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<p>The effect of minor compositional variations on the martensitic transformation and the magnetic properties of CoNiAlFe alloys was investigated using X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, magnetic measurements, and in situ SEM observations. Samples taken from the same bulk material with the nominal composition Co<sub>35</sub>Ni<sub>35</sub>Al<sub>28</sub>Fe<sub>2</sub> (at. − %) were analyzed, exhibiting slight process-induced differences in composition. The results show that even small deviations of less than 1&#xa0;at. − % lead to significant shifts in the transformation temperatures and the Curie temperature. The following trends are identified: An increased Ni:Al ratio raises the martensite start temperature <i>M</i><sub>s</sub>, whereas a higher Co content lowers <i>M</i><sub>s</sub>, with the Ni:Al ratio showing the dominant effect within small compositional deviations. The <i>M</i><sub>s</sub> further shows a clear increase with increased valence electron concentration (<i>e</i>/<i>a</i>). Considering these trends, <i>M</i><sub>s</sub> varies significantly among the alloys ranging from − 73 to 83&#xa0;°C. The Curie temperature <i>T</i><sub>c</sub> increases with decreasing Al content and with decreasing Ni content when Ni was substituted by Co. While Co is the most effective element for raising <i>T</i><sub>c</sub>, it simultaneously reduces <i>M</i><sub>s</sub>, requiring a careful balance since the functional window of the alloy is defined by the temperature interval between <i>M</i><sub>s</sub> and <i>T</i><sub>c</sub>. For the current samples, a variation in <i>T</i><sub>c</sub> from − 31 to 109&#xa0;°C is observed. The results indicate the remarkable sensitivity of structural and magnetic transformation to minor compositional deviations and highlight the potential to tune <i>M</i><sub>s</sub> and <i>T</i><sub>c</sub> through targeted composition control.</p>

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Compositional Sensitivity of Martensitic Transformation and Magnetic Properties of CoNiAlFe Ferromagnetic Shape Memory Alloys

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年4月2日

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  • Elisa Holzmann,
  • Christian Hinte,
  • Gregory Gerstein,
  • Sebastian Herbst,
  • Hans Jürgen Maier

摘要

The effect of minor compositional variations on the martensitic transformation and the magnetic properties of CoNiAlFe alloys was investigated using X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, magnetic measurements, and in situ SEM observations. Samples taken from the same bulk material with the nominal composition Co35Ni35Al28Fe2 (at. − %) were analyzed, exhibiting slight process-induced differences in composition. The results show that even small deviations of less than 1 at. − % lead to significant shifts in the transformation temperatures and the Curie temperature. The following trends are identified: An increased Ni:Al ratio raises the martensite start temperature Ms, whereas a higher Co content lowers Ms, with the Ni:Al ratio showing the dominant effect within small compositional deviations. The Ms further shows a clear increase with increased valence electron concentration (e/a). Considering these trends, Ms varies significantly among the alloys ranging from − 73 to 83 °C. The Curie temperature Tc increases with decreasing Al content and with decreasing Ni content when Ni was substituted by Co. While Co is the most effective element for raising Tc, it simultaneously reduces Ms, requiring a careful balance since the functional window of the alloy is defined by the temperature interval between Ms and Tc. For the current samples, a variation in Tc from − 31 to 109 °C is observed. The results indicate the remarkable sensitivity of structural and magnetic transformation to minor compositional deviations and highlight the potential to tune Ms and Tc through targeted composition control.