First-Ever Clarification of Mechanism of Shape Memory Effect Driven by Magnetic Field - Breakthrough towards Practical Use of High-Output Actuators (Press Release)
- Release Date
- 27 Apr, 2010
- BL15XU (WEBRAM)
Hiroshima University
Tohoku University
National Institute for Materials Science
Tohoku Gakuin University
Significance of this research
• The mechanism of structural phase transition in ferromagnetic shape memory alloys, which has been unclear, was first clarified from the viewpoint of electron structure.
• The results of this research are expected to provide powerful guidelines to materials design for next-generation actuators mainly using ferromagnetic shape memory alloys with advanced performance.
|
The research group where Akio Kimura, an associate professor, and Ye Mao, a student at the Graduate School of Science, Hiroshima University; Masafumi Shirai, a professor, and Yoshio Miura, an assistant professor at the Research Institute of Electrical Communication, Tohoku University; Keisuke Kobayashi, a research fellow, and Shigenori Ueda, a fellow at the National Institute for Materials Science (NIMS); Ryosuke Kainuma, a professor at the Institute for Multidisciplinary Research for Advanced Materials, Tohoku University; and Takeshi Kanomata, a professor at the Faculty of Engineering, Tohoku Gakuin University, are the main members, has clarified the mechanism of structural phase transition in ferromagnetic shape memory alloys by hard X-ray photoelectron spectroscopy at SPring-8 and by first-principles calculation, a theoretical method, for the first time in the world. This achievement is expected to lead to the provision of strict guidelines for the material design of next-generation actuators mainly using ferromagnetic shape memory alloys. This research was supported by Grants-in-Aid for Scientific Research and Cooperative Research Projects at the Research Institute of Electrical Communication, Tohoku University. The results were published online as a highlighted paper in the American scientific journal Physical Review Letters on 26 April 2010, prior to the printed version on 30 April 2010. Publication: (Physics Viewpoint) |
<Figure>
Fig. 1 Variant conversion in shape memory alloys
The state of shape memory alloys switches between high- and low-temperature phases upon changing the temperature. In most cases, these alloys have a cubic structure in the high-temperature phase and a low-symmetry structure in the low-temperature phase. The low-temperature phase is composed of several regions with different crystallographic orientations, called variants. When an alloy is cooled to induce phase transition, all variants are generated in almost equal amounts and arranged to minimize the shape change due to phase transition. In such an arrangement, the boundaries between variants form twin interfaces that move relatively easily. Therefore, when a stress is applied to the interfaces in the low-temperature phase, they move and variant conversion occurs, resulting in large distortion. This large distortion disappears upon transition to the high-temperature phase, and then the alloy returns to its original shape. This is the shape memory effect.
Fig. 2 Crystal structures of Ni2Mn1-xSn1-x in high-temperature phase (cubic phase)
and low-temperature phase (martensitic phase)
Ni2Mn1-xSn1-x has a cubic structure (cubic crystal) with high symmetry in the high-temperature phase, whereas it has a low-symmetry structure (orthorhombic crystal) in the low-temperature phase after the martensitic transformation.
Fig. 3 Hard X-ray photoelectron spectra of Ni2Mn1-xSn1-x
When the hard X-ray photoelectron spectra of Ni2Mn1-xSn1-x were measured while decreasing the temperature from 300 K (room temperature) to 20 K, marked changes in the electron structure near the Fermi energy were observed at the temperature of the martensitic transformation [Fig. 3(a)]. Also, the peak of spectra observed in the high-temperature phase shifted towards the low-energy side with increasing Mn concentration [Fig. 3(b)].
Fig. 4 Electron state density of Ni2Mn1-xSn1-x obtained by first-principles calculation (a)
and dependence of total energy on lattice constant ratio (c/a) (b)
The electron state density obtained by first-principles calculation revealed that the Ni 3d minority-spin electron state shifted towards the low-energy side with increasing Mn concentration [Fig. 4(a)]. The first-principles calculation excellently explains that the cubic crystal becomes unstable in terms of energy when Mn concentration increases [Fig. 4(b)].
|
For more information, please contact: Prof. Masafumi SHIRAI (Tohoku University) Dr. Keisuke KOBAYASHI (NIMS) Prof. Takeshi KANOMATA (Tohoku Gakuin University) |
- Previous Article
- Development of a New Experimental Method to Clarify Atomic Orbitals of Conduction Electrons in Solids
- Current article
- First-Ever Clarification of Mechanism of Shape Memory Effect Driven by Magnetic Field - Breakthrough towards Practical Use of High-Output Actuators (Press Release)