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

Abstract

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.

Effects of cyclic heat treatment and aging on superelasticity in oligocrystalline Fe-Mn-Al-Ni shape memory alloy wires

“…The superelastic stress-strain curves in Fig.3a, 3b and 3c show serrated response, which is more pronounced during the forward transformation. Similar serrated stress strain behaviors were observed previously in other FeMnAlNi alloys as well as in magnetic SMAs such as NiMnGa [24,25]. Omori et al reported serrated stress-strain behavior in FeMnAlNi polycrystalline wires with a d/D ratio greater than one [19].…”

“…Serrated deformation behavior has also been observed in NiMnGa [24,25], however the serrations were linked to the surface roughness of the samples. The damaged surface layer resulting from spark plasma cutting or introduced via surface preparation can become stress concentrators, act as finely dispersed pinning sites for twin boundaries, and limits the ability of the martensite to coarsen.…”

Fatigue life and fracture mechanics of unconstrained Ni–Mn–Ga single crystals in a rotating magnetic field

“…When defects such as scratches are below 10 nm, the average switching field (and hence twinning stress) is lowered and twin nucleation occurs more easily [27]. When surface defects are much larger there is a tendency for dislocations to become pinned and interact with other passing dislocations [18,26]. Polishing with coarse slurry leads to a rougher surface and a stressed surface layer which allows for increased number of crack initiation points where dislocations will pile up and induce high stress concentration points.…”

“…Constraining the FSMA element inhibits twin reorientation locally within the constrained volume. Surface defects and residual stresses limit twin boundary motion and increases twinning stress [18]. Constrained samples exhibit reduced strain and prolonged fatigue life [19].…”

Material Design, Processing, and Engineering Requirements for Magnetic Shape Memory Devices

“…Surface damage stabilizes a fine twin structure, as shown by Chmielus et al [79] and Ezer et al [80]. 10M samples with fine twin structure, whether oriented or lamellar, exhibits good fatigue life [71,83]. In [76], Straka et al propose a phenomenological model, which describes the mechanism of reducing fatigue.…”