Magnetic-field-induced twin boundary motion in magnetic shape-memory alloys

2000 ◽  
Vol 61 (22) ◽  
pp. R14913-R14915 ◽  
Author(s):  
Harsh Deep Chopra ◽  
Chunhai Ji ◽  
V. V. Kokorin
Keyword(s):  
Magnetic Field ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Twin Boundary ◽  
Boundary Motion ◽  
Magnetic Shape Memory ◽  
Magnetic Shape Memory Alloys

Numerical Simulation of Twin-Twin Interaction in Magnetic Shape-Memory Alloys

2008 ◽  
Vol 1090 ◽  
Author(s):  
Markus Chmielus ◽  
David Carpenter ◽  
Alan Geleynse ◽  
Michael Hagler ◽  
Rainer Schneider ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Shear Stress ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Shear Strain ◽  
Twin Boundary ◽  
Boundary Motion ◽  
Magnetic Shape Memory ◽  
Magnetic Shape Memory Alloys ◽  
Line Defects

AbstractTwin boundary motion is the mechanism that drives the plastic deformation in magnetic shape memory alloys (MSMAs), and is largely dependent on the twin microstructure of the MSMA. The twin microstructure is established during the martensitic transformation, and can be influenced through thermo-magneto-mechanical training. For self-accommodated and ineffectively trained martensite, twin thickness and magnetic-field-induced strain (MFIS) are very small. For effectively trained crystals, a single crystallographic domain may comprise the entire sample and MFIS reaches the theoretical limit. In this paper, a numerical simulation is presented describing the twin microstructures and twin boundary motion of self-accommodated martensite using disclinations and disconnections (twinning dislocations). Disclinations are line defects such as dislocations, however with a rotational displacement field. A quadrupole solution was chosen to approximate the defect structure where two quadrupoles represent an elementary twin double layer unit. In the simulation, the twin boundary was inclined to the twinning plane which required the introduction of twinning disconnections, which are line defects with a stress field similar to dislocations. The shear stress - shear strain properties of self-accommodated martensite were analyzed numerically for different initial configurations of the twin boundary (i.e. for different initial positions of the disconnections). The shear stress - shear strain curve was found to be sensitive to the initial configuration of disconnections. If the disconnections are very close to boundaries of hierarchically higher twins – such as is the case for self-accommodated martensite, there is a threshold stress for twin-boundary motion. If the disconnections are spread out along the twin boundary, twinning occurs at much lower stress.

Effects of Surface Pinning, Locking and Adaption of Twins on the Performance of Magnetic Shape-Memory Alloys

2011 ◽  
Vol 684 ◽  
pp. 177-201 ◽  
Author(s):  
Markus Chmielus ◽  
Peter Müllner
Keyword(s):  
Magnetic Field ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Surface Damage ◽  
Magnetic Shape Memory ◽  
Magnetic Shape Memory Alloys ◽  
Fatigue Lifetime ◽  
Twin Density ◽  
External Constraints ◽  
The Magnetic Field

We study the effect of surface modifications and constraints on the mechanical properties of Ni-Mn- Ga single crystals, which are imposed by (i) structural modifications near the surface, (ii) mounting to a solid surface, and (iii) guiding the stroke. Spark eroded samples were electropolished and characterized before and after each polishing treatment. Surface damage was then produced with spark erosion and abrasive wearing. Surface damage stabilizes and pins a dense twin-microstructure and prevents twins from coarsening. The density of twins increases with increasing degree of surface deformation. Twinning stress and hardening rate during mechanical loading increase with increasing surface damage and twin density. In contrast, when a damaged surface layer is removed, twinning stresses, hardening rate, and twin density decrease. Constraining the sample by mounting and guiding reduces the magnetic-field-induced strain by locking twins at the constrained surfaces. . For single-domain crystals and for hard magnetic shape-memory alloys, external constraints strongly reduce the magnetic-field-induced strain and the fatigue lifetime is short. In contrast, for selfaccommodated martensite and for soft magnetic shape-memory alloys, the twin-microstructure adapts well to external constraints and the fatigue lifetime is long. The performance of devices with MSMA transducers requires managing stress distributions through design and control of surface properties, microstructure, and constraints.

Magnetic-field-induced changes in magnetic shape memory alloys

2008 ◽  
Vol 481-482 ◽  
pp. 258-261 ◽  
Author(s):  
P. Entel ◽  
M.E. Gruner ◽  
W.A. Adeagbo ◽  
A.T. Zayak
Keyword(s):  
Magnetic Field ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Magnetic Shape Memory ◽  
Magnetic Shape Memory Alloys ◽  
Induced Changes

A Memory Variable Approach to Modeling the Magneto-Mechanical Behavior of Magnetic Shape Memory Alloys

2013 ◽  
Author(s):  
Doug LaMaster ◽  
Heidi Feigenbaum ◽  
Isaac Nelson ◽  
Constantin Ciocanel
Keyword(s):  
Magnetic Field ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Mechanical Strain ◽  
Magnetization Vector ◽  
Unit Cell ◽  
Magnetic Shape Memory ◽  
Magnetic Shape Memory Alloys ◽  
Short Side ◽  
Recoverable Strain

Magnetic shape memory alloys (MSMAs) have attracted interest because of their considerable recoverable strain (up to 10%) and fast response time (1 kilohertz or higher). MSMAs are comprised of martensitic variants that have tetragonal unit cells and a magnetization vector that is innately aligned with the short side of the unit cell. These variants rotate either to align the magnetization vector with an applied magnetic field or to align the short side of the unit cell with an applied compressive stress. This reorientation leads to a mechanical strain and an overall change in the material’s magnetization, allowing MSMAs to be used as actuators, sensors, and power harvesters. This paper builds upon the work of Kiefer and Lagoudas [4,5] as well as improvements proposed by LaMaster et al. [1] to present a thermodynamic based model to predict the response of an MSMA to axial mechanical loading and transverse magnetic loading. This work is unique, however, in its use of a memory variable, which references the last stable configuration. This is similar to the approach used by Saint-Sulpice [2] in modeling SMA wires. The resulting model has zero driving force for reorientation of variants at the beginning of any load and again when the load is removed. Thus the model predicts what is seen physically, that the material is stable when no magneto-mechanical load is present. Furthermore, this model is more physical and less empirical than others in the literature, having only 2 material parameters associated with the stress-strain or stress-field response. In addition, this model includes evolution rules for the magnetic domain volume fractions and the angle of rotation of the magnetization vectors based on thermodynamic requirements. The resulting model is calibrated and predictions are compared with both the more established Keifer and Lagoudas model as well as experimental data. Results show decent correlation with experiments. The model can be further improved by calibrating the demagnetization factor to experimentally measured changes in magnetic field.

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