Investigating TEM sample preparation methods for Ti-Ni based shape memory alloy wires for use in in-situ TEM

TEM and particularly in-situ TEM investigations of shape memory alloys (SMAs) have the potential to produce an experimental understanding of some of the unknown fundamental problems faced in SMA research, such as fatigue and forward transformation mechanisms at a fundamental level. Such experiments are underrepresented in SMA research due to the difficulty faced in preparing electron transparent SMA samples. This thesis seeks to investigate the most widespread TEM sample preparation methods used for the preparation of Ti-Ni based SMAs and will show a comparative study of these methods to identify the most appropriate and reproducible method for use in in-situ TEM experiments.

This thesis will outline in detail step by step guides on how to produce electron transparent TEM samples from commercially available Ti-Ni based SMA wires through electropolishing and focused ion beam methods. The advantages and disadvantages of both methods from a literature standpoint will be detailed followed by an investigation of these advantages and disadvantages from an experimental standpoint. Both methods will be compared using HRTEM images and diffraction patterns using a number of different Ti-Ni based SMA wire samples, as well as a general discussion on the difficulties faced using both methodologies. Each methods applicability for use in in-situ TEM experiments and an in-depth discussion on future in-situ work that can be conducted using the prepared samples will also be given.

The thesis presents unexpected results in which FIB methods proved to be more reliable and reproduceable in comparison to electropolishing methods. Various problems were encountered with electropolishing such as maintaining the electrolyte at a low temperature, maintaining a high current and difficulties in relation to imaging such as charging and not reaching eucentric height within the microscope from produced electropolished samples. FIB methods produced no signs of Ga+ induced defects or dislocations which were expected based on available literature, and a lot of HRTEM and FFT analysis could be conducted in comparison to the electropolished samples.