In-Situ Diffraction Studies Of Ferromagnetic Shape Memory Alloys Using High Energy Syn- Chrotron X-Rays

Ferromagnetic Shape Memory Alloys (FSMAs) are a new kind of functional mate-rials developed since 1990s. When compared with conventional shape memory alloys, FSMAs show a magnetically controlled shape memory effect in addition to a ther-mally controlled shape memory alloy. They have received much attention in recent years. The structural transition of FSMAs plays an important role in determining their properties. It is necessary to in-situ investigate their structures and transitions in order to improve their performance under magnetic fields.In this thesis, two typical FSMAs, Heusler alloys Ni2MnX (X=Sn and Sb) and half-Heusler alloys CoMnZ (Z=Si and Ge), were chosen as the model material, and their structural evolutions during cooling were in-situ investigated using high-energy synchrotron X-rays. The effects of temperature, magnetic field and bulk composition on the structures of the model materials were analyzed and discussed. The main con-clusions are summarized as follows.1. The Heusler alloys Ni50Mn37Sn13, Ni5oMn37Sb13 and Ni45Cu5Mn37ySn13 show a martensitic transition from a L21-type cubic austenite to a 40-type modulated ortho-rhombic martensite during cooling. The half-Heusler alloys CoMnSi, CoMnGe, CoMnSi0.33Geo.67 and CoMnSio.67Ge0.33 show a martensitic transition from a Ni2In-type hexagonal austenite to a TiNiSi-type orthorhombic martensite during cooling. The bulk composition does not alter crystal structures either of the austenites or of the martensites, but has a significant effect on the onset temperature of the martensitic transitions.2. The magnetic field of 2 T stabilizes the austenite of the Heusler alloys, and hinders the martensitic transitions of the alloys. The magnetic fields of 2 T and 6 T promote the martensitic transitions of the half-Heusler alloys. It was concluded that that the effects of the magnetic field on martensitic transitions are related to changes of magnetic moments of the alloy samples during transition.3. The structural transitions of the Heusler alloys and the half-Heusler alloys both are accompanied by magnetic phase transitions. The effects of the magnetic fields on the crystal structures depend on the type of the alloy samples. The magnetic field of 2 T does not affect the lattice parameters of the Heusler alloys significantly. In contrast, the magnetic fields of 2T and 6 T affect the lattice parameters of some of half-Heusler alloy compositions. The unit-cell volume of the CoMnSi and CoMnSi0.67Geo.33 alloys decrease under the magnetic fields, but that of the CoMnGe and CoMnSi0.33Geo.67 al-loys does not change under the magnetic fields.