Which Microscope is Often Used to View Metal Surfaces

Ways to Examine Metals by Light Microscopy

March 7, 2018

Wayne D. Niemeyer, Senior Research Scientist, McCrone Associates

Low-cal microscopy imaging techniques, such equally brightfield, darkfield, and Nomarski differential interference dissimilarity (NDIC), are used to examine metallic surfaces and polished/etched metallographic cross sections. Grain size and shape, inclusions, and internal stress patterns are some of the common items of interest for light microscopy imaging of metals. Numerous examples are shown in this webinar. 36 minutes.

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Charles Zona (CZ):
My name is Charles Zona, and I would similar to welcome everyone to today’s McCrone Group webinar. Our presenter is Wayne Niemeyer of McCrone Associates. Wayne is a Senior Research Scientist with McCrone Assembly who has over 40 years of experience in both electron microscopy and light microscopy. In today’s webinar, Wayne is going to talk about
Ways to Examine Metals by Light Microscopy. Wayne volition field questions from the audition immediately following the webinar. This webinar is being recorded and will be available on The McCrone Group website nether the Webinars tab. Now I will hand the program over to Wayne.

Wayne Niemeyer (WN):
Thank you lot, Chuck and welcome everybody. I certainly capeesh you taking time out from your busy schedules to join us today, so hopefully I can provide some useful information for you in examining metals by light microscopy.

These are what I call common light microscopy methods that nosotros use for metals:

  • Reflected (episcopic) illumination
    • Brightfield
    • Darkfield
    • Crossed polarizers
    • Nomarski differential interference contrast (NDIC)

The reflected light because they’re opaque, of class, and it’s too known every bit episcopic illumination, and the four types that I like to employ are brightfield, darkfield, crossed polarizers, and Normarski differential interference dissimilarity, ameliorate known as NDIC.

At present the brightfield, darkfield, and crossed polarizers are probably the most mutual illumination methods you’ll run across in various publications and books, and so forth.

Normarski is a fiddling bit less known, but certainly not here; I use Nomarski quite a bit, and I’ll show you some examples of these various illumination techniques. I’m not going into a lot of detail at all virtually the setups for brightfield, darkfield, and Normarski, and and then along, and the microscope accessories that are needed for things like that; I’grand only going to requite you the applied uses of these illumination techniques.

Here’southward an case of some brown stains that were noticed underneath the clear coating on the inside of a food can. The clear coating was fortunate because we were able, and so, to look through the coating down to the base steel—tin can plated steel, and to run across these dark-brown regions that were causing the stain. Notice they’re circular; they have blackness lines going through them, and when I initially saw this, I idea for certain that these were starch grains. That’s a typical morphology of a starch grain; the blackness lines are known equally isogyres.

Stains nether a food can’s internal clear coating. Episcopic illumination with crossed polarizing filters.

We found out later through some analysis, by isolating some of these things from that can, that the material was actually iron carbonate. Information technology was caused past some corrosion of the base of operations steel underneath the tin that was oozing upwards onto the surface of the metallic and oozing out underneath the clear coating to produce these features.

Let’southward have a expect at some contamination on tin plated steel. The surface of tinplate steel is quite shine and brilliant, and in brightfield episcopic illumination it appears quite vivid and you sometimes have to reduce the intensity of the illumination to see the structures a picayune bit better.

You can also insert a polarizing filter into the light path and that’ll help reduce some of the glare, only like it would from polarizing sunglasses that reduce glare on ripples and waves from a lake. Only under the episcopic illumination, we see in the center there are two large very night materials, and kind of in betwixt them at the centre the prototype is something that’due south a little lighter grey, and so the rest of the surface is just the tin coating material.

If nosotros cross the polarizers, at present we can see a lot more than information. The black material is now showing up equally orangish-brown material and there’due south a lot more of it—smaller features kind of scattered around that image—that we actually wouldn’t even notice with the brightfield illumination. But the colour of that material would suggest that information technology’s rust; atomic number 26 oxide.

The center part of it (merely to the left of heart)—at present we meet that that lighter grayness material is at present kind of a bright white cloth under the crossed polarizers, so nosotros can do one more than matter: we can insert a first order red compensator into the light path, and that will also now allow u.s.a. to see that these bright materials that are laying there on the surface are actually particulates that are crystalline materials; they are quite birefringent.

