Crystal structure and formation mechanism of the secondary phase in Heusler Ni-Mn-Sn-Co materials
In the present work, crystal structure and formation mechanism of the secondary phase in Heusler Ni-Mn-Sn-Co materials were investigated using X-ray diffraction, scanning/transmission electron microscopy and selected-area electron diffraction techniques. Experimental results showed that the secondary phase presented in both Ni44.1Mn35.1Sn10.8Co10 as-cast bulk alloy and melt-spun ribbon, possessing a face-centered cubic (fcc) Ni17Sn3-type structure. The secondary phase in the as-cast bulk alloy was resulted from a eutectic reaction after the formation of a primary dendritic β phase during cooling. However in the melt-spun rapidly solidified ribbon, the secondary phase was largely suppressed as nano-precipitates distributed along the grain boundaries, which was attributed to a divorced eutectic reaction. The secondary phase exhibited partial amorphous state due to high local cooling rate.
KeywordsHeusler Ni-Mn-Sn-Co materials Secondary phase Melt spinning Divorced eutectic reaction Microstructure
In the past decades, alternative solid-state refrigeration methods have increasingly received attentions which provide high efficiencies, and thus they can contribute to a more efficient use of resources and lower greenhouse gas emissions . Heusler Ni-Mn-(Ga,In,Sn,Sb) with magnetocaloric effect represents a promising class of candidate materials based on two solid-state transitions including the first-order martensitic transformation and the second-order magnetic transition of austenite [2, 3, 4, 5]. Nowadays a lot of efforts have been made on the doping of the fourth element since it has been considered as an effective way to improve the magnetic properties [6, 7, 8, 9, 10]. Meanwhile, it was found that the secondary phase would form with these additions and produce strong effects on the martensitic transformation and magnetic properties as well. Feng et al. observed that face-centered cubic (fcc) particles appeared both along the grain boundaries and inside the grains after doping 5 % Fe to Ni-Mn-In alloys, and enhanced mechanical properties were obtained with the precipitation of the secondary phase particles . A Dy(Ni,Mn)4Ga phase with a hexagonal CaCu5-type structure was identified by Gao et al. in Ni50Mn29Ga21−x Dy x alloys. They found that the crystal structure of martensite evolved from five-layered to orthorhombic, and then to non-modulated structures. The martensitic transition temperatures also notably increased with increasing Dy content due to the presence of the Dy(Ni,Mn)4Ga precipitates . Very recently, a fcc γ phase in Ni43Co12Mn41Sn9 alloy was identified based on conventional X-ray diffraction technique, which could be fully suppressed under melt-spun condition, and thus the martensitic transformation shifted to higher temperature with positive effects on magnetic properties in the Ni43Co12Mn41Sn9 ribbon . Additionally, melt-spinning technique could generate highly textured homogeneous polycrystalline ribbons [11, 12] with substantially improved magnetic properties , and it is being widely used for the fabrication of high-performance magnetocaloric materials [14, 15, 16, 17]. Considering that Ni-Mn-Sn is highly potential for large-scale engineering applications among the Heusler Ni-Mn based family, and Co doping is effective to enhance the magnetocaloric effect through intensifying the magnetization discrepancy between the austenite and the martensite [18, 19], it is necessary to better understand the crystal structure and formation mechanism of the secondary phase in the Heusler Ni-Mn-Sn-Co materials. However, fewer reports on the secondary phase could be found in the literature. The main objective of the present work is to provide more information on this point based on Ni44.1Mn35.1Sn10.8Co10 alloy.
As-cast Ni44.1Mn35.1Sn10.8Co10 ingot with mass about 80 g was arc melted from Ni, Mn, Sn, Co with purities of 99.99 % (mass fraction) in argon gas atmosphere. Additional w (Mn) = 5 % was added to compensate for its evaporation loss. The ribbons were fabricated using melt spinning technique at a linear speed of 10 m/s and the resultant melt-spun ribbons were about 20 mm long, 4–6 mm wide and 30–35 μm thick.
X-ray diffraction (XRD, DLMAX-2200) was employed to detect the phases and crystal structures. Microstructural observations were conducted using a field-emission scanning electron microscopy (SEM, JSM-6700F). Transmission electron microscopy (TEM) analyses were performed in a JEOL JEM-2010F microscope equipped with an energy dispersive spectrometer (EDS), operated at 200 kV using a double-tilt stage. High-temperature differential scanning calorimetric measurement (NETZSCH DSC 404C) was carried out at heating/cooling rates of 10 K/min.
3 Results and discussion
The secondary phase is identified as fcc Ni17Sn3-type structure. The as-cast bulk alloy firstly forms dendritic β matrix, and then undergoes a eutectic reaction during cooling. While in case of melt-spun rapid solidification, the secondary phase is resulted from a divorced eutectic reaction in limited residual liquid phase after the formation of a large amount of primary β phase.
The secondary phase formed in melt-spun ribbons was largely suppressed as nano-precipitates, which distributed along the grain boundaries compared with the as-cast bulk alloy and meanwhile the secondary phase exhibited partial amorphous state due to high local cooling rate.
The authors gratefully acknowledge the supports from the National Natural Science Foundation of China (Grant Nos. 51201096 and 51474144), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20123108120019), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.