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

Preparation of bulk single crystals of the ferromagnetic shape memory alloy Ni2MnGa has been accomplished using the Bridgman method. Magnetic and magn etostrictive properties from samples sectioned from different portions of t he crystal showed high variability, indicating significant chemical segrega tion occurred during single crystal growth. This chemical segregation durin g crystal growth implies that the compound Ni2MnGa does not melt congruentl y. Partial evaluation of the liquidus diagram of Ni-Mn-Ga by thermal analys is and microstructural evaluation has determined that the primary solidific ation surface for the Heusler alloy phase exists over a wide range of compo sitions. The primary solidification surface falls modestly in temperature w ith either increasing Mn or Ga concentration. The composition Ni50Mn25Ga25 melts incongruently over a temperature range of approximately 18 degreesC. Characterization of the compositional variation along the growth direction in the single crystals was done using energy dispersive spectrometry and th e results normalized against the fraction solidified. Compositional changes during solidification followed constant electron/atom (e/a) ratios over mo st 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 e lemental constituents and the overall compositions were determined. The par titioning coefficients were found to be uniformly consistent between differ ent growth runs, even though the overall compositions differed between crys tals. The martensite transition temperatures along the length of the crysta l were measured by differential scanning calorimetry and compared to predic ted transition temperatures based on the e/a concentration. (C) 2000 Elsevi er Science B.V. All rights reserved.