So, there are a lot of things that nosotros can see with these types of illuminations that’ll help u.s.a. identify or isolate materials for further identification.

Hither’south an example of an electrolytic tin plated steel surface with crossed polarizers. Here we can come across the tin grains; in fact, in the center there’s a big tin grain that’s kind of a bluish-greyness color, and if we rotate the stage ninety°, we tin can change that color into a blueish color and then have the surrounding material exist more of the grey fabric. This is oftentimes referred to as optical staining techniques to highlight diverse features on the surface of the metals that are anisotropic-type materials.

Hither is an examination of some polished beryllium metal. With normal brightfield episcopic illumination—pretty characterless, non much to see there. Information technology’south very reflective; fifty-fifty turning down the intensity of illumination to darken information technology a lot more than, yous can run into very little material. Merely if we cross the polarizers, now nosotros can meet the grain structure of the beryllium. Beryllium is one of those metals that’due south an anisotropic material, therefore, information technology will show up under the crossed polarizers with the grains that are in different orientations, and we can come across the unlike colorations of the grains in those orientations. Now yous can take an image similar this—a digital prototype similar this, and take information technology into some image analysis software and do some, maybe, grain sizing or frequency distributions, per centum coverage of specific size of the grains, and things like that.

And so nosotros’ll switch now from surface imaging to cantankerous sections of the base metals of materials, and this is known as metallography techniques. This is where samples are cut out of the metal and mounted in an epoxy medium. Sometimes it’s a thermal set epoxy, other times it can be a two component epoxy that cures at room temperature. So they’re ground and polished. They’re ground to get past the impairment from the cut area shown by the two arrows here—the two lines. We desire to get past that cutting damage into the more pristine base metallic, then nosotros tin can last polish these cross sections, and mayhap even etch them like I accept shown on the right-hand side here with the aluminum from this drink can cease.

The carving material here is i% hydrofluoric acid. This is brightfield illumination, and you see the grain structure of the aluminum as it has wrapped effectually to the outside of the body claw, or flange, and support inside the flange, and then it’s all pressed together. And then we can see the strain structures of the aluminum as information technology’s going around these bending and pressing processes. Y’all’ll discover at the lesser of the image there is a slight kink that has formed here, at the very bottom, and nosotros can see that in that location’s some strain pattern going on, and this is a possibility at present for some residual stresses that could be in this area that might somewhen lead to a crack.

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Again, on the aluminum cease, we tin can cut through the score area of the apartment— that’southward what opens up the pigsty so that you can pour the liquid out when you pull that tab up. These score lines are in that location to have controlled fractures of that flap so that it fractures very uniformly around the circumference and folds downward into the can. Nosotros cutting a cross section through the score lines, and again, we can etch them with hydrofluoric acid, and we tin encounter the flow patterns of the aluminum from that stamping functioning to produce those score lines.

In the right-hand image, we can run into that there’s a trivial notch, a fiddling opening, at the very base of operations of the “V.” That’s a potential site for corrosion attack—crevice corrosion attack—from the outside if the tin gets involved with some sort of liquid on the outside of the stop. These, over again, are brightfield illumination with the one% hydrofluoric acrid etch.

Here’s an example of brightfield illumination from a polished longitudinal section of a cold-worked carbon steel. This is 2% nital etch. Nital is nitric acid in methanol, and this is 2% by volume nitric acrid in the methanol. That’southward used quite often for carbon steel to highlight the grain structure so that nosotros can meet the catamenia of the grains, and so forth.

This is a longitudinal section, and so we tin can run across the elongation of the grains that occurred during the cold-working process to thin the metal sheet down to the final thickness. We can likewise meet in hither that at that place are some regions that look like they might be stringer inclusions in the metal, and those are often of import to visualize.

Polished longitudinal section of cold worked carbon steel; ii% nital compose, brightfield illumination.

Hither is one of my favorite cross department images. This is a polished aluminum canvass section, cantankerous section, that has one huge, major inclusion in information technology. You can also run across that around that inclusion the metallic is already fracturing because it’s and so big in in that location. Obviously, with the metal forming operation that these sheets go through, they encountered a lot of fracturing problems because of these inclusions. These inclusions were produced at the aluminum mill.

