The present investigation was undertaken to investigate the effect of quenching rate on the microstructure of Ni 50 Mn 50 À x Sn x ribbon alloys and its influence on the martensitic starting phase-transition temperature M S . The study was performed on melt-spun ribbons of a ferromagnetic shape memory alloy (FSMA) of composition Ni 50 Mn 37 Sn 13 produced by varying the speed of the rotating copper wheel between 15 and 50 m s À1 . The resulting samples have a chemical composition close to the nominal one and at room temperature crystallize in a cubic single-phase austenite with the highly ordered L2 1 -type crystal structure without a significant variation in the cell parameter. The average grain size varies between 1.4 and 7.3 mm, while M S increased from 212 to 258 K. Our results suggest that in these materials the average grain size of the parent austenite phase limits the size of the martensite variants, giving rise to its stabilization, while a multivariant transformation mode is maintained.
The kinetic arrest of martensitic transformation (MT) has been observed in as-solidified Ni52.2Mn34.3In13.5 melt spun ribbons. The main characteristics of this unusual field-induced magneto-structural phenomenon have been determined through a dc magnetization study. The sample studied was fabricated by rapid solidification using the melt spinning technique at a high quenching rate of 48 ms-1. At room temperature, it is a single phase austenite (AST) with the bcc B2-type crystal structure and Curie temperature of TCA=285 K. With decreasing temperature, the austenite phase transforms into the martensite phase (MST) with TCM≈185 K at a starting martensitic transition temperature of MS=275 K. A moderate but progressive kinetic arrest of the AST to MST transformation has been observed for magnetic field values above H=10 kOe and was studied up to Hmax= 90 kOe. The metastable character of the non-equilibrium field-cooled state is revealed by the decreasing behavior of the saturation magnetization under a large magnetic field of 50 kOe after temperature cycling from 10 K to 150 K. The total magnetization difference Δσ between the zero field-cooling and field-cooling pathways of the temperature dependence of magnetization shows irreversible and reversible components and the former decreases with decreasing temperature.
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