Characterization of shape memory alloys via transmission electron microscopy
Shape memory alloys are a unique class of metal alloys that can recover from permanent strain when they are heated above a certain temperature. These alloys also exhibit pseudoelastic properties: they can sustain large deformations at a constant temperature and return to their original undeformed shape when the stress is released. Ferromagnetic shape memory materials can undergo similar shape changes in response to a magnetic field; however, their magnetic properties change following physical deformation by an external mechanical stress (the so-called ferromagnetic shape memory effect). These ferromagnetic materials are now used in a variety of applications, including military, medical, safety, and sensor uses. In the last decade, they have also been considered for use in ferroic cooling devices. However, several problems have to be overcome in order to achieve high efficiency in functional devices. This thesis addresses these problems with the aid of advanced transmission electron microscopy (TEM) techniques, and explores the structure-function interrelationship of ferromagnetic shape memory materials. Two ferromagnetic shape memory alloys, Fe70Pd30 and Fe-Pd-Pt thin films, were structurally and chemically investigated at the nanoscale by TEM, including in situ measurements for evaluating specific phase transformations. Ferroic cooling systems were examined using Ni-Ti and Ni-Mn-In-Co thin films, which were selected for their high potential for use in magnetocaloric and elastocaloric devices. In situ TEM measurements were combined with ex situ studies in order to mimic the in-function status. Some TEM techniques, such as automated crystal orientation mapping and electron tomography in the scanning TEM mode, were also performed for the first time on these alloys.