Polished section of aluminum sail with major inclusion. Brightfield illumination.

Hither is a brightfield examination of a polished cross section of stainless steel that has very severe stress corrosion cracks. This river-like blueprint that y’all see in in that location is quite typical stress corrosion cracking. This is unetched; we don’t want to see the grain structure and so much—we’re more interested now in really seeing the corrosion cracks rather than the grain structure of the metal.

Brightfield illumination of polished section of stainless steel with severe stress corrosion cracks. Unetched.

I am going to spend some time talking well-nigh this type of sample: it is a side seam joint weld from a tin plated steel food tin. If you take a food can, like a soup can or something like that, and take off the paper wrapper, you’ll see that down the sidewall of the tin there is a stripe that’due south virtually a millimeter broad and dark gray. That’s the weld that occurs from the original canvas stock that has been rolled into the cylinder, and then overlapped slightly at the edges and arc welded to produce the final cylinder. Now the weld, in this case, is covered with an internal blanket—that’s called the side seam stripe—to protect the weld from corrosion by the production that’due south in the can. This particular can does non have a blanket around all of the surfaces of the tin can; merely around the weld, and that coating is nigh a one-half inch or then in width to protect the weld.

In welding operations with metals, the weld volition become very susceptible to corrosion assail compared to the surrounding matrix metal. In other words, in the electrochemical sense, the weld becomes anodic, the base steel metal becomes cathodic, and now when you have a corrosion reaction process starting, the corrosion reacts very rapidly within the anodic region; in this example, the weld.

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Here we accept plane polarized light illumination; in other words, I have brightfield illumination with one polarizer in the low-cal path to help reduce some of the glare, and we can see the base of operations metallic in the weld and the interior blanket. At present if we cross the polarizers—and these are unetched again—now we can just highlight the interior coating if we want to by adjusting brightness and contrast, and take the metallic out of the image, if we’d similar.

Or, we tin can go ahead and insert that first-club blood-red compensator and just highlight the base metal itself, separate from the coating, and accept the two side by side. It’south just a affair what you want to try to show as to the types of accessories and filters, and and then forth, that yous want to put into the arrangement.

Let’s talk a picayune bit most Normarski differential interference contrast on that aforementioned side seam joint weld; again, it’s unetched. But when we use the NDIC we can highlight the base metal surface, and we can run into very, very fine detail on the surface of metals with NDIC. In this detail example, we tin encounter that there are lots of piffling scratch marks on the metal, and that’due south from the polishing operation that was done on this one, and for the metallographers out there, if you expect at this sample just visually afterwards it has been polished, you’ll say “Wow, that’southward a nice polish mirror stop on that metal.” Put it under NDIC sometime and you lot’ll see that maybe it’s not quite as smooth as you thought. Really, a good style to monitor the quality of your polishing operations is to look at it with NDIC.

Nosotros can also use NDIC to colorize samples. The NDIC organisation has in the light path what’s known as Wollaston crystals, or Nomarski crystals, which are basically quartz wedges that are glued together, and we can move those wedges back and forth in the lite path and so that the lite will go through different thicknesses of those ii wedges to produce these different colorizations. So if we look on the right-hand side, the colorization contrast with the blue for the base of operations metal, and now we can come across that there’s another layer, where the arrows are pointing to, on pinnacle of that metallic betwixt the metallic and the coating. That’southward the can coating; the tin plating showing up now that we can’t run across any other mode by the calorie-free microscopy techniques. This is the only manner that I know of that you can actually see tin coating that sparse. Can, being an anisotropic metal, will show up because of the crossed polarizers, and this volition assistance highlight information technology.

NDIC for colorization dissimilarity to bear witness the presence of the base metal tin plating layer (arrows). Non etched.

If we take that aforementioned area and just increase magnification from left to right here, nosotros tin can highlight that tin can plating layer a little bit more than, and we can come across that information technology’s approximately 2 µm thick, then that’s pretty thin for looking at it past light microscopy. Information technology’s actually better to wait at it by scanning electron microscopy. But, nosotros tin can certainly come across it in this imaging technique with NDIC. Notation also that in the blue colorization layer nosotros’re not seeing the scratches then much from the polishing, and that’due south because of the mode it’due south colorizing it.

