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

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resourcesThe articles and presentations that can be down-loaded from this web site are based upon work done by GFV while employed at Bethlehem Steel (1967-1983), Carpenter Technology (1983-1996), Buehler Ltd. (1996-2009) and Struers (2009-Present) and from the authors consulting work for companies such as, Latrobe Steel, Scot Forge, etc., and from his litigation work. GFV's bylined articles appearing in various issues of the ASM Handbook series have been listed here courtesy of ASM International, Materials Park, Ohio.

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