Effect of Mn incorporation for Ni on the properties of melt spun off-stoichiometric compositions of NiMnGa alloys

The investigation addresses the effect of Mn incorporation for Ni on the properties of a series of Ni77−xMnxGa23 (x=22–29; at%) ferromagnetic shape memory alloys prepared in the form of ribbons by a melt spinning technique. Phase transformation studies in these ribbons by differential scanning calorimetry revealed that austenitic start and martensitic start temperatures decreased with the increase in Mn content. The Curie temperature (TC) of these alloys determined from thermal variation of magnetisations was found to rise with increasing Mn content. The martensitic transformation temperatures were above TC in low Mn containing (x=22 and 23) alloys. Morphology observed through transmission electron microscopy manifested complex martensitic features in the alloy with x=22 while x=29 had an austenitic phase. The alloys with intermediate Mn content (x=24, 25) had overlapping magnetic and martensitic transformations close to room temperature. The thermal lag between austenitic and martensitic characteristic temperatures in these alloys has been corroborated to their structural state. X-ray diffraction indicated a predominant martensite phase and austenite phase in low and high Mn containing alloys respectively. In-situ diffraction studies during thermal cycle indicate martensite–austenite transformations.

Research highlights
► The present work is an attempt to investigate the corresponding stoichiometric variations of Ni to Mn in the series of Ni77-xMnxGa23 at% ferromagnetic shape memory alloys.
► Some empirical fittings have been incorporated to show the effect because most of the literatures quote on the individual effect of either Ni or Mn with respect to Ga. Though Ni and Mn are both considered to have similar effects but in the present work a number of experimental evidences have been given regarding their separate entities.
► The effect of magnetic and structural transformation lags have also been shown in the present document which have not been generally focussed.
► Therefore, we expect that the present work will be an important scientific contribution to the area of ferromagnetic shape memory alloys designed for room temperature applications.