So now let’s have the same weld and let’due south etch information technology with the nital—the 2% nital. On the left-hand side, we have the plane polarized light illumination and we have the weld zone, which is highlighted hither: you can see that much dissimilar crystal structure, or grain structure, of the weld area versus the surrounding base metal to the correct and left of the weld. Then the within side seam protective polymer coating is nonetheless there, but we can barely run across it in these particular images. That weld zone is likewise referred to as a oestrus affected zone in welding terms. On the right paw side we take a higher magnification prototype of the weld zone under the plane polarized light and so you lot can encounter the grain construction much better. It’s a very complex grain structure, for sure, compared to the elongated grains that are in the base metal next to information technology.

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Now nosotros tin can exercise another things. On the left we have the aeroplane polarized light again, and on the right—now we’ve crossed the polarizers. Now we tin can see a couple of things that really leap out at united states of america.

Outset of all, that internal protective coating is now very vivid white. The protective coating has fillers and pigments in it that are quite birefringent materials, and that’s why they show upward so well now with the crossed polarizers.

I want to draw your attention to what’s happening within that yellow box. We tin can see that there’south evidence of recrystallized material in that fusion region of the weld. This is where the two overlapping pieces of the tinplate have been melted and welded together. I suspect that the recrystallized material that nosotros’re seeing in there—from the birefringence with the crossed polarizers—is probably from the can that melted during the welding process and diffused into that fusion zone from both of those plates equally they were welded together. Tin can, again, is an anisotropic metal; the iron base of operations steel is not. That’south what I think that is; it’s probably recrystallized tin grains in that fusion region.

We can also go to colorizing the features in here with the compose. Nosotros can colorize information technology blueish like we have on the left, or nosotros can colorize information technology mainly with orange, on the right. Here nosotros can see that at that place are dissimilar—perhaps phases present, especially in the blue. From my eye, I tin can encounter that a trivial better with the blue. There is a nighttime kind of webbing blazon net in the structures that is interwoven in in that location among the flatter grains in the sample. So, here you can take these images, digital images, and put them into some prototype analysis software, and possibly do some analysis for percent area coverage of the phase that is represented by the web-similar material, something like that.

Allow’due south take a await now at the base of operations steel versus the weld zone, and the structure of that textile. On the left-hand side we have the plane polarized light illumination. Then we can increase the magnification a piddling chip more than and look at it with NDIC. Notice the NDIC is giving you some three dimensionality perspective of those weld grains from the etching process. NDIC is actually, really good for topography data on reflected opaque subjects like this. It’south normally used in transmitted calorie-free mode for biological samples, for cell structures where you have very little contrast; the transmitted light Normarski differential interference contrast gives y’all much ameliorate contrast on depression-contrast-blazon materials. But, in metals or surface imaging like this, nosotros are actually seeing topography differences, and that tin can go very useful.

Notation too in the NDIC image that all the lilliputian scratch marks that nosotros saw before with the NDIC unetched are gone. The etching does remove those very fine scratches. It’s dissolving the surface away and penetrating down into the grains of the metal. You can also utilise this equally sort of a quality test on your metallography to observe out if you’ve actually removed the scratches and gotten away from the surface, and gone downwards more into the base metal.

I too want to bring your attention to the detail that this tin bring upward. Look inside the ruby box here, you might not be able to run across information technology also well, but there’s a grain in there that has parallel structures spaced at about at a 45° angle, and spaced only a micrometer or two apart from each other. Very, very fine item that you tin can run into with NDIC that you’re just not going to run into in any way other than calorie-free microscopy.

NDIC illumination of weld zone and adjaent base steel, two% nital etch.

At present we can proceed on and take a wait at what darkfield illumination presents for us. On the left we accept our plane polarized lite, and on the correct is the darkfield illumination of the weld zone. Here we tin run across the grain structure outlines very hands—the nice light outlines, and and then the dark regions are the flat regions of the grains that are reflecting straight support to the objective and to the eyepiece. Those reflections coming directly back up are really blocked in the objective system, and so that we wouldn’t run across that. We’re seeing the scattering of light due to angles and topography differences, and so forth. Annihilation that’due south really flat is going to look quite dark.

