Abstract 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 35 Ni 35 Al 28 Fe 2 (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.
Holzmann et al. (Thu,) studied this question.