Modeling magnetoelasticity and magnetoplasticity with disconnections and disclinations
- PDF / 586,914 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 34 Downloads / 176 Views
1050-BB02-01
Modeling magnetoelasticity and magnetoplasticity with disconnections and disclinations Peter Mullner1, Alan Steward Geleynse1, David Robert Carpenter1, Michael Scott Hagler1, and Markus Chmielus1,2 1 Materials Science and Engineering, Boise State University, 1910 University Dr., MS 2075, Boise, ID, 83725 2 Smart Magnetic Materials Group, SF1, Hahn-Meitner-Institut, Berlin, 14109, Germany Abstract The magneto-mechanical properties of magnetic shape-memory alloy single crystals depend strongly on the twin microstructure which is established during the martensitic transformation, and through thermo-magneto-mechanical training. For self-accommodated martensite, twin thickness and magnetic-field-induced strain are very small. For effectively trained crystals, a single twin may comprise the entire sample and magnetic-field-induced strain reaches the theoretical limit. Furthermore, the deformation of self-accommodated martensite is pseudoelastic (magnetoelasticity) while the deformation of effectively trained crystals is plastic (magnetoplasticity). Twin microstructures of self-accommodated martensite were modeled using disclinations which are line defects such as dislocations, however with a rotational displacement field. The defect structure was approximated in a quadrupole solution where two quadrupoles represent an elementary twin double layer unit. The twin boundary was inclined to the twinning plane which required the introduction of twinning disconnections. 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 is sensitive to the initial configuration indicating that disconnection nucleation is controlling the magneto-mechanical properties of self-accommodated martensite. INTRODUCTION AND BACKGROUND Magnetic Shape Memory Alloys (MSMAs) tend to deform up to 10% upon the application of a magnetic field [1-3]. The magnetic-field-induced strain is due to twin boundaries moving under the influence of an internal stress produced by magnetic anisotropy energy [4]. The amount of strain varies from about 0.002% for fine-grained polycrystalline Ni-Mn-Ga [5] up to 10% for single-variant 14M Ni-Mn-Ga single crystals [2]. For Ni-Mn-Ga single crystals, the magneticfield-induced strain strongly depends on training, i.e. on thermo-magneto-mechanical treatments [6, 7]. Training biases the twin microstructure leading to a predominant twin variant. Very effective training leads to the formation of a single-variant crystal. After ineffective training, the microstructure contains various twin variants with almost equal fractions. For effectively trained Ni-Mn-Ga, magnetic-field-induced deformation tends to be large and permanent upon removal of the magnetic field. The term ‘magnetoplasticity’ is used for this type of deformation [8, 9]. For ineffectively trained Ni-Mn-Ga, magnetic-field-induced deformation tends to be small a
Data Loading...
