Papers by Keyword: Ni-Mn-Ga Alloys

Abstract: The effect of composition on martensitic phase transformation and structure in polycrystalline NiMnGa was investigated through Ni substitution for Mn, Ga and Mn substitution for Ga. The martensitic transformation temperatures almost linerly increase with increasing Ni content in Ni_50+xMn_50-xGa₂₅, Ni_50+yMn₂₅Ga_25-y and Mn content in Ni₅₀Mn_25+zGa_25-z. The increases in rate of the martensitic transformation temperatures are different for the three conditions. It is large for Ga substituted by Ni, slow for both Mn substituted by Ni and Ga substituted by Mn. Analysis shows that electronic concentration is an important factor to influence the martensitic transformation temperature in the Ni-Mn-Ga alloys. The results show that Ni excess stabilizes the NM martensitic structure and Mn excess stabilizes the 7M martensitic strcuture in broad composition range.

Abstract: For off-stoichiometric Ni₂MnGa ferromagnetic shape memory alloys, a large shape change could be induced through the rearrangement of martensitic variants under an external magnetic field. Insight into the orientation relationships of martensitic variants and the characteristics of variant boundaries is thus essential for understanding the magnetic shape memory performance. In this paper, a thorough crystallographic investigation was made on the incommensurate 7M modulated martensite in one polycrystalline Ni₅₀Mn₃₀Ga₂₀ alloy by means of X-ray diffraction and SEM electron backscattered diffraction (EBSD). Locally, there are four differently-oriented martensitic variants, being twin related to one another. The twin interface planes are coherent and they are in coincidence with the respective twinning planes (K₁). A primary exploration was performed to improve the microstructure by repeated magnetic field training during phase transition. The present investigation could offer useful guidance to develop specific technique for microstructure optimization.