Magnetic shape memory alloy Ni50Mn27Ga22Fe1 with commensurate and incommensurate modulations : thermal and stress-induced transformations

This dissertation advances the understanding of crystal structure, phase transformations, and mechanical properties in Ni50Mn27Ga22Fe1 magnetic shape memory alloy by combining experimental investigations with the development of new models to interpret the results. Motivated by the need to better understand the structure of modulated martensites, the work introduces a method to obtain martensitic phases with a simplified microstructure through the application of sample constraints. This approach enables precise determination of lattice parameters across five-layered modulated (10M), sevenlayered modulated (14M) and non-modulated (NM) phases. It further proposes the constant plane shift (CPS) model to describe the average lattice of modulated martensites, overcoming the limitations of previous tetragonal building block models. The CPS model accurately reflects both continuous and discontinuous changes in the lattice parameters with temperature. The research further focuses on the investigation of the incommensurately (IC) modulated crystal structure, explores temperature- and stress-induced transitions between commensurate (C) and IC modulated structures, examining the stability of the IC structure and impact on twin boundary mobility. Direct measurements of the elastic matrix for IC 10M martensite indicate that nano-twin boundaries become immobile under high stress, fundamentally altering the elastic response compared to C-modulated structures. Together, these results bridge critical gaps in the understanding of structure-property relationships in Ni-Mn-Ga-based alloys. Furthermore, they provide a foundation for future atomic-scale modelling and the development of advanced magnetic shape memory devices.