Rapid Examination of Common Engineering Alloys using the Phenom - Application Note by Phenom-World

:: AZoNanotechnology Article

Topics Covered

Introduction
Metallurgical Inspection Using the Phenom
Providing Information About an Alloy

Introduction

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 Phenom 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.

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.

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 alloy

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 7.