We tin besides run across the protective coating is a squeamish bright white; nosotros can see that much more conspicuously at this point. Only again, these things tin can be used, and then, for other imaging manipulations with software, and and then along, if y’all are trying to practise some quantitative work with that.

Plane polarized low-cal illumination of weld zone, 2% nital etch.

I am going to cease up a footling bit here to give you iv unlike images of the same region—the aforementioned field of view—using each of these 4 techniques to show y’all what they can really tell y’all.

Here’s a polished brass. It’s final polished with 1 µm diamond paste slurry, and the brightfield illumination is what most metallographers would be looking at to await at their polishing system. We tin can run into that this one is non polished quite well enough nonetheless. Some of these nighttime specks have what we call “comet tails” coming off of them like they are smeared a little scrap; a smearing action. We can as well see some heavier scratches that also are kind of parallel to these little smearing things. This sample needs to be polished a little chip more than. Information technology hasn’t removed all of the artifacts from the previous grinding and polishing steps before information technology got to the one micron diamond paste, but it illustrates what you come across with this type of illumination.

Let’south go ahead and cantankerous the polarizers. Well, in that location are our scratches, again. For the metallographers out in that location, if yous’re looking at how well you lot’ve done, you may not want to look at it with crossed polarizers because information technology volition exist a sticker daze for you compared to the brightfield arrangement, just it certainly can exist used as a quality test for how well your polishing has been going.

If we accept a look at information technology with darkfield illumination, nosotros don’t see the scratches quite every bit much, but it’south really highlighting all the picayune inclusions that are on the surface or within the surface of that metal. Now, you can take something like this image and take it offline into an image processing software and do particle sizing, particle distribution, particle frequency, you can practice particles per unit surface area, and things similar that to aid characterize the sample.

Finally, if we look at it with NDIC, yous can come across at present that nosotros can really see the very, very fine features from the polishing operation, and nosotros tin can likewise run into some of these inclusions that are nonetheless there, only not quite as well as we could with darkfield—but if you really want to see very fine particular on the surface of materials that are highly polished, NDIC is probably the way to become and then that yous can really see what’southward going on on the surface.

I desire to close with giving you some references that y’all can expect for. If you’re still interested in optical microscopy of metals, this is a cracking reference volume:
Structure of Metals through Optical Microscopy
past Tomer, copyright 1991 by ASM International. It’s probably non available now. I did non bank check information technology on the ASM website. This is an outstanding volume considering the author goes through some preparation techniques, metallography training techniques, optical microscopy stuff, different alloys, different cross sections from metal forming, deep drawing operations, welding operations, other joining type systems, then forth. It’southward a very, very good reference book for all kinds of dissimilar things. If you are a generalist working with different types of metals, alloys, and then along, and want to compare what they look similar under light microscopes, this is an outstanding resource for that.

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Then we have the book
Images Of Materials. This is copyright 1991 by Oxford University Press. It’s edited by Williams, Pelton, and Bronsky, and there are various authors that have contributed to this book. The first affiliate is optical microscopy of metals and it is written past George Vander Voort. George is a globe-renowned expert metallurgist, metallographer, fracture analyst, and and so along, and he does a lot of webinars besides for these types of things. He’s an excellent resource. The outset 50 pages are his chapter, and and so after that information technology has all kinds of images from dissimilar techniques: scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, and and then forth; lots of unlike material images by various other instrumental techniques. Another outstanding reference book.

Finally, this is one of my favorites. It’southward an old book copyright 1972 by the American Society for Metals, or ASM International as information technology is known now. It is the
Metals Handbook Volume 7, Atlas of Microstructures of Industrial Alloys. This particular volume has all kinds of unlike alloys and is but loaded with pictures of microstructures, surfaces, and cantankerous sections, and then along. ASM International at present produces books like this, but they are more geared for specific alloy materials. In that location might exist books just for zinc alloys, and other books for copper alloys, and other books for steel alloys, and and so forth, which is fine, and they are more up-to-engagement of grade, but this particular book has them all. Information technology has tons of different materials: refractory metals, stainless steels, nickel alloys, copper alloys, titanium alloys, electrical contact materials, carbon steels, steel castings, and tool materials—everything all in one book. Information technology is an admittedly outstanding book. If yous want to get it, you probably will have to find something on eBay, because I know ASM doesn’t publish this one anymore.

