Thermal characteristics of Ni-Fe-Ga-Mn and Ni-Fe-Ga-Co ferromagnetic shape memory alloys

The martensitic transformation (MT) characteristics of three quaternary polycrystalline alloys with compositions Ni53.5Fe16.5Ga27Co3, Ni54Fe16Ga27Mn3 and Ni52Fe18Ga27Co3 have been studied in order to investigate the effects of additions of Mn or Co to Ni-Fe-Ga, in substitution of Ni or Fe. The shifts in the MT temperatures, compared to the corresponding ternary alloys, are in agreement with the e/a ratio changes, although an atomic size factor seems to be also at play when Co substitutes Fe. The increase in the degree of L21 order brings about a decrease of the MT temperatures, the behaviour being equivalent to that of ternary alloys. The martensitic phases observed in the quaternary alloys differ slightly from those of the ternary; in particular, an unusual four-layered phase has been found in the alloy containing Mn. Different martensite stabilization degrees, depending on the quenching rate, have been observed in the Ni53.5Fe16.5Ga27Co3 alloy transforming above room temperature. For intermediate quenching rates, the effect is concomitant with the quenched-in disorder, which is gradually recovered when heating above the end of the MT. Annealing treatments at 1270 K promote the formation of ordered γ′ (L12 structure) precipitates in the Ni52Fe18Ga27Co3 alloy. The particles are richer in Fe and poorer in Ga than the matrix, leading to a slight reduction in the e/a ratio of the matrix, which results in a decrease of MT temperatures, as confirmed by DSC runs.