Chemical segregation during bulk single crystal preparation of Ni–Mn–Ga ferromagnetic shape memory alloys

Abstract

Preparation of bulk single crystals of the ferromagnetic shape memory alloy Ni 2MnGa has been accomplished using the Bridgman method. Magnetic and magnetostrictive properties from samples sectioned from different portions of the crystal showed high variability, indicating significant chemical segregation occurred during single crystal growth. This chemical segregation during crystal growth implies that the compound Ni 2MnGa does not melt congruently. Partial evaluation of the liquidus diagram of Ni–Mn–Ga by thermal analysis and microstructural evaluation has determined that the primary solidification surface for the Heusler alloy phase exists over a wide range of compositions. The primary solidification surface falls modestly in temperature with either increasing Mn or Ga concentration. The composition Ni 50Mn 25Ga 25 melts incongruently over a temperature range of approximately 18°C. Characterization of the compositional variation along the growth direction in the single crystals was done using energy dispersive spectrometry and the results normalized against the fraction solidified. Compositional changes during solidification followed constant electron/atom (e/a) ratios over most of the length of the ingot. Under the single crystal growth conditions, the segregation pattern corresponds to a fully mixed condition and the data was fitted to a complete mixing model. Partitioning coefficients for the elemental constituents and the overall compositions were determined. The partitioning coefficients were found to be uniformly consistent between different growth runs, even though the overall compositions differed between crystals. The martensite transition temperatures along the length of the crystal were measured by differential scanning calorimetry and compared to predicted transition temperatures based on the e/a concentration.