We present results of an extensive neutron study of the martensitic transformation in Shape Memory Alloys of nominal composition Ni_55-xFe_19+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 T_M = 250 K and Curie temperature T_C = 289 K . Neutron measurements for this alloy were performed between 300 K (austenite phase) and 150 K (martensitic phase). At temperatures T>T_C 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< T_M 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 =90.83^o. 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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