Martensitic phase transformation in rapidly solidified Mn 50 Ni 40 In 10 alloy ribbons

Heusler alloy Mn50Ni40In10 was produced as preferentially textured ribbon flakes by melt spinning, finding the existence of martensitic-austenic transformation with both phases exhibiting ferromagnetic ordering. A microcrystalline three-layered microstructure of ordered columnar grains grown perpendicularly to ribbon plane was formed between two thin layers of smaller grains. The characteristic temperatures of the martensitic transformation were MS=213 K, Mf =173 K, AS =222 K, and Af =243 K. Austenite phase shows a cubic L21 structure a=0.6013 3 nm at 298 K and a Curie point of 311 K , transforming into a modulated fourteen-layer modulation monoclinic martensite. © 2008 American Institute of Physics. DOI: 10.1063/1.2827179

Heusler alloy Mn 50 Ni 40 In 10 was produced as preferentially textured ribbon flakes by melt spinning, finding the existence of martensitic-austenic transformation with both phases exhibiting ferromagnetic ordering.A microcrystalline three-layered microstructure of ordered columnar grains grown perpendicularly to ribbon plane was formed between two thin layers of smaller grains.The characteristic temperatures of the martensitic transformation were M S = 213 K, M f = 173 K, A S = 222 K, and A f = 243 K. Austenite phase shows a cubic L2 1 structure ͑a = 0.6013͑3͒ nm at 298 K and a Curie point of 311 K͒, transforming into a modulated fourteen-layer modulation monoclinic martensite.© 2008 American Institute of Physics.͓DOI: 10.1063/1.2827179͔Since Sutou et al. 1 reported the occurrence of martensitic transformation in the ferromagnetic Heusler system Ni 50 Mn 50−x In x , considerable attention has been dedicated to study magnetism and magnetic shape memory effect, [2][3][4] magnetic entropy change, [4][5][6][7][8] and magnetotransport properties [9][10][11] of these alloys.Nevertheless, ferromagnetism in both phases is only observed in the narrow composition range of 15ഛ x ഛ 16 2 .The characteristic temperatures of the reversible first order structural transformation between both phases, referred as martensitic and austenitic starting and finish temperatures ͑i.e., M S , M f , A S , and A f , respectively͒, strongly vary upon small changes in the chemical composition.7][8] Ni-Mn-In Heusler alloys are therefore of significant prospective importance for applications in both magnetically driven actuators due to magnetic shape memory effect and as working substances in magnetic refrigeration technology.
Until now, the investigated alloys are usually bulk polycrystals obtained by arc or induction melting followed by a high temperature annealing, 1-8,10 or single crystals grown by Czochralski method. 9,11Present investigation was carried out to employ rapid quenching by melt spinning to produce Mn-Ni-In Heusler alloys.This technique offers two potential advantages for the fabrication of these magnetic shape memory alloys: the avoiding, or reduction, of the annealing to reach a homogeneous single phase alloy, and the synthesis of highly textured polycrystalline ribbons.Ribbon shape can be also appropriate for use in practical devices.We fabricated the alloy Mn 50 Ni 40 In 10 by melt spinning.Its valence electronic concentration per atom e / a is 7.801, allowing the existence of martensite-austenite transformation with both phases exhibiting ferromagnetic ordering, opening its potential use as a magnetic shape memory alloy. 3We report in this letter a preliminary characterization of the microstructural features and magnetic behavior.
As-cast pellets of nominal composition, Mn 50 Ni 40 In 10 were prepared by Ar arc melting from pure elements ͑Ͼ99.9%͒.The samples were melt spun in argon atmosphere at a wheel linear speed of 48 ms −1 .X-ray diffraction ͑XRD͒ analyses were performed using Cu K␣ radiation ͑ = 1.5418Å͒ with a low temperature device.Microstructure and elemental composition were examined by using a scanning electron microscope ͑SEM͒ equipped with a microanalysis system.Magnetization measurements were performed in the temperature interval of 4.2-350 K, using a physical properties measuring system ͑Quantum Design͒ platform with the vibrating sample magnetometer module.Zero-field cooling ͑ZFC͒, field cooling ͑FC͒, and field heating ͑FH͒ thermomagnetic curves were recorded at 50 Oe, and 50 kOe, with a temperature heating or cooling rate of 2 K/ min.The magnetic field was applied along the ribbon axis.Curie point T C was inferred from the minimum in the dM / dT versus T curve.Hysteresis loops were measured up to 10 kOe using a superconducting quantum interference device magnetometer.
