Study Of The Microstructure And Phase Transition In Mn-Ni-Ga Ferromagnetic Shape Memory Alloy

Ferromagnetic shape memory alloys (FSMA) have received great attention in recent decades due to their magnetic-field-induced shape memory/strain effect (MSME), high frequency response, magnetoresistance, hall effect, and magnetocaloric effect. In recent years, a new ferromagnetic shape memory alloy-Mn2NiGa has been developed. Many problems of the crystal structures have not been solved in Mn2NiGa alloy, which greatly hinders further improvement of the functional performances of this MSMA.In this work, the martensitic transformation and different microstructure of Mn-Ni-Ga alloys have been studied by Scanning electron microscopy (SEM), X-ray diffraction (XRD), Transmission electron microscopy (TEM) and High resolution transmission electron microscopy (HREM).The microstructures of the Mn-rich Mn2+xNiGa (x=0,0.02,0.06) alloys at room temperature were investigated. The results show that the main phases of all the three alloys are austenites. The Ni2In-type hexagonal MnNiGa phase was found in Mn2NiGa alloy. The crystal orientation relationship between the hexagonal MnNiGa and cubic austenite was observed as [001]A//[0111]H,(040)A//(2202)H and [110]A//[2110]H,(220)A//(0112)H. Beside MnNiGa phase, a few of layered non-modulated martensites appeared in matrix of Mn2.02NiGa alloy. Non-modulated martensites appeared conspicuously in Mn20.06NiGa alloy, meanwhile the MnNiGa phase disappeared in this alloy. Using HREM observation, the local lattice distortions which form nano-scaled domains with a characteristic of strain glass were found in the austenites of these three alloys at room temperature.The microstructure of Mn50+xNi25-xGa25(x=-3,0,3,5) alloys were investigated. Single phase of austenitic with cubic structure is present for Mn4.7Ni28Ga25alloy at room temperature. With the increase of the Mn content and the Ni content reduce, the electron concentration decreases, cell volume increases, increasing the martensitic transformation point, as a result, the non-modulated martensitics phase increases gradually.The microstructure and phase transformation of Mn2.04NiGa alloys that aged at low temperature have been investigated. The results show that the parent phase of alloy has transformed into martensite after aging treatment. The alloy which aged at423K/72h has become the non-modulated martensite completely. Partial martensitic transformation occurred in the alloys that aged at523K/72h and623K/72h. In addition, the crystal orientation relationship between the tetragonal martensites and cubic austenite was observed as (001)A//(001)M,[100]A//[110]M. After aging at573K/72h and673K/72h, the microstructures of alloy have great changes, many nano-scale MnNiGa precipitated in the matrix. Furthermore, the size of these nano-scale MnNiGa phases grow up with increasing aging temperature, and it will hinder the martensite shear movement, which make the transition point much lower. As a result, the composition of the martensites become Mn-rich, Additionally, there exist some16M modulation martensites in Mn2.04NiGa alloy aged at623K/72h, which is observed by the electron diffraction analysis for first time in Mn-Ni-Ga alloys.