Low‐temperature martensite relaxation in Co–Ni–Ga shape memory alloy monocrystal revealed using in situ cooling, transmission electron microscopy and low rate calorimetry

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Structural science, crystal engineering and materials, 2020-08, Vol.76 (4), p.563-571
• Main Authors: Żak, Andrzej, Dańczak, Anna, Dudziński, Włodzimierz
• Format: Article
• Language: English
• Subjects: Calorimetry; CoNiGa; Cooling; Cooling rate; Differential scanning calorimetry; Enthalpy; Heat measurement; Heating; in situ TEM; Literature reviews; Martensite; Martensitic transformations; Phase transitions; relaxation; shape memory alloy; Shape memory alloys; Single crystals; Temperature; Transmission electron microscopy
• ISSN: 2052-5206; 2052-5192; 2052-5206
• DOI: 10.1107/S2052520620006794

This work presents the results of research on a Co49Ni21Ga30 magnetic shape memory single crystal. Based on a literature review, it was identified that analyses of phase transformations have been limited to specific heating and cooling rates, which could lead to an incomplete description of the resulting phenomena. Differential scanning calorimetry (DSC) performed with different heating/cooling rates enabled the precise determination of enthalpy values, which deviate from literature values. Weak and previously unnoticed thermal phenomena at temperatures below 190 K were also observed. Their presence was confirmed by low‐temperature in situ transmission electron microscopy (TEM). Through DSC measurements and TEM observations, a model of the discovered phenomenon was proposed, which may have an impact on a better understanding of the physics of magnetic shape memory materials. A Co49Ni21Ga30 magnetic shape memory single crystal was studied using in situ imaging and low‐temperature calorimetry. Both methods confirmed the occurrence of local reconstruction of the crystal lattice at temperatures below 190 K.