SEM images at different magnifications of fracture cross section and the free surface of ribbons are presented in Fig. 1.As the micrographs demonstrate, the samples are fully crystalline showing fracture surfaces of a cleavage type revealing the fast crystallization and growth kinetics of the alloy.The ribbon thickness was around 7 -10 m.A thin layer of small equiaxed grains crystallizes in the ribbon surface in contact with the wheel ͓Fig.1͑b͔͒.Furthermore, an abrupt change takes place by the formation of ordered columnar grains growing perpendicularly to ribbon plane.This suggests that the rapid solidification process induces a direc-tional crystalline grain growth.At the free surface, another thin layer of small grains appears.It cuts and covers the oriented columnar structure.The granular microstructure exhibited by the free surface is shown in Fig. 1͑c͒.
A careful study by energy dispersive spectroscopy microanalysis was carried out to estimate the average elemental chemical composition.After numerous analyses performed on the cross section and both ribbon surfaces of different samples, we found a nearly homogeneous distribution of the chemical elements and the average composition Mn 49.5 Ni 40. 4 In 10.1 .SEM examinations in backscattering emission mode also confirmed that the chemical elements are homogeneously distributed in the alloy.Segregation of minor or secondary phases was not observed.
The ZFC, FC, and FH thermomagnetic curves recorded at 50 Oe are shown in Fig. 2. In the ZFC curve, the magnetization keeps low values and a small positive slope with the increase in temperature, and has a sudden raise at around 220 K suggesting a structural martensite-austenite phase transformation; a similar behavior is observed in the FH curve.The FC curve simultaneously confirms its reversible and thermally hysteretic character.group Fm3m and a lattice parameter a = 0.6013͑3͒.This reveals that ribbons are fully single phase with a cubic austenite parent phase.Below the phase transformation, the crystal structure of the martensite phase formed was identified as monoclinic with a fourteen-layer modulation ͑14M͒ and lattice parameters a = 0.4281 nm, b = 0.5789 nm, c = 2.966 nm, and ␤ = 93.84°.From XRD patterns, it is also established that ribbons exhibit crystallographic texture in agreement with their grain-oriented columnar microstructure.The crystal directions ͓400͔ and ͓040͔ are preferentially oriented perpendicular to the ribbon plane at 298 and 150 K, respectively.Hysteresis loops measured at 270 and 150 K ͑Fig.4͒ confirm the ferromagnetic ordering exhibited by both phases.Saturation magnetization is higher for austenite phase as was previously seen from thermomagnetic curves of Fig. 2. M͑H͒ curve for austenite approaches to saturation at a lower field value being magnetically softer than the martensite, as a result of its higher symmetry cubic structure and hence lower magnetocrystalline anisotropy.The inset of Fig. 4 zooms into the low magnetic field region.Coercive field H C for austenite is negligible within the uncertainty of the measurement ͑±2 Oe͒, while H C for martensite is 93 Oe.
In conclusion, rapid solidification by melt spinning is an effective preparation route to obtain, in a single-step process, ribbons of the Mn 50 Ni 40 In 10 alloy with an homogenous chemical composition, preferential texture, and a single cubic L2 1 austenite phase at room temperature.As-quenched ribbons exhibit a three-layered microcrystalline microstructure.At low temperature, austenite transforms into a fourteen-layered monoclinic martensite with martensiteaustenite phase transformation temperatures M S = 213 K, M f = 173 K, A S = 222 K, and A f = 243 K. Further studies are in course for understanding the role of the different factors affecting the manufacture of this Heusler alloy as melt spun ribbon with optimal textured microstructure and magnetostructural behavior, which may be a subject of significant scientific and technological interests.

͑Received 11
October 2007; accepted 1 December 2007; published online 9 January 2008͒ FIG. 1. SEM micrographs of the different regions of as-quenched Mn 50 Ni 40 In 10 ribbons: ͓͑a͒ and ͑b͔͒ fractured cross section at different magnification and ͑c͒ free surface.