Effects of surface damage on twinning stress and the stability of twin microstructures of magnetic shape memory alloys

Chmielus, Markus; Witherspoon, Cassie Lynne; Ullakko, K.; Müllner, Peter; Schneider, R.

doi:10.1016/j.actamat.2011.01.035

Cited by 46 publications

(28 citation statements)

References 24 publications

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“…Subsequently, the release of this internal stress can drive the single variant to transform back into the initial state prior to applying stress, i.e., re-creating multi-variants upon mechanical unloading. A similar hypothesis was advanced to explain the $4% recovery observed in a 5M Ni-Mn-Ga single crystal, where obstacles to the twins were attributed the rough surface of the sample [36]. Thus, the superelastic behavior of the present wires can be attributed to the existence of the c precipitates (and possibly also surface roughness).…”

Section: Mechanism For Fluctuations and Superelasticitymentioning

confidence: 57%

Superelasticity by reversible variants reorientation in a Ni–Mn–Ga microwire with bamboo grains

et al. 2015

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a b s t r a c tThe link between microstructure and mechanical properties is investigated for a superelastic Ni-Mn-Ga microwire with 226 lm diameter, created by solidification via the Taylor method. The wire, which consists of bamboo grains with tetragonal martensite matrix and coarse c precipitates, exhibits fully reversible superelastic behavior up to 4% tensile strain. Upon multiple tensile load-unload cycles, reproducible stress fluctuations of $3 MPa are measured on the loading superelastic stress plateau of $50 MPa. During cycles at various temperatures spanning À70 to 55°C, the plateau stress decreases from 58 to 48 MPa near linearly with increasing temperature. Based on in situ synchrotron X-ray diffraction measurements, we conclude that this superelastic behavior is due to reversible martensite variants reorientation (i.e., reversible twinning) with lattice rotation of $13°. The reproducible stress plateau fluctuations are assigned to reversible twinning at well-defined locations along the wire. The strain recovery during unloading is attributed to reverse twinning, driven by the internal stress generated on loading between the elastic c precipitates and the twinning martensite matrix. The temperature dependence of the twining stress on loading is related to the change in tetragonality of the martensite, as measured by X-ray diffraction.

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Section: Mechanism For Fluctuations and Superelasticitymentioning

confidence: 57%

Superelasticity by reversible variants reorientation in a Ni–Mn–Ga microwire with bamboo grains

et al. 2015

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“…Measured transient thrusts with a 10M Ni-Mn-Ga single crystal rod with aspect ratio 5 were much higher than those from biological equivalents of the same size, e.g., tadpole tails. By contrast, in MSMA samples with aspect ratio close to or below unity, uniaxial strain due to MFIS, rather than bending, is the dominant response [17] albeit with kinking if only one twin boundary carries the deformation [18,19]. Chmielus et al [20] reported large MFIS in polycrystalline Ni-Mn-Ga foam with single crystalline or bamboo struts.…”

Section: Introductionmentioning

confidence: 83%

“…3(b), the unloading part of the stress-strain curve exhibits a large recovery of nearly 4% when the stress falls below about 5 MPa. Chmielus et al [19] reported that, for a monocrystalline 14M sample with 2.4 Â 3.0 Â 4.4 mm dimensions, large compressive unloading strain recovery of $1% could be attributed to the relatively rough surface of a sample polished with 6 lm diamond slurry. A deformed surface layer impedes twin boundary motion near the surface.…”

Section: Uniaxial Compressive Deformationmentioning

confidence: 98%

Magnetic-field-induced bending and straining of Ni–Mn–Ga single crystal beams with high aspect ratios

et al. 2015

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a b s t r a c tSmall monocrystalline beams of the magnetic shape-memory alloy (MSMA) Ni-Mn-Ga, with a square 1Â1 mm 2 cross section and length between 2 and 10 mm, with the 10M martensite structure and all faces parallel to {100}, were subjected to rotating magnetic fields while being held at one end. The beams deform by both magnetic-field-induced straining (MFIS) and magnetic-torque-induced bending (MTIB), in directions parallel and perpendicular to the beam's longitudinal axis, respectively. With the field parallel to the beam axis, the beams were straight and short. Upon field rotation, the beam elongated and bent in the direction of the field. When the field reached 90°, the beam deflected rapidly and took a bent shape oriented in the opposite direction. Upon further field rotation, bending strain and axial strain decreased until the beam was short and straight again with the field at 180°. With an increase in beam aspect ratio, the bending component increases while the total axial strain remains constant. MTIB -a natural but so far neglected response of long MSMA samples exposed to a transversal magnetic fieldoccurs during switching of current linear (one-dimensional) actuators, thus causing friction losses, wear, and fatigue. However, MTIB provides the opportunity to actuate high aspect ratio MSMA continuously and smoothly in all directions (in three dimensions), thus mimicking slender biological actuating structures such as microorganism flagella tails and fins of fish, heart valves, leaves and petals of plants, and wings of birds or insects.

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“…In that case a hysteresis as large as 9 K was measured [28]. The small hysteresis seen in the present alloy shall be rather attributed to an artifact resulting from the thermal mass of the sample [29]. This effect can be assumed to increase also the apparent hysteresis of the structural phase transformation, but is neglected here, because this work focuses on the structural and magnetic behavior across the transformation.…”

Section: Transformation Temperatures Derived From Dsc Measurementsmentioning

confidence: 99%

Structural behavior and magnetic properties of a Ni–Mn–Ga single crystal across the martensite/austenite two-phase region

et al. 2015

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Effects of surface damage on twinning stress and the stability of twin microstructures of magnetic shape memory alloys

Cited by 46 publications

References 24 publications

Superelasticity by reversible variants reorientation in a Ni–Mn–Ga microwire with bamboo grains

Superelasticity by reversible variants reorientation in a Ni–Mn–Ga microwire with bamboo grains

Magnetic-field-induced bending and straining of Ni–Mn–Ga single crystal beams with high aspect ratios

Structural behavior and magnetic properties of a Ni–Mn–Ga single crystal across the martensite/austenite two-phase region

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