Microstructure and precipitates in annealed Co38Ni33Al29 ferromagnetic shape memory alloy

• Micron-sized non-twinned, single and triple twinned precipitates characterized.
• Precipitates and austenite matrix fit K–S orientation relationship.
• The precipitates are Co-rich and Al- and Ni-deficient with respect to matrix.
• The twinned precipitates have a plate-like shape.
• The non-twinned precipitates have an often bent lath-like shape.

Transmission electron microscopy was performed to investigate the microstructure and precipitates in the annealed Co38Ni33Al29 ferromagnetic shape memory alloy. Apart from the dendritic secondary phase in the austenite matrix, micron-sized (up to 100 μm) fcc-based precipitates with partial γ′ L12 ordering and containing none, one or three {1 1 1}p parallel twin planes were found. The orientation relationship between the precipitates and matrix was found to be Kurdjumov–Sachs. STEM–EDX analysis indicates that twinned and non-twinned precipitates are Co-rich and Al- and Ni-deficient with respect to the matrix and with a lower Co/Al ratio for the latter. The 3D morphologies of precipitates were reconstructed with focused ion beam/scanning electron microscope dual-beam slice-and-view imaging, showing that the single {1 1 1}p plane twinned precipitates have a plate-like shape while the non-twinned precipitates are lath-like and often bent.