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

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 M_s, whereas a higher Co content lowers M_s, with the Ni:Al ratio showing the dominant effect within small compositional deviations. The M_s further shows a clear increase with increased valence electron concentration (e/a). Considering these trends, M_s varies significantly among the alloys ranging from − 73 to 83 °C. The Curie temperature T_c 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 T_c, it simultaneously reduces M_s, requiring a careful balance since the functional window of the alloy is defined by the temperature interval between M_s and T_c. For the current samples, a variation in T_c 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 M_s and T_c through targeted composition control.