Abstract
The off-stoichiometric ferromagnetic Ni-Mn-Ga SMAs have recently gained noticeable interest mainly due to a large reversible magnetic-field induced strain (MFIS). This effect is mostly reported for five-layered 10M Ni-Mn-Ga because the single crystals of this composition exhibit very low twinning stress yielding up to 7% of strain. Additionally, the small periodic displacements of atoms in a crystal lattice, referred to as modulations appear frequently in a number of functional materials. The remarkably low twinning stress enables variant reorientation, under moderated magnetic fields, and it appears to be linked to modulation. Nevertheless, the origin of modulated crystal structure of Ni-Mn-Ga alloys is still unknown. Therefore, in this study, the role of modulation and the sequence of twin boundary formation such as type I, type II, modulation, and a/b boundaries in the self-accommodation process upon martensitic transformation are investigated in five-layered 10M and seven-layered 14M martensites. In-situ experiments combined with a high-resolution scanning electron microscope and synchrotron radiation show a specific sequence of phase formation during cooling as well as provide significant insight into the view of the adaptive character of martensitic transformation. In-situ experiments reveal the transformation phase sequence upon martensitic transition as a function of distance from the austenite/martensite interface confirmed both by synchrotron diffraction and EBSD experiments. As a result, at least six different martensitic structures can be formed in a specific order before the final NM martensite is established. Furthermore, 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 an elastic change of lattice parameters. The results are discussed with respect to the shuffling of atomic layers. A similar analysis is performed on the hexagonal metals and fcc-hcp transition in HEAs.
About the Speaker
His primary research area is dealing with smart metallic materials exhibiting a martensitic transformation and related effects such as shape memory effect, magnetocaloric effect and super and pseudoelasticity. This activity has been manifested in various research projects and publications in the field of martensitic transformation. Concurrent his research interests lie in the area of microstructural, mechanical and texture examination of metals, alloys and intermetallic phases subjected to sever plastic deformation (SPD).
Furthermore, the experience gained doing study with synchrotron radiation and EBSD was used for the investigations of explosively joint clads such as AlTi, AlMg, CuTi, stainless steel with Ta and Zr and multilayer systems.
Host: Professor Huseyin Sehitoglu
