Localized deformation in Ni-Mn-Ga single crystals

Abstract

The magnetomechanical behavior of ferromagnetic shape memory alloys such as Ni-Mn-Ga, and hence the relationship between structure and nanoscale magnetomechanical properties, is of interest for their potential applications in actuators. Furthermore, due to its crystal structure, the behavior of Ni-Mn-Ga is anisotropic. Accordingly, nanoindentation and magnetic force microscopy were used to probe the nanoscale mechanical and magnetic properties of electropolished single crystalline 10M martensitic Ni-Mn-Ga as a function of the crystallographic c-axis (easy magnetization) direction relative to the indentation surface (i.e., c-axis in-plane versus out-of-plane). Load-displacement curves from 5-10 mN indentations on in-plane regions exhibited pop-in during loading, whereas this phenomenon was absent in out-of-plane regions. Additionally, the reduced elastic modulus measured for the c-axis out-of-plane orientation was $50% greater than for in-plane. Although heating above the transition temperature to the austenitic phase followed by cooling to the room temperature martensitic phase led to partial recovery of the indentation deformation, the magnitude and direction of recovery depended on the original relative orientation of the crystallographic c-axis: positive recovery for the in-plane orientation versus negative recovery (i.e., increased indent depth) for out-of-plane. Moreover, the c-axis orientation for out-of-plane regions switched to in-plane upon thermal cycling, whereas the number of twins in the in-plane regions increased. We hypothesize that dislocation plasticity contributes to the permanent deformation, while pseudoelastic twinning causes pop-in during loading and large recovery during unloading in the c-axis in-plane case. Minimization of indent strain energy accounts for the observed changes in twin orientation and number following thermal cycling.

Grain Growth, Porosity, and Hardness Changes in Sintered and Annealed Binder-jet 3D Printed Ni-Mn-Ga Magnetic Shape Memory Alloys

“…It can be seen that low-load nanoindentation testing can be used in assessing the twin boundary movement behavior of porous Ni-Mn-Ga matrices, as the primary mode of plastic deformation can be limited to that of local glissile dislocation movement of the 14M martensitic phase. As noted by Aaltio et al[14], these discontinuities are representative twin boundary movements in this experiment which has overcome the necessary external force threshold to travel about the sample[15]. Behavior of the forcedisplacement loading curve observed in our samples contain values and shape similar to that observed by Aaltio et al, indicating similar micro-mechanical properties of binder-jet 3D-printed and sintered Ni-Mn-Ga.…”

Pop-In Phenomenon as a Fundamental Plasticity Probed by Nanoindentation Technique

“…Oliver et al demonstrated a similar phase transformation behavior in Ge [ 216 ], while Bradby et al showed no phase transformation in GaAs, InP, and GaN, but a slip deformation for the pop-in behavior [ 217 ]. Shape-memory alloys also exhibit transformation-associated pop-in behavior [ 218 , 219 , 220 , 221 ]. Caër et al demonstrated the pop-in and pop-out behavior in CuAlBe and discussed the phase transformation and deformation mechanisms [ 218 ].…”