<Abstract>

A combinatorial approach using diffusion couples and TEM analyses was carried out to

investigate the composition-dependent martensitic transformation in NiMnGa alloys.

The compositions cover a large portion of the off-stoichiometric Ni₂MnGa compositions

and some Mn-rich compositions. Crystallographic variations of the martensitic phase,

including non-modulated (NM) martensite, modulated (5M or 7M) martensite, and

austenitic phase were identified in the diffusion couples and investigated with respect to

their microstructure and crystallography. The 5M and 7M martensitic structures were

only found near the interphase boundary between austenite and martensite, while the

NM martensitic structures were found mostly away from the interphase boundary. The

tetragonality ratio (c/a) for NM martensite generally increases with e/a ratio, but was

also dependent on the composition. The habit plane and martensitic microstructure that

consists of twinned variants with differing orientations were documented using electron

diffraction. The pre-martensitic state was observed in the austenitic phase that was

located near the interphase boundary between austenite and martensite, with distinctive

tweed microstructure and a strain field originating from the local lattice distortions.

The combinatorial approach proves to be efficient and systematic in studying the

composition-dependent martensitic transformation in NiMnGa alloys and can be

potentially applied to other shape memory alloys.
<全文連結>