Preparation of TEM samples of ferritic alloys

Characterisation of lattice damage formation in tantalum irradiated at variable temperatures

The formation of radiation-induced dislocation loops and voids in tantalum at 180(2), 345(3) and 590(5)°C was assessed by 3MeV proton irradiation experiments and subsequent damage characterisation using transmission electron microscopy. Voids formed at 345(3)°C and were arranged into a body centred cubic lattice at a damage level of 0.55 dpa. The low vacancy mobility at 180(2)°C impedes enough vacancy clustering and therefore the formation of voids visible by TEM. At 590(5)°C the Burgers vector of the interstitial-type dislocation loops is a<100>, instead of the a/2 <111> Burgers vector characteristic of the loops at 180(2) and 345(3)°C. The lower mobility of a<100> loops hinders the formation of voids at 590(5)°C up to a damage level of 0.55 dpa.

Preparation of micro-foils for TEM/STEM analysis from metallic powders

A technique has been developed which facilitates the preparation of electro-polished micro-foil transmission electron microscopy (TEM) specimens, which have previously been machined out of ≈100 μm diameter metallic powder particles using a Focussed Ion Beam (FIB) instrument. The technique can be used to create small volume TEM specimens from most metallic powder particles and bulk metal samples. This is especially useful when the matrices are ferritic steels, which are often difficult to image in the electron microscope, since the necessary aberration corrections change as the sample is tilted in the magnetic field of the objective lens. Small samples, such as powder particles, were attached to gold support grids using deposited platinum and were then ion milled to approximately 2 μm thickness in a focussed ion beam (FIB) instrument. Subsequently, the specimen assemblies were electropolished for short durations under standard conditions, to produce large (5 μm×5 μm) electron transparent regions of material. The specimens produced by this technique were free from FIB related artefacts and facilitated atomic resolution scanning-TEM (STEM) imaging of ferritic and nickel matrices containing, for example, yttrium rich oxide nano-dispersoids.

Novel techniques of preparing TEM samples for characterization of irradiation damage

Focus ion beam preparation of transmission electron microscopy (TEM) samples has become increasingly popular due to the relative ease of extraction of TEM foils from specific locations within a larger sample. However the sputtering damage induced by Ga ion bombardment in focus ion beam means that traditional electropolishing may be a preferable method. First, we describe a special electropolishing method for the preparation of irregular TEM samples from ex-service nuclear reactor components, spring-shaped spacers. This method has also been used to prepare samples from a nonirradiated component for a TEM in situ heavy ion irradiation study. Because the specimen size is small (0.7 × 0.7 × 3 mm), a sandwich installation is adopted to obtain high quality polishing. Second, we describe some modifications to a conventional TEM cross-section sample preparation method that employs Ni electroplating. There are limitations to this method when preparing cross-section samples from either (1) metals which are difficult to activate for electroplating, or (2) a heavy ion irradiated foil with a very shallow damage layer close to the surface, which may be affected by the electroplating process. As a consequence, a novel technique for preparing cross-section samples was developed and is described.

Fabrication of a TEM sample of ion-irradiated material using focused ion beam microprocessing and low-energy Ar ion milling

Cross-section-view TEM samples of ion-irradiated material are successfully fabricated using a focused ion beam (FIB) system and low-energy Ar ion milling. Ga ion-induced damages in FIB processing are reduced remarkably by the means of low-energy Ar ion milling. There are optimized ion milling conditions for the reduction and removal of the secondary artifacts such as defects and ripples. Incident angles and accelerated voltages are especially more important factors on the preservation of a clean surface far from secondary defects and surface roughing due to Ga and Ar ion bombardment.