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Metallurgical inspection using the Phenom
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(Nanowerk NanoBusiness Article) The properties of many engineering materials are mostly governed by a combination of metal composition and the morphology and distribution of key microstructural features. These features can be observed with conventional optical microscopy. However, when higher magnification and 3-D detail is required, a scanning electron microscope (SEM) is best suited.
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FEI’s Phenom is a desktop SEM that exceeds the resolution of optical microscopes (30nm v. 200nm, respectively) and eliminates the expense, delay and difficulty associated with operating a traditional SEM. Rapid examination of common engineering alloys (e.g. Al, Ti, Fe and Ni) can be performed with the Phenom in areas such as routine metallurgical analysis, quality control, failure analysis, and research studies.
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Metallography provides information about an alloy linking its composition and processing to its properties and performance. For example, in titanium alloys the yield strength has been shown to be related to the thickness of the α-laths (Influence of processing on microstructure and mechanical properties of (α+β) titanium alloys).
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| Left: Image 1. BSE Mode of β-processed
Ti-6-4; Right: Image 2. BSE Mode of Ti-550.
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Precise measurement of the α-laths is important for input into emerging models that are capable of predicting alloy properties. Image 1 shows the α-laths (darker phase) as well as the β-ribs (lighter phase) between the laths of β-processed Ti-6-4. Using established measuring techniques it is possible to calculate the average thickness of these laths with an uncertainty of approximately 50 - 100 nm, which is less than the resolution limit afforded by conventional optical microscopes (i.e. 200nm) (Quantification of microstructural features in α/β titanium alloys).
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Often in titanium alloys there is a second nucleation event that will result in the formation of sub-micron sized α-laths (e.g., secondary alpha) as shown in Image 2. This characteristic can often strengthen the alloy. It is extremely challenging to image such fine features using an optical microscope, or even to confirm their presence. However, they are readily observed when using the Phenom.
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| Left: Image 3. Intermetallics and porosity of an automotive Al alloy; Right: Image 4. Intermetallics of an automotive Al alloy.
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Additional microstructure observations of a cast aluminum alloy are shown in Image 3 and Image 4. The size and distribution of the intermetallic phases (lighter phases) and shrinkage porosity impacts the alloy properties. Image 3 illustrates an important feature of the Phenom – the ability to gather topological information (e.g. details of the shrinkage porosity) due to the intrinsic depth of focus, while concurrently collecting information about the distribution of phases within the microstructure indicated by contrast variations in the image.
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| Left: Image 5. Nickel-base superalloy; Right: Image 6. Nickel-base superalloy.
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Ni-based superalloys, shown in Image 5 and Image 6, are an excellent example of an "engineered" alloy, and the current state-of-the-art alloys are the product of decades of development. The alloy, consisting of a disordered gamma matrix and ordered gammaprime, exhibits a very attractive combination of strength and creepresistance, enabling it to be used as the material of choice for certain regions of a gas-turbine engine (jet engine). The gamma prime is typically enriched in Al, giving it a darker appearance in BSE mode as shown in Image 6.
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Source: FEI Company. Find your Phenom contact information on www.fei.com/phenon
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