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Structure and mechanical training of 5M and 7M Ni-Mn-Ga magnetic shape memory alloys
|Uwe Gaitzsch , Martin Pötschke , Stefan Roth , Bernd Rellinghaus , Ludwig Schultz|
Leibniz-Institute for Solid State and Materials Research, P.O.Box 270116, Dresden D-01171, Germany
Magnetic shape memory materials gained a large research interest owing to their capability to deform by some percent via twin boundary motion under the influence of a magnetic field. Concurrently, they are supposed to react faster than conventional shape memory materials because neither heating nor cooling are involved. The predominant material system is Ni-Mn-Ga with compositions around Ni2MnGa. In our case Ni50Mn30Ga20 and Ni50Mn29Ga21 alloy samples are used to produce polycrystalline textured samples. Upon cooling these alloys transform martensitically at 100 °C and 55 °C, respectively. The evolving martensitic structure is either orthorhombic (7M) or tetragonal (5M, NM) and depends on the thermomechanical history of the sample as well as the composition. Since only two of the three possible structures are suitable of providing the mandatory highly moblile twin boundaries, it is important to understand and control the phase formation process by appropriate thermal and mechanical treatment. Once the sample is given a suitable structure, samples for training are hot mold cast for directional solidification. X-ray diffraction techniques are applied to investigate the texture in these samples. After being cut erosively they are trained thermomechanically and in successive compression tests to lower the (de)twinning stress. The stresses in the training process have to be kept as low as possible to avoid brittle fracture of the samples during the cyclic compression process. An Instron testing machine has been modified in order to allow for magnetomechanical tests in magnetic fields of up to 0.8 Tesla.
Presentation: Oral at E-MRS Fall Meeting 2007, Symposium E, by Uwe Gaitzsch
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-10 14:45 Revised: 2009-06-07 00:44