Loss of grain boundary segregant during ion milling

Abstract

It is shown that material segregated to grain boundaries can be lost during ion milling. This specimen preparation artifact has been studied in the case of bismuth in copper and has also been observed for phosphorus in stainless steel. The loss is associated with specimen heating during ion milling and can be alleviated by good clamping and cooling of the specimen during milling. Specimen heating permits grain boundary diffusion of the segregating element to the specimen surfaces with subsequent loss of segregant from the specimen by evaporation or sputtering during ion milling. Loss of bismuth during in situ heating to 200-300 degrees C is demonstrated. Therefore, care must be taken in specimen preparation for analytical electron microscopy measurement of such segregation. Similar effects may occur during ion milling of other materials, especially those where low thermal conductivity will result in high beam heating. In these cases, care must be taken to avoid loss of segregant during specimen preparation. Additional tests showed that no significant loss of segregant was observed during X-ray microanalysis, even at nominal room temperature and probe currents five-fold higher than that normally used for microanalysis.

Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials

“…Beam-induced heating was considered to play a critical role in accelerating hydrogen in-diffusion and enhancing hydride formation by both conventional broad ion beam (Ar ions) milling 11 and focused ion beam (Ga ions) milling 23 . In earlier work, it has been reported that conventional argon ion milling could heat TEM specimens up to around 400 °C 45 and further cause both microstructural and chemical instability in a wide range of materials including Au/Si specimens 46 and metallic alloys such as aluminium 47 , copper and stainless steel 48 . Conducting liquid-N 2 (LN 2 ) cooling of the specimen stage during Ar + ion milling has proved to be an effective pathway to prevent artificial chemical instability by decreasing diffusivity in such cases.…”

“…For example, the Au/Si interface migration caused by beam-induced heating could be eliminated by using a LN 2 cooled specimen stage 46 . Kenik 48 also showed that low temperature milling could effectively alleviate the loss of grain boundary segregating elements, such as bismuth in copper or phosphorous in stainless steel. With the same consideration, Carpenter et al 11 suggested that cryo-cooling of specimen could be of significant benefit on preventing hydrogen pick-up and hydride formation during broad Ar ion milling of Ti and Zr materials.…”

Measurement and estimation of temperature rise in TEM sample during ion milling

“…Sample heating by the ion milling has been a well-known phenomenon and many reported about related issues, loss of grain boundary segregation [5], micro-structural change [6,7], estimation of the temperature using low-meltingpoint metals [8], and measurement using temperature probe [9][10][11]. The operational manuals of ion-milling system provide only a rough estimate of the thermal power, which can be as much as 300 mW at a high milling rate [12].…”