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Characterizing Iron Based Historical Specimens – Buehler

Roman Nail, Broken below Head, Found in Vicenza, Italy – Montage of the broken Roman nail (left, Klemm’s I) and view of a typical large slag stringer (2% nital).

Patches of as-quenched martensite were found in the carburized nail head. Picral will not etch as-quenched martensite, but it will etch tempered martensite.
Patches of as-quenched martensite within the pearlitic carburized head revealed using 10% sodium metabisulfite.
Most of the head, but not in the center of the top face, was carburized. Etched with Beraha’s sulfamic acid 3/1 reagent (left) and Klemm’s I (right) and imaged with polarized light and sensitive tint.
Montage of the microstructure starting from under the head at the left corner (C –carburized) and going inward at the top of the shaft (AF –acicular ferrite; S –slag; EF –equiaxed ferrite). Klemm’s I, polarized light and sensitive tint used to image the structure.
Microstructure in the shaft (left, center and right side, respectively) revealing numerous mechanical twins (Neumann Bands) and slag stringers in a ferritic matrix. Beraha’s sulfamic acid (3/1) solution used with polarized light and sensitive tint to image the microstructure.
Higher magnification view of the mechanical twins in the ferrite in the shaft above the break (Beraha’s 3/1 sulfamic acid reagent, XP+ST).

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Characterizing Iron Based Historical Specimens – Struers

George’s Rules for Pain-free Specimen Preparation

  • Use the gentlest possible sectioning equipment: abrasive cut off saw or precision saw only
  • Use blades developed for metallography, not for production cutting
  • Avoid shrinkage gaps when mounting
  • Start grinding with the finest possible abrasive
  • Keep the polishing surface uniformly covered with abrasive and lubricant
  • Use proper loads

Revealing the Microstructure

  • Nital and picralare good to assess the general structure
  • Selective etchants are excellent for phase identification and measurements, e.g., alkaline sodium picrate to color cementite
  • Prior-austenite grain boundaries in martensite can be revealed using aqueous saturated picric acid plus HCland a wetting agent at 80-90°C
  • Color “tint” etchants reveal details about grain orientation, chemical segregation and residual deformation and are phase specific
  • Choose the best illumination mode, e.g., dark field to reveal grain boundaries, polarized light for inclusions, and NomarskiDIC to reveal height differences between constituents that polish at different rates.

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Influence of Specimen Preparation on SEM Images and Comparison to LOM Images

Influence of Specimen Preparation on SEM Images and Comparison to LOM Images

SEM and LOM are sometimes viewed as competitive methods; but, they are actually complementary. Some people believe that specimen preparation is less critical for SEM work than for LOM work; but, the reverse is true. Image forming mechanisms for the LOM and the SEM are different – we should make the most of these differences. The LOM is a more efficient tool for fast, low magnification (≤ 2000X) examination of a specimen – use it first to determine if, and where, one should follow with higher magnification SEM examination and EDS or EBSD work.

Prepare every specimen for SEM examination with the quality required for EBSD work; this is actually quite easy with the right equipment, consumables and methods. Examples of preparation methods for many metals/alloys and other materials can be found in Struers publications and web site. Examples of LOM and SEM images of specimens that were properly prepared or poorly prepared will be shown.

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Etching to Reveal Microstructure

Guidelines

  • Low magnification evaluation; deeper etch
  • Time is less reliable than appearance
  • Best results occur when etching right after polishing, time can create a passive surface
  • High magnification evaluation; shallow etch
  • If you are not satisfied, an etch-repolish-etch sequence may improve the results

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Equilibrium Phase Diagrams

Hexagonal-Close Packed Crystal Lattice

Each of the interior atoms has a slice in an adjoining cell. In turn, there are three slices within this cell. Thus the three whole atoms are contributed to the cell. The two atoms centered on the top and bottom faces are each half within the cell. These two halves combined contribute one whole atom. Each of the twelve corners has one-sixth of an atom. Thus all twelve corner atoms combined contribute two whole atoms.

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Deformation Determined by Electron Backscatter Diffraction (EBSD)

Towards A More Quantitative Measurement of the Deformation During Metallographic Specimen Preparation Using EBSD and FIB

By Philippe T. Pinard, Pierre Hovington, Marin Lagacé, George F. Vander Voort, Raynald Gauvin

  • Materials and Mining Engineering Department, McGill University
  • Institut de recherche d’Hydro-Québec, Varennes, Québec
  • Consultant, Struers Inc., Westlake, Ohio, USA

Surface deformation during metallographic preparation have been previously studied using light optical microscopy (LOM) and transmission electron microscopy (TEM) [1]. With its submicron resolution, electron backscattered diffraction (EBSD) can provide quantitative deformation analysis at a smaller length scale than LOM while provide higher statistics than TEM. This work aims to determine the level of deformation produced during different metallographic preparation steps of common materials. As a first iteration, the deformation profile induced by 80, 240 and 600 ANSI grit SiC papers on commercially pure iron (BCC), copper (FCC) and titanium (HCP) was measured.

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Ceramic Preparation

New technologies have improved the speed and accuracy of specimen preparation for materials such as ceramics, cermets, nitrides, borides and sintered carbides.

Microscopic examination is an important inspection technique widely used in the research, quality control and failure analysis of very hard materials. To properly reveal the microstructure of these very hard materials, it is critical to use the proper equipment (usually automated), along with suitable high-quality consumable products, such as cutting blades, mounting compounds, abrasives, lubricants and working surfaces with proven preparation methods.

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Advances in Metallography

Sectioning of Specimens

“Chop” style cuttershave variable pressure over the cut which degrades the microstructure. Minimum area of contact cutting (MACC) maintains pressure more constantly yielding less damage.

Chop vs. Orbital Cutting

With the chop cutter the contact area, when cutting a round bar,is very small initially, increases to a maximum, then decreases to a point contact. Thus, the pressure varies through the cut. This reduces cutting efficiencyand increases heat generation. Orbital cutting reduces this variation. When the wheel lifts out of the cut, coolant flows into the cut which reduces sectioning damage.

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A Layman’s View of Plasma Spray Coating Metallography

12_wsThe paper describes the approach taken in the author’s first metallographical study of plasma sprayed coating. After consideration of the preparation problems known to be associated with porous materials, the coating materials and the substrate material, and coatings in general, literature on metallographic studies of plasma sprayed coatings was identified, obtained and reviewed.

A coating section of each type was carefully removed from the substrate, refrigerated in liquid nitrogen, broken and examined with the SEM to reveal the nature and degree of porosity and bonding defects. Specimens were prepared using two different approaches and two variations of one of the approaches. Polishing was conducted using DP-Plan cloths and two different diamond sizes followed by two stages of diamond polishing with conventional cloths and finally using colloidal silica on a vibratory polisher. Vibratory polishing time must be critically controlled for two of the three coatings. Conventional etchants were tried to enhance microstructural detail but the Pepperhoff interference layer method was found to be more useful. By George Vander Voort