Research On The Mechanism Of The Twin Boundary Motion Of Ni-Mn-Ga Single Crystals And The Simulation Of Stress-induced Martensitic Transformation

As a new class of shape memory alloy, Ni₂MnGa ferromagnetic shape memory alloy (FSMA) has both large reversible strain and higher response frequency, which has attracted considerable attention and has been widely investigated. Progress has been made in the crystal structure, magnetic performance, phase transitions and mechanism of magnetically induced strain. However, the mechanism of the twin boundary motion, which controls the shape memory effect has not been well known. In this thesis, the different interfaces in Ni₂MnGa martensite were investigated through internal friction (IF) and high revolution transmission electron microscope (HRTEM), the factors that influence the movement of twin boundaries and the condition to get better shape memory effect had been discussed. Finally, the stress-induced martensitic transformation of Ni₂MnGa was simulated and discussed by the finite element method (FEM).In the observation of Ni₂MnGa martensitic structure, the sub-structure of stripe twins were found to be located in each of the variant. After the heating-cooling process with reverse and martensitic transformation, the orientation of new variants were different from the former ones and the interpenetrated structure was observed. It was estimated that if the stress was applied, the single variant martensite could be formed and the maximum shape memory effect could be obtained.The interfaces between different variants were not twin-related and belonged to the random boundaries, which was explained by the coincidence site lattice (CSL) calculation. A large amount of edge dislocations were found in the HRTEM observation. It was forecasted that when the field was applied, the twin boundaries could move through the movement of the twinning dislocations on the {112} twinning planes along the <111> directions. However,...