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Twin structure and adaptive phase formation in Ni-Mn-Ga modulated single crystals

by Robert Chulist (Institute of Metallurgy and Materials Science, Polish Academy of Sciences)




The crystal structure and transformation path from austenite to 10M martensite in Ni-Mn-Ga single crystal were examined employing high energy synchrotron radiation and scanning electron microscopy. Using temperature gradient an austenite/twinned martensite interface was stabilized revealing the crystal structure and microstructure of both phases as well as the transformation sequence across the interface. Three distinct types of martensite crystal lattice namely simple tetragonal, and two monoclinic modulated i.e. 10M’ to 10M are confirmed, depending on the distance from the interface. In-situ measurements show that the lattice mismatch formed at a habit plan is compensated by the formation of micro-twinned and branched martensite along with change of lattice parameters. Furthermore, it is shown that the characteristic for periodic shuffling twin boundaries such as modulation twins or inverting stacking faults in the form of a/b boundaries are installed in a later stage of transformation. Thus, the transformation takes place not by one shear process but by a sequence of at least three shears, first along the elements (202)[10], then (100)[00] and finally a minor shear along the (110)[10] occurs. Overall the results show the crystal lattice do not adapt a modulated phase at the habit plane but rather a simple tetragonal structure providing significant insight into the view of the adaptive character of martensitic transformation.