Transitions Between Austenite and Martensite Structures in Ni ₅₀Mn ₂₅Ga ₂₀Fe ₅ Thin Foil

Abstract

Using common and high resolution transmission electron microscopy, we study the evolution of crystal structure with increasing thickness in a thin Ni₅₀Mn₂₅Ga₂₀Fe₅ foil. Iron alloying enables to observe the complete spectrum of known martensitic phases and respective transitions simultaneously. Starting from cubic austenite at about 30 nm foil thickness, the structure evolves via interleaved stripes of austenite and five-layered modulated 10M martensite to pure 10M phase with low density of stacking faults. With increasing thickness the 10M phase transforms gradually to seven-layered modulated 14M by increasing density of stacking faults. Finally the non-modulated tetragonal NM phase appears within the 14M phase by detwinning of nanotwins. We found large local variation of lattice parameters, which is ascribed to elastically very soft austenite and 10M phase and faulty 14M lattice. Our experiments show clearly that nanotwinning and stacking faults are inherent structure features tightly connected with lattice modulation and intermartensite transformations. This is important finding in broader attempt to bridge the gap between the nanotwinning and modulation period development with temperature and composition. From the application point of view, the observed instability of modulation or of particular phases in thin films imposes certain limits on the use of material on microscale.

Keywords: Structural modulation; Austenite-to-martensite phase transformation; Nanotwins; HRTEM; Ni-Mn-Ga-Fe