analyze the direct martensitic and reverse transformation paths in detail. The additional residual phase fraction only facilitates the forward martensitic transformation, but not the reverse transformation. This pronounced asymmetry between cooling and heating demonstrates that the forward transformation to martensite is controlled by nucleation and the associated excess energy contributes to hysteresis losses. For the reverse transformation, however, nuclei of austenite are always present and accordingly this transformation is controlled by growth of the austenitic phase. The experiments reveal that only the use of heating minor loops is useful to improve the efficiency of magnetocaloric epitaxial films and by this, the ratio of usable magnetization difference divided by hysteresis loss can be increased by 64 %.

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Section: Resultssupporting

confidence: 92%

Section: Nucleation and Growthmentioning

confidence: 99%

Section: Nucleation and Growthmentioning

confidence: 99%

Section: Nucleation and Growthmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

et al. 2018

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Crystallography of Martensitic Transformation in Epitaxial Ni₅₀Mn₃₀Ga₂₀ Thin Film

et al. 2017

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In the present work, scanning electron microscopy and electron backscatter diffraction technique revealed that the microstructure of epitaxial Ni 50 Mn 30 Ga 20 thin films are composed of six different orientated NM martensite variant colonies. In each martensite variant colony, there are eight different orientated NM martensite variants. The six NM and 7M martensite variant colonies are transformed from six (101) A planes of austenite, with the transformation consequence from Austenite to 7M martensite and then to NM martensite. Cross section TEM examination reveals the coexistence of 7M martensite and NM martensite. The habit plane between 7M martensite and NM martensite can be determined as (001) mono of 7M martensite and (112) Tetr of NM martensite.

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Microfabrication Approaches on Magnetic Shape Memory Films

Kar,

Nielsch,

Fähler

et al. 2023

Adv Eng Mater

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Magnetic shape memory alloys are emerging multifunctional materials that enable applications like high‐stroke actuation, solid‐state refrigeration, and energy harvesting of waste heat. Thin films of these alloys promise integration in microsystems to exploit their multifunctional properties at the microscale. However, the microfabrication process of these Heusler alloys is difficult. Here, we investigate different etching techniques for the microfabrication of epitaxial Ni‐Mn‐Ga films, explain the encountered challenges, and demonstrate ways to overcome them. Our results show that wet chemical etching is suitable for large patterned structures, while reactive ion etching of Ni‐Mn‐Ga films is unsuitable due to redeposition. For patterning structures below 10 µm with clean and sharp edges, the best results are obtained by ion‐beam etching with adjusted sample‐stage tilt. Finally, we demonstrate a microfabrication process using Si microtechnology to fabricate partially free‐standing structures. Our findings give guidelines for the fabrication and integration of these smart materials in Si‐based microsystems.This article is protected by copyright. All rights reserved.

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Nucleation and growth of hierarchical martensite in epitaxial shape memory films

Cited by 47 publications

References 43 publications

Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

Crystallography of Martensitic Transformation in Epitaxial Ni₅₀Mn₃₀Ga₂₀ Thin Film

Microfabrication Approaches on Magnetic Shape Memory Films

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Nucleation and growth of hierarchical martensite in epitaxial shape memory films

Cited by 47 publications

References 43 publications

Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

Crystallography of Martensitic Transformation in Epitaxial Ni50Mn30Ga20 Thin Film

Microfabrication Approaches on Magnetic Shape Memory Films

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Crystallography of Martensitic Transformation in Epitaxial Ni₅₀Mn₃₀Ga₂₀ Thin Film