Effect of selective substitution of Co for Ni or Mn on the superstructure and microstructural properties of Ni50Mn29Ga21

Results from a systematic study of the effect of cobalt, selectively substituted for Ni and Mn, in the modulated orthorhombic (7 M) Ni50Mn29Ga21 alloy are contrasted. Substitution of Co for Mn resulted in the stabilization of a non-modulated tetragonal (NM) phase at higher Co content. At lower Co contents, a mixture of 7 M and NM phases were found to co-exist. Increasing in Co content caused suppression of long-range twin deformation leading to sporadic islands within which twin variants were confined. These effects are attributed to the considerable stresses generated by an atomic volume change of 9.5% caused locally when Co is substituted for Mn. On the other hand, substitution of Co for Ni, which causes only 1.7% atomic volume change locally, was observed not to alter either the superstructural ordering or the long-range twin deformation. Our results show that the microstructures resultant on Co doping have a strong correlation to the structural ordering, which in turn depends on whether Co is substituted for Mn or for Ni, and also on the concentration of Co.

► The effect of cobalt, selectively substituted for Ni and Mn, in Ni50Mn29Ga21 alloy with 7 M orthorhombic structure are contrasted.
► Substitution of Co for Mn resulted in the stabilization of a non-modulated tetragonal phase at higher Co content.
► Substitution of Co for Mn also caused suppression of long range twinning, which is confined to sporadic islands.
► Substitution of Co for Ni has not altered either the 7 M superstructural ordering or the long-range twin deformation.
► Microstructures resultants on Co doping have a strong correlation to the structural ordering.