So, give thanks you lot for joining us today, and yous tin can start putting your questions in the chat box if you haven’t already done so.

Yep, we will kickoff taking questions hither, simply get alee and blazon them into the questions box, and Wayne volition start going through these:

“Could y’all delight explain more than well-nigh stress corrosion cracking?”

Stress corrosion is caused past, obviously, a corrosion reaction on metals that have been stressed in some way or another. They’re either bent, or they’re deep drawn, things similar that, where they’re not in the normal relaxed position. Those stress areas are going to exist much more than susceptible to corrosion attack. When the corrosion attack starts, it doesn’t just go effectually the grain boundaries; it goes right through everything—all the grains, and so along. It’south a very rapid peachy machinery. It simply occurs, of course, with metals that are under a stress load of some sort.

“Tin Normarski be retrofit to a standard calorie-free microscope?”

I judge it depends on the microscope—if the Nomarkski accessories are bachelor for it. You’d have to check with the microscope manufacturer for that, but certainly information technology can. Information technology’southward going to have to exist a compound microscope, though, not a stereomicroscope.

“What kind of configuration is needed for NDIC?”

There are accessories for the NDIC. Y’all’re going to need the Wollaston prisms to be inserted into the light path, and you’re going to need the polarizing filter and analyzing filter along with it to do the cantankerous polarization that y’all need to exercise with NDIC. Again, those are things to check with your manufacturer to notice out what accessories you are going to have to accept, but that would be the basic setup for it.

“For cross polarization, are the two polarizer filters upstream of the sample, or downstream, or what? How are the cross polarizers planned in the optical transmitted path, and does it matter which ane rotates?”

Typically, and certainly in transmitted light, i polarizer is put underneath the sample, and and then the low-cal comes through information technology and passes upwardly through the sample, then the analyzer, which is the other polarizing sheet, is up in the light path leading to the eyepieces. In reflected light, there is a polarizing filter inserted into the calorie-free path, and then the analyzer is placed elsewhere there, and y’all rotate the analyzer. The polarizing filter is generally stock-still in identify and then the analyzer is rotated versus it.

“I joined a footling tardily. I do PLM fiber identification and I am new to NDIC. Is information technology something software based, or simply accessories based?”

Information technology is accessories based. It is something that you would use in the microscope to take images of your samples from there. It’s actually pretty skillful for cobweb use, looking especially at the natural fibers, to look at the surface structure of natural fibers.

“Do you lot have a particular brand of microscope that you use?”

The microscopes that I have in my function are Olympus. My polarizing light microscope is the Olympus BX51.

“Which method would you recommend to highlight transfer of metal from one source to another via contact?”

I presume y’all’re talking about transfer of textile from the metals as they make contact, if they scraped against each other or something similar that. Certainly, you lot look at light microscopy first, that’s usually always the offset thing that we would practice, and then you can put it into—if y’all tin—you can get it into an scanning electron microscope, and get a trivial more detailed information about it, and possibly elemental analysis of material that has been transferred from one to the other to endeavour to verify if that fabric was consistent with that metallic having touched it.

“Could image software like Photoshop exist used to identify percent carbon in steel, ultrahigh carbon steel?”

No, not that I’m aware of. To place the percent carbon in steel, that more often than not has to be done past other instrumental methods, non by light microscopy.

Someone else has a BX51. “Are there NDIC accessories available and compatible for it?”

At that place certainly are, because I have them on mine. And so, yeah, those accessories are certainly available. McCrone Microscopes & Accessories would sell that type of thing for that microscope and also for the Nikon microscopes.

I think that’south nearly it. I think y’all got through all of the questions, Wayne, lots of good questions out there. Definitely go to our McCrone Microscopes & Accessories webpage or contact ane of our sales representatives. They would be happy to assistance y’all out with some of the accessories that Wayne was talking about. We hope to see you, and then. Thanks.

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Which Microscope is Often Used to View Metal Surfaces

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