On the formation of temperature-induced defects at the surface of TEM specimens prepared from TiAl using high-energy Gallium and low-energy Argon ions

Specimens for transmission electron microscopic (TEM) investigations were prepared from gamma titanium aluminide alloys with Gallium and Argon ions using a focused ion beam (FIB) and a precision ion polishing system (PIPS). Preparation to electron transparency by Gallium ions alone leads to the formation of crystalline platelets of the alpha titanium phase at the specimen surfaces, revealed through conventional and high resolution (HR) TEM analysis. The platelets are assumed to precipitate from priorly formed amorphous layers. The required crystallisation temperature of about 480 degrees C is generated through the ion bombardment implying that the gamma titanium aluminides can be heated substantively during sputtering. The primary reason for this is the restricted transfer of heat away from the beam impact point when the specimen thickness comes close to the beam diameter. The formation of the platelets can be avoided by terminating the Gallium ion treatment prior to that, while providing for a sufficient thermal bonding of the specimen to the grid as well and polishing off the remaining material by Argon ions, which are much less focused and less energetic, so that the local heat peaks are reduced.