Zelený Martin, Straka Ladislav, Sozinov Alexei, Heczko Oleg
NETME Centre, Faculty of Mechanical Engineering, Institute of Materials Science and Engineering, Brno University of Technology, Brno, CZ-61669, Czech Republic.
Faculty of Mathematics and Physics, Charles University, Prague, CZ-12116, Czech Republic.
Sci Rep. 2018 May 8;8(1):7275. doi: 10.1038/s41598-018-25598-z.
In order to explain the formation of low-temperature phases in stoichiometric NiMnGa magnetic shape memory alloy, we investigate the phase transformation paths from cubic austenite with Heusler structure to low-symmetry martensitic structures. We used ab initio calculations combined with the generalized solid state nudged elastic band method to determine the minimum energy path and corresponding changes in crystal lattice. The four-, five-, and seven-layered modulated phases of martensite (4O, 10M, and 14M) are built as the relaxed nanotwinned non-modulated (NM) phase. Despite having a total energy larger than the other martensitic phases, the 10M phase will spontaneously form at 0 K, because there is no energy barrier on the path and the energy decreases with a large negative slope. Moreover, a similar negative slope in the beginning of path is found also for the transformation to the 6M premartensite, which appears as a local minimum on the path leading further to 10M martensite. Transformation paths to other structures exhibit more or less significant barriers in the beginning hindering such a transformation from austenite. These findings correspond to experiment and demonstrates that the kinetics of the transformation is decisive for the selection of the particular low-symmetry structure.
为了解释化学计量比的NiMnGa磁性形状记忆合金中低温相的形成，我们研究了从具有赫斯勒结构的立方奥氏体到低对称马氏体结构的相变路径。我们使用从头算计算结合广义固态推挤弹性带方法来确定最小能量路径以及晶格的相应变化。马氏体的四层、五层和七层调制相（4O、10M和14M）被构建为弛豫纳米孪晶非调制（NM）相。尽管10M相的总能量高于其他马氏体相，但它将在0 K时自发形成，因为路径上没有能垒且能量以很大的负斜率下降。此外，对于向6M预马氏体的转变，在路径开始时也发现了类似的负斜率，6M预马氏体在通向10M马氏体的路径上表现为局部最小值。向其他结构的转变路径在开始时或多或少都表现出明显的能垒，阻碍了从奥氏体的这种转变。这些发现与实验结果相符，并表明转变动力学对于特定低对称结构的选择起决定性作用。
