Phase Transition Behavior Of Ni-Mn-Based Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory alloys, which exhibit "sensing" and "actuating" are one of the new functional materials. Ferromagnetic shape memory Ni-Mn-Ga alloys are discovered firstly. However, they are fragile, their threshold values of the driving magnetic field are high, Ga elements are more expensive, magnetostriction of stability and reproducibility is low. In order to overcome these shortcomings, Ni-Mn-X (Sn,Sb) alloys showing many intriguing physical properties such as their magneto caloric effect, the giant magnetoresistance, the shape memory effect and low cost are receiving ever-increasing attention. The main methods for preparation of Ni-Mn-X (Sn,Sb) alloys include arc melting, induction melting and melt spinning, but they have not been systematically studied with respect to the phase transition behavior of the as-prepared alloys.In this paper, firstly, three different melting methods of arc melting, induction melting and melt spinning were used to prepare Ni50Mns50xSnx (x=10,13) and Ni50Mn50-xSbx (x=10,13) alloys, respectively. Then these samples were annealed by different annealing processes in vacuum. The transformation temperature and hysteresis temperature were studied by DSC. The crystal structure and phase composition were studied by X-ray diffraction (XRD) at different temperature. The main conclusions obtained are as follows:1. The XRD analysis demonstrated that Ni-Mn-Sn and Mn-Ni-Sb alloys have cubic L2i structures at high temperature (austenitic phase) and orthorhombic four-layered (40) structures at low temperature (martensitic phase).2. Different solidification process demonstrated that:the annealing time of arc-melted Ni-Mn-X (Sn,Sb) alloys need to be longer than that of induction melted ones, while melt-spun alloys without annealing can also lead to martensitic transformation. The melt spun method can decrease the martensitic transformation temperature (except for Ni50Mn40Sn10) and thermal hysteresis (except for Ni50Mn38S13). With the faster cooling rate, the unit cell volumes decreased. For Ni50Mi40Sb10 alloy, the unit cell volume of induction melted alloy is maximum, arc-melted alloy following (relatively reduced about 0.6%), melt spun alloy is minimum (relatively reduced about 2%).3. Different compositional variations demonstrated that transformation temperature of Ni-Mn-Sn and Ni-Mn-Sb alloys increase with increasing values of e/a. With Sn content increasing, the lattice constant of Ni-Mn-Sn alloys increase. These arc-melted Ni-Mn-Sn alloys increased by 0.5% and melt-spun one by annealing at 900? for 24h alloy increased by 0.7%.This is because the radius of Sn atom is larger than that of Mn atom.4. Different annealing process demonstrated that appropriate annealing times can reduce the thermal hysteresis. The thermal hysteresis of Ni5oMn37Sn13 alloy is only 10K. Afer annealing, the lattice parameters decreased. For Ni-Mn-Sn alloys, the lattice parameters decreased by 0.1% and Ni50Mn50-xSbx (x=10,13) alloys by induction melting and annealing at 900?for 24h decreased by 1.5% and 2.3%. Because of homogenizing annealing, these point defects are improved, then leading to a decrease in the Mn-Mn distance.