Unraveling phase transformation mechanism in next-generation shape memory alloys: Toward applications leveraging shape recovery and energy absorption properties
Cu-Al-Mn-based shape memory alloys (SMAs), expected to be next-generation SMAs, are promising for a wide range of applications, such as seismic-resistant structural materials and medical devices, due to their low cost, ease of processing, and excellent superelasticity. Recent studies reported large shape recovery in single crystals of this alloy, though the underlying mechanism remained unclear.
Collaborative research group of Assoc. Prof. Hiroshi Akamine (Graduate School of Integrated Science and Technology, Nagasaki University; formerly Assist. Prof. at Faculty of Engineering Sciences, Kyushu University), Ryo Takamatsu (formerly master’s student at Graduate School of Engineering Sciences, Kyushu University, now at DENSO Corporation), Prof. Emeritus Minoru Nishida (Kyushu University), Assist. Prof. Sheng Xu, Profs. Toshihiro Omori and Ryosuke Kainuma (Graduate school of Engineering, Tohoku University), Assoc. Prof. Kakeru Ninomiya and Prof. Maiko Nishibori (International Center for Synchrotron Radiation Innovation Smart, Tohoku University), and Dr. Sumio Kise (Furukawa Techno Material Co., Ltd.) clarified that multi-step martensitic transformations(*4) occur in single-crystal Cu-Al-Mn alloys via electron microscopy and synchrotron X-ray diffraction. Furthermore, they demonstrated that energy absorption increases as the multi-step martensitic transformation progresses. These findings suggest the potential for controlling energy absorption property in Cu-Al-Mn SMAs by manipulating phase transformations, enabling applications in seismic damping and civil engineering materials.
The results were published online in the journal Acta Materialia on April 15, 2025 (U.S. time).
Cu-Al-Mn-based SMAs are considered promising next-generation SMAs to replace popular Ti-Ni SMAs due to their lower material cost, workability, and excellent superelasticity. A challenge has been their tendency to fracture along grain boundaries. The recent development of low-cost single-crystal growth technique(*5) has opened practical use and accelerated research of single-crystal Cu-Al-Mn SMAs. It was recently discovered that large shape recovery can be achieved by applying tensile stress along specific crystallographic directions in single crystals, but the mechanism remained unclear.
Through transmission electron microscopy (TEM) and synchrotron X-ray diffraction, the research group for the first time revealed that single-crystal Cu-Al-Mn alloys undergo multi-step martensitic transformations (see Fig. 1).
Figure 1 shows the results of TEM observations. It was revealed at the atomic scale that the initial L2₁ structure transforms under stress into 18R (6M) and subsequently 6R (2M) structures. Synchrotron X-ray diffraction verified the same transformation at the bulk state under applied stress.
Fig. 1 TEM observation of phase transformation:
(a, b) Electron diffraction patterns from 6M (18R) and 2M (6R) structures
(c, d) Atomic-resolution images of 6M (18R) and 2M (6R) structures
It was also found that as the second-stage transformation (6M→2M) progresses, the area enclosed by the stress–strain hysteresis loop increases, indicating greater energy absorption (see Fig. 2). Materials with high energy absorption are suitable for damping applications and are expected to be used as seismic-resistant structural materials. Furthermore, the alloy exhibits superelasticity and can recover its shape even after more than 10% strain, offering potential to prevent structural collapse.
Fig. 2 (a) Stress–strain curves of cyclic tensile test under incremental strain. The first-stage transformation (L2₁→6M) occurs up to ~11%, followed by a second-stage transformation (6M→2M).
(b) Energy absorption calculated from stress–strain curves (blue line). Energy absorption increases sharply beyond 12% strain where the second-stage transformation occurs.
This research was supported by JSPS KAKENHI grants (19H00829, 23K13540, 23H00231, 23H00206). Synchrotron X-ray diffraction measurements were conducted at SPring-8 (proposal nos. 2020A1197, 2021B1270, 2021B3740). The paper is open access thanks to APC support from Nagasaki University.
*1: Cu-Al-Mn SMAs
Developed by a research group led by Professors Ishida, Kainuma, Sutou, and Omori at Tohoku University (Kainuma et al., 1996; Sutou et al., 2008).
*2: Superelasticity
A property where a material deforms via phase transformation and returns to its original shape upon unloading, without heating. Unlike shape memory effects requiring thermal input, this behavior occurs at constant temperature.
*3: Energy absorption capacity
When force is applied to a material, some energy is dissipated through internal friction or heat emission. This dissipated energy indicates the material’s ability to absorb vibrations.
*4: Martensitic transformation
A transformation involving coordinated atomic shifts without atomic diffusion, also called displacive or diffusionless transformation.
*5: Low-cost single crystal growth technique
A method of obtaining single crystals via cyclic heat treatment without large-scale equipment. Developed by Tohoku University, Professor Araki (Kyoto University, formerly Nagoya University), and Furukawa Techno Material Co., Ltd. (Omori et al., 2013; Kusama et al., 2017).
Title: Successive stress-induced phase transformations with large stress-strain hysteresis in single crystal Cu-Al-Mn shape memory alloys
Authors: Hiroshi Akamine, Ryo Takamatsu, Sheng Xu, Toshihiro Omori, Ryosuke Kainuma, Sumio Kise, Kakeru Ninomiya, Maiko Nishibori, Minoru Nishida
Journal: Acta Materialia
DOI: 10.1016/j.actamat.2025.121054
