The Martensitic Transformation And Crystallography Of Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory alloys (FSMA) are a kind of alloy possessing the thermoelastic martensitic transformation, which can produce strain under an external magnetic field. Because of its high frequency response and the large output strain, a number of scholars are researching the performance of the FSMAs, especially the Ni2MnGa alloy. Lots of sensors and actuators prepared by FSMAs can be used essentially in the electronics, aerospace and military power. Therefore, the research of the thermoelastic martensitic transformation in FSMA has been intensively studied to improve the properties of the alloys.In this paper, the alloy of Ni_52.5Mn_23.5Ga₂₄ added by Co were smelted to observed the optical microstructure, the temperature of martensitic transformation, the Curie temperature, the crystal structure, mechanical properties and magnetization at room temperature. Then the yield strengthσ0.2 (Ms) of austenite at martensitic transformation start temperature obtained by extrapolation was used to calculate the martensitic transformation driving force. After the introduction of the theory of energy minimization and its application, the twinning plane, the orientation angle between different twinning plane, the twins within twins structure, the habit plane and the orientation relationship between austenite and matensite can be predicted by the theory according to the crystal structure and the lattice parameters of the alloys. Finally, in order to find the FSMAs with high toughness, the alloys of Ni56.5Fe17Ga26.5 added by Pd were prepared to study the various properties. The results show that:(1) The martensitic transformation temperature Ms, Mf, As and Af of the alloys of (Ni_52.5Mn_23.5Ga₂₄) 100-xCox (x=0,2,4,6,8) enhance with the increasing of Co content. This shows that the martensitic transformation temperature is linear with the valence electronic density. The Curie temperature is very sensitive to the composition of the alloy. The coupling between Co and Mn and the distance between the Mn atoms adjacently are important factors to affect the Curie temperature and the magnetization. The martensitic transformation, which is from cubic to tetragonal structure, can produce three martensitic variants in different directions, resulting with twinning martensite, and the twinning plane is {112}M obtained by TEM. the mechanical properties of the alloy added with Co at room temperature can prevent the sliding of twinning plane, and the cracks is along the grain boundary. The atomic disordering made by grinding lead the disappearance of martensitic transformation, but after the heat treatment for 3h at 600℃, the powder can restore the same martensitic transformation as the block sample.(2) The relationship between the martensitic phase transformation driving force alloy of (Ni_52.5Mn_23.5Ga₂₄)100-xCox (x=2,4,6 and 8) and the yield strengthσ0.2 at the martensitic transformation start temperature Ms isΔGnon-chem P→M=1.19σ0.02 (Ms)+1.63. We also found that the hysteresis temperatureΔT, which is the difference between the equilibrium temperature and the start temperature of martensitic transformation, increases with the yield strengthσ0.2 of the alloy.(3) The martensitic variants arrange stagger in the grains of the (Ni_52.5Mn_23.5Ga₂₄) 98Co2 alloy. The twinning plane is{110}p in the crystal lattice of austenite before martensitic transformation, and becomes{112}M after martensitic transformation. The calculated results from the continuum theory are conformed to the results obtained by experiment. The angle between different orientation twinning planes in the same grain is 85.04°, which is very close to the value observed by experiments. The structure of twins within twins is also observed by the optical microscopy. The twinning planes should be perpendicular to each other in the structure of twins within twins, calculated from the continuum theory, which is actually consistent with the results observed results. In Ni_52.5Mn_23.5Ga₂₄ alloy, the habit planes calculated by the continuum theory are{0.70811,-0.04348,0.70476},10.6°different with Cong's results calculated by WLR theory. The orientation relationship between austenite and martensitic variants is close to the Bain relationship. The calculated results are that the angle between (001)A and (001)M1 is 45.0°and the angle between [100] A and [100] M1 is 4.21°as shown in Fig.6.2 (a), while the experimental observation are 44.8°and 3.95°.(4) The addition of Pd in Ni56.5Fe17Ga26.5 can promote the formation of y phase, play a role in grain refinement and increase the temperature of martensitic transformation and the Curie temperature slightly. The structure of martensitic phase is 7 layer modulated structure. The formation of y phase increases the toughness of the alloy founding in the stress and strain tests.