Neutron study of the martensitic transformation in Ni-Fe-Ga alloys

Jon Gutierrez 2Patricia Lázpita 2Vasudeva Siruguri 2Jose Manuel Barandiarán 2Paul F. Henry 3Volodymyr A. Chernenko 1T. Kanomata 4

1. Institute of Magnetism NASU (IMag), Vernadsky 36-b, Kyiv 03142, Ukraine
2. Facultad de Ciencia y Tecnología (UPV/EHU), P.Box 644, Bilbao 48080, Spain
3. Institut Laue Langevin (ILL), 6 Rue Jules Horowitz, Grenoble, France
4. Tohoku Gakuin University, Tagajo, Japan

Abstract

We present results of an extensive neutron study of the martensitic transformation in Shape Memory Alloys of nominal composition Ni55-xFe19+xGa26 (x = 0, 1, 2 at.%) transforming close to room temperature. This study was performed at instrument D20 of the ILL, working at high resolution mode (λ =1.88 Å). Neutron experiments were performed by cooling and afterwards by heating the samples, so the temperature of the structural transitions was also determined.

For each composition, a piece cut from the ingot was crushed to eliminate their strong texture, and the resulting powder was carefully annealed to recover from the defects induced by crushing. Magnetic characterisation before and after processing revealed no changes in the martensitic tra nsition and Curie temperatures.

For instance, the x=0 sample shows a martensitic transformation temperature TM = 250 K and Curie temperature TC = 289 K . Neutron measurements for this alloy were performed between 300 K (austenite phase) and 150 K (martensitic phase). At temperatures T>TC we have determined the coexistence of two phases: a L21 one with space group Fm3m and cell parameter a=5.7300 Å, and a γ fcc with cubic simmetry , space group Pm3m, with cell parameter a=3.5859 Å. This second pha se was also detected by magnetic measurements, giving rise to a second Curie temperature of about 363 K.

At temperatures T< TM the γ fcc phase remains unchanged, while the L21 phase transforms into a majority monoclinic seven-layer modulated s tr uct ure with parameters a= 4.0361 Å, b=2.8792 Å, c=28.135 Å and β =90.83o. Some contributions arising from a five-layer modulated structure have been also found.

Analysis of the structural results for all compositions, plus concordance between all st ruct ural and magnetic transitions elucidated from different experimental techniques are discussed.

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Presentation: poster at E-MRS Fall Meeting 2005, Symposium C, by Jon Gutierrez
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-19 07:42
Revised:   2009-06-07 00:44
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