Metallurgical origin of the effect of Fe doping on the martensitic and magnetic transformation behaviours of Ni50Mn40-xSn10Fex magnetic shape memory alloys

This study investigated the metallurgical origin of the effects of Fe substitution for Mn on the martensitic and the magnetic transformation behaviours of Ni50Mn40-xSn10Fex (x   = 0, 3, 4, 5, 6) alloys. Substitution of Fe for Mn at above 3 at% introduced an fcc γ phase in the microstructure. Formation of the γ phase influenced the composition of the bcc/B2 matrix, leading to decrease in martensitic transformation temperatures and transformation entropy change. The Curie temperature of the parent phase increased slightly, whereas the Curie temperature of the martensite increased rapidly with increasing Fe addition. Changes in the temperatures of the martensitic and magnetic transformations are confirmed to directly relate to the e/a ratio of the matrix caused by formation of γ phase. The minimum e/a ratio value for the occurrence of the martensitic transformation is estimated to be 8.045 for the alloy system studied. A narrow e/a ratio range of 8.113–8.137 is estimated for the occurrence of metamagnetic transformationM(para)⇔A(ferro)M(para)⇔A(ferro). This metamagnetic reverse transformation was induced by a magnetic field at 225 K within a range of 3–7 T in the Ni50Mn35Sn10Fe5 alloy. The magnetic work required to induce the transformation is estimated to be ∼176 J/kg, comparable to the thermodynamic energy deficit for the transformation at the testing temperature estimated from thermal measurement. These findings clarify the origin of the effects of Fe doping in Ni50Mn40-xSn10 alloys and provides reference on alloys design for this system.

Substitution of Fe for Sn in Ni50Mn40-xSn10 at above 3at%Fe causes formation of an fcc γ phase. Formation of the γ phase alters the matrix composition, which in turn changes to the e/a ratio. Based on the experimental evidences obtained, several important critical conditions for alloy design have been identified. These include the minimum e/a ratio (8.045) for the alloy to exhibit martensitic transformation and a narrow e/a ratio window (8.113–8.137) for the alloy to exhibit metamagnetic martensitic transformation M(para)⇔A(ferro)M(para)⇔A(ferro). Such knowledge is critical for the development of magnetically activated shape memory alloys.Figure optionsDownload as PowerPoint slideResearch highlights
► This study clarifies and question of fundamental issue on the metallurgical origin of the effects of Fe doping on the properties of NiMnSn alloys.
► The findings and the view point expressed are novel.
► Based on the experimental evidences a few important critical parameters have been determined, including the minimum e/a ratio (8.045) for the alloy to exhibit martensitic transformation and a narrow e/a ratio window (8.113–8.137) for the alloy to exhibit metamagnetic martensitic transformationM(para)⇔A(ferro)M(para)⇔A(ferro).
► Such knowledge is critical for alloy design.