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Application of neutron powder diffraction for the study of non-stoichiometric Ni2MnGa based alloys

Pnina Ari-Gur 2Giora Kimmel 1James W. Richardson 3Ashfia Huq 4

1. Institutes for Applied Research, Ben Gurion University of the Negev, Beer-Sheva 84105, Israel
2. Western Michigan University (WMU), Kalamazoo 49008, United States
3. Argonne National Laboratory (ANL), 9700 South Cass Avenue, Argonne, IL 60439, United States
4. Oak Ridge National Laboratory (ORNL), 1 Bethel Valley Rd, Oak Ridge, TN 37831, United States


One important group of shape memory alloys is the ferromagnetic Ni2MnGa.  At room temperature, the alloy has a Heusler alloy L21 type structure.  A martensitic phase transition occurs upon cooling.  At the stoichiometric composition, the martensitic transformation temperature is below room temperature. Since the magnetic shape memory effect (MSM) occurs in the LT martensitic structure, non-stoichiometric alloys have been developed to enable the MSM effect at ambient conditions. One of the possibilities is the non-stoichiometric Ni2Mn1+xGa1-x alloys providing modification of transformation temperature maintaining the Curie point.  The deviation from stoichiometry yields non-homogenous materials after casting due to the coring effect. Another problem is that the exchange between Mn and Ga reduces the degree of order which makes neutron diffraction more sensitive to the crystallographic characterization. In this work the crystal structure of several non-stoichiometric polycrystalline alloys were investigated at several temperatures using pulsed neutron source at Argonne National Laboratory. The following parameters were studied: The deviation from stoichiometry; temperature of isothermal heat treatment for homogenization and ordering; cooling rate after homogenization. It was found that the homogenization process at high temperature following by fast cooling is essential in order to obtain single non-stoichiometric phase. There is no indication to B2 to L12 transition.    




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Related papers

Presentation: Oral at 11th European Powder Diffraction Conference, Microsymposium 9, by Giora Kimmel
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-04-30 22:12
Revised:   2009-06-07 00:48