AFM in Metallurgy
Metallurgy is important in industry for developing and producing products from metals, alloys, composites, and other metal related materials. Typically product development and quality control processes utilize microstructure analysis for characterization of metals. The microstructure of metals can vary depending on heat treatment, composition, fatigue or stress distribution. For failure analysis it is critical to examine fracture surfaces. Physical metallurgy research focuses on the changes in microstructure of metals under various conditions. For example, the thermodynamics of defect formation, grain boundaries migration and precipitation are understood by studying morphology evolution. Also, morphological studies are helpful in understanding stress-strain distribution, dislocation flow, and crack propagation. The most commonly used tools for studying metals in both industry and research are the optical microscope, the scanning electron microscope and the transmission electron microscope. The Atomic Force Microscope is emerging as an important tool for metallurgy. The advantages of the AFM for metallurgical studies are substantial. The AFM gives: - Extremely high magnification: Magnifications of up to 1,000,000X are easily obtained with an AFM.
- Three dimensional surface topograms: Unlike the other types of microscopes, the AFM gives magnification in the X-Y and Z axes. This is very helpful in understanding the depth of surface features.
- Easy to use: In comparison to other microscopes with extreme magnification, the AFM is relatively easy to use. With the advanced technology used in AFM systems, it is possible to be proficient in measuring images in less than one day.
- Simplified Sample Preparation: Little or no sample preparation is required before metal samples are examined with an AFM.
- Optical Microscope: AFM instruments include a video optical microscope for rapidly visualizing the surface of metals.
Below are two of the many applications for an AFM in metallurgical studies. Dendrite Structure:
Dendrite (from Greek dendron - a tree) - a crystal that has a treelike branching pattern most evident in cast metals slowly cooled through the solidification range. Often it is possible to visualize dendrite structure in an optical microscope. For example, optical microscope images of dendrites are shown in figures 1 and 4. However, the resolution of the video optical microscope is limited to 1 micron. Other microscope methods such as the SEM do not facilitate the visualization of dendrite structure as shown in figure 2. With the AFM it is very easy to visualize the structure of dendrites with the resolution far exceeding 1 micron as illustrated in figures 3 and 5. Such studies can provide useful information for better understanding interface instability during the solidification process.  Figure 1: Nano-R™ optical microscope image of NiAl, 23 at% Al, dendrite structure. Field of view 1000 µm × 750 µm  Figure 2: SEM image of NiAl, 23 at% Al dendrite structure. Field of view is 5 mm × 4.5 mm. Dendrite structure is not apparent for this sample in SEM. Sample and picture are courtesy of Dr.S.V.Prikhodko (UCLA)  Figure 3: Nano-R™ AFM images of NiAl, 23 at% Al dendrite structure. Left-hand image 97 µm × 97 ,µm right-hand image 70 µm × 70 µm.  Figure 4: Nano-R™ optical microscope image of NiAl, 26 at% Al, dendrite structure. Field of view 1000 µm × 750 µm.  Figure 5: Nano-R™ AFM images of NiAl, 26 at% Al dendrite structure. Left-hand image 95 µm × 95 µm, right-hand image 23 µm × 23 µm . Precipitation Study
Precipitate hardening (aging) is hardening caused by the precipitation of a constituent from a supersaturated solid solution. Aging is a microstructure change, usually by precipitation, that occurs in some alloys after a preliminary heat treatment or cold working operation. Precipitate hardening alloys (superalloys) have excellent mechanical properties at elevated temperatures and are broadly used for industrial applications. Observation of the precipitate's size of less than 1 micron is not possible in an optical microscope due to a limitation in resolution as demonstrated in figure 6. SEM visualization is adequate as shown in figure 7, although it provides neither a 3D image nor precise line profile measurements for the features of interest. TEM is the most commonly used technique for precipitation study as shown in figure 10; however it requires complex sample preparation as well as help of a highly skilled operator. AFM use and sample preparation are very simple. AFM images have great lateral and vertical resolution as shown in figures 8 and 11. Figure 9 illustrates that the AFM is able to perform precise line profile measurements along X, Y, Z as well as any arbitrary direction in the XY plane. Studies and measurements can help in understanding the fundamental problems such as the kinetics of coarsening and the influence of external and internal stresses on the equilibrium shape of inclusions.  Figure 6: Nano-R™ optical microscope image of NiAl with Ni3Al precipitates. Field of view 600 µm × 450 µm Ni3Al precipitates cannot be resolved by an optical microscope for this sample.  Figure 7: SEM image of Ni3Al precipitates. Field of view is 20 µm × 18 . µm inclusions appear white on the photograph. Sample and picture are courtesy of Dr. S.V. Prikhodko (UCLA).  Figure 8: Nano-R™ AFM images of Ni3Al precipitates. Microstructure appears to have much finer features compared to SEM image. AFM image of a single particle shown on the right-hand side. Left: 15.3 µm × 15.3 µm, Center: 2.91 µm × 2.91 µm, Right: 0.73 µm × 0.73 .µm  Figure 9: Height measurements of the line profile of a single Ni 3Al inclusion. The height is 47.9 nm.  Figure 10: Dark field TEM image of Ni3Ga precipitates in NiGa, 25.7 at% Ga. Contrast inverted, inclusions appear white. Plane of view is 1.5 µm × 1 µm . Sample is oriented in a way that plane of view is (001) plane.  Figure 11: Nano-R™ AFM images of Ni3Ga precipitates in NiGa, 25.7 at% Ga. Inclusions etched out and appear dark. Sample is oriented in a way that plane of view is (001) plane. AFM image of a single particle shown on the right-hand side. Discrepancy in volume fraction compared to TEM could be due to etching process as well as foil thickness. Left: 21.1 µm × 21.1 µm, Center: 5.8 µm × 5.8 µm, Right: 2.2 µm × 2.2 µm .
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