Ferromagnetic shape memory alloys (FSMAs) are presently intensively studied because of their potential applications as smart materials. Martensitic transformations and lattice reorientation processes in FSMAs can be triggered not only by changes in temperature and stress, as in conventional paramagnetic SMAs, but also by applying a magnetic field. Recently, Co-Ni-Al alloys have attracted increasing interest as new FSMAs [1,2].

Conventional transmission electron microscopy and electron diffraction studies together with advanced microscopy techniques (HRTEM, EFTEM) were carried out in order to characterize the microstructure of annealed (1275°C/4h and 1100°C/72h) Co₃₈Ni₃₃Al₂₉. In addition to the major constituents (B2 type ordered β-phase and f.c.c. ordered γ-phase), rod shaped precipitates ranging from 20 to 75 nm of h.c.p. ε-Co were observed in the 1275°C/4h sample (Fig.1). EFTEM measurements confirm that the precipitates are Co-rich (Fig.2). The orientation relationship between the nanosized precipitates and the B2 matrix is found to be the Burgers orientation relation. Martensitic transformation to the tetragonal L1_o structure was observed at -40°C which was confirmed by in-situ experiment and DSC measurements. Large precipitates (Co,Ni)₃Al with size up to several microns were found in specimen annealed at 1100°C/72h. These precipitates show the L1₂ ordered structure while a majority contain a single twin boundary (Fig.3).

References:

[1] Karaca H.E. et al., 2004, Scripta Mater. 51, 261-266

[2] Kainuma R. et al., 1996, Intermetallics 4, S151-S158

Acknowledgements: B. Bartova and S. Ignacova like to thank MULTIMAT “Multi-scale modeling and characterization for phase transformations in advanced materials”, a Marie Curie Research Training Network (MRTN-CT-2004-505226) for supporting this work.

Fig.1

Fig.2

Fig.3

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/11217 must be provided.

1. Electron microscopy studies of Ni-based shape memory materials revealing (un)expected microstructural features
2. Transformation and twinning processes in CoNiAl alloy single crystals
3. Magnetic field induced strain and training process in NiMnGa single crystal
4. Modeling superelastic NiTi wire structures
5. Static and dynamical functional properties of ultra thin NiTi wires
6. Microstructure, martensitic transformation and shape memory effect in Ni₃Ta – novel high temperature SMA
7. Composition gradients surrounding Ni₄Ti₃ precipitates in a Ni-rich NiTi alloy studied by EELS, EFTEM and EDX