Directional Solidification And Microstructure Analysis Of Ni-Fe-Ga-Co Ferromagnetic Shape Memory Alloys

Ni-Fe-Ga-Co is a new ferromagnetic shape memory alloy system. Because of its enhanced ductility,high magnetocrystalline anisotropy energy and wide transformation temperature range, this alloy systemattracts much attention. Based on the interplay of magnetic-mechanical-thermal variables, this alloyexhibits magnetic shape memory, magnetostriction and magnetocaloric effect. Therefore, it is promising formagnetically controlled actuators and refrigerators.In this work, directionally solidified Ni₅₂Fe₁₇Ga₂₇Co₄alloys were focused. They were prepared byzone melting liquid metal cooling （ZMLMC） method. The preferred crystal orientation and microstructurewere systemically studied. Meaningfully, the martensitic single-variant set was reported. In addition, themartensitic microstructure was analyzed by the EBSD method. The coupling of the magnetic domains andmartensitic lamellas were revealed.ZMLMC method is an effective way to prepare highly preferred columnar crystals. It is deduced thathigh temperature gradient and low crystal growth velocity are beneficial for stable solidification. Under thiscondition, coarse columnar crystals with uniform composition and highly preferred orientation are obtained.Under low temperature and high growth velocity, the solidified crystals grew along <100> direction andprimary γ phase was formed during the unstable growth; when increasing temperature gradient anddecreasing growth velocities, the coarse columnar crystals with uniform sizes grow along <110> directionand well-developed preferred orientation, i.e. along （222） planes of the martensite.The martensite of Ni₅₂Fe₁₇Ga₂₇Co₄alloy has2-layered L10structure. The martensitic lamelas cangrow transcrystalline on the crystal boundary with small misorientation. Giant martensitic single-variant setis obtained under high temperature gradient and low growth velocity. Because of its well-alignedmartensitic lamellas, it is meaningful for large magnetic-field-induced strain. EBSD result reveals that thesingle-variant set has integral inner structure and no microtwin. Moreover, the twinning planes are （110）and （110）.However, microtwins are formed in local area of the martensitic lamellas by internal stress. It isrevealed by EBSD result that the microtwins are “twins within twins”, which are formed by hierarchicaltwinning. The internal stress results from the thermal effect, the lattice distortion energy on the boundariesand the interaction of the magnetic domains and the martensitic lamellas.The magnetic domains intersecting with the martentic lamellas are directly observed by opticalmicroscope without polarization. These magnetic domains have similar morphology with the martensite lamellas but can be verified by EBSD method. The interaction of the two structures can be revealed by thesurface relief induced by internal stress. It is revealed by EBSD that the twin boundaries have highmisorientation of80-85°while the domain walls are small angle interface with misorientation of1-8°. Themisorientation of the twin boundaries is due to the twin relationship, while the misorientation of the domainwalls is caused by surface relief.