Metallurgical Inspection Using
Providing Information About an Alloy
The Phenom is a new tabletop scanning electron microscope (SEM)
which combines the high magnification of electron microscopy with the ease of
use of optical microscopy to improve performance in a tabletop instrument.
The Phenom, a tabletop SEM provides useful magnifications up to
20,000x, is easy to use as the typical laboratory-grade optical microscopes. The
cuts away the time, difficulty, and expense of the conventional SEM. The
operator simply places the sample in the specially designed holder on the
microscope. Due to its unique design there is no risk of damaging the lens. The
automatically focused image is displayed in less than 30 seconds later, with the
resolution and depth of focus typical belonging to SEMs.
Figure 1. Phenom Desktop SEM
Metallurgical Inspection Using the Phenom
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.
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.
Providing Information About an Alloy
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 a-laths.
Precise measurement of the a-laths is important for input into emerging models
that are capable of predicting alloy properties. Figure 2 shows the a-laths
(darker phase) as well as the ß-ribs (lighter phase) between the laths of
ß-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). Often in titanium alloys there is
a second nucleation event that will result in the formation of sub-micron sized
a-laths (e.g., secondary alpha) as shown in Figure 3. 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.
Figure 2.BSE Mode of ß-processed Ti-6-4
Figure 3.BSE Mode of Ti-550
Additional microstructure observations of a cast aluminum alloy are shown in
Figure 4 and Figure 5. The size and distribution of the intermetallic phases
(lighter phases) and shrinkage porosity impacts the alloy properties. Figure 4
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.
Figure 4.Intermetallics and porosity of an automotive Al
Figure 5.Intermetallics of an automotive Al alloy
Ni-based superalloys, shown in Figure 6 and Figure 7, 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 gamma-prime, exhibits a very attractive combination of
strength and creep-resistance, 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
Figure 6. Nickel-base superalloy
Figure 7. Nickel-base superalloy
A complete set of references is available be referring to the
Source:"Metallurgical Inspection Using the Phenom" Application
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