Analytical Methods for Materials - Weaver Research .Analytical Methods for Materials Lesson 5

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  • Analytical Methods for Materials

    Lesson 5The Optical Microscope and Imaging Modes

    Suggested ReadingY. Leng, Materials Characterization, 2nd Edition, (2013), Wiley, Hoboken, NJ Chapter 1.

    ReferenceGoodhew, Humphreys and Beanland, Chapter 1Brandon and Kaplan, Chapter 3, pp. 123-177K. Geels, D.B. Fowler, W-U. Kopp, and M. Rckert, Metallographic and Materialographic Specimen Preparation, Light Microscopy, Image Analysis and Hardness Testing, (2007) ASTM International, West Conshohocken, PA.G.F. Vander Voort, Metallography Principles and Practice, (1999) ASM International, Materials Park, OH. 107

  • Introduction Differences in properties of light reflected from a

    specimen allow us to identify microstructural features.

    Light waves change amplitude or phase when they interact with matter.

    108

    From Leng, p. 25

  • Introduction

    The eye can only distinguish differences in amplitude and wavelength. Bright-field imaging Dark-field imaging

    Phase differences must be converted into amplitude differences. Phase contrast imaging Polarized light imaging Nomarski contrast

    109

  • Imaging Modes Bright-field (BF) and dark-field (DF) are the most

    commonly used examination modes.

    110

    BF

    DF

    Figures from Leng, pages 26,27.

  • 111

    Examination ModesBright Field

    Surfaces of the sample that are perpendicular to the incident light appear bright.

    Surfaces oblique to the incident light reflect less light back into the objective and appear darker.

    The natural colorsof materials can also be observed in this mode.

    Brandon & Kaplan

  • 112

    Examination ModesDark Field

    Figure 1.29 Dark-field illumination in a reflected light microscope (from Leng, page 27).

    Dark field microscopy is usually used to examine things such as cracks, pores, voids, and inclusions.

    It works opposite of bright-field microscopy in that no light that is directly reflected from the sample contributes to the image.

    Only the light rays that are deflected by diffusescattering go back intothe objective lens for image formation.

  • 113

    Examination ModesPhase Contrast

    For specimens with little inherent contrast in bright-field (e.g., polymers).

    Phase changes caused by light diffraction by an object is converted into an amplitude change.

  • Polarized light waves are light waves in which the vibrations occur in a single plane.

    It is used to examine materials that are optically anisotropic.

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    Examination ModesPolarized Light

    http://en.wikipedia.org/wiki/Polarizer

  • 115

    Examination ModesPolarized Light

    Orient polarizer and analyzer crystals in crossed positions.

    Often used to image anodized aluminum.

  • 116

    Examination ModesDifferential Interference Contrast (DIC)

    Also known as Nomarski microscopy.

    Produces images with emphasized topographic details.

    DIC requires several optical components, therefore it can be very expensive to set up. Figure 2 Prior austenite grain boundaries revealed by thermal etching after heating to 1000C for 600 s and cooled at a rate of 5 C/s; (a), (c) are

    brightfield optical images, while (b), (d) are the same images after using Nomarski microscopy.

    Figure from D. San Martin, Y. Palizdar, R.C. Cochrane, R. Brydson, and A.J. Scott, Materials Characterization, v. 61, n. 5 (2010) pp. 584-588.

    THIS PAPER IS WORTH READING!!!

  • 117

    Examination ModesDifferential Interference Contrast (DIC)

    First, light is passed through a polarizer.

    Second, it is passed through a prism that breaks the beam into two parts with a small phase difference between the two.

    Both beams then pass through the condenser and then the specimen.

    http://www.microscopyu.com/articles/dic/images/desenarmontintrofigure1.jpg

  • 118

    Examination ModesDifferential Interference Contrast (DIC)

    First, light is passed through a polarizer.

    Second, it is passed through a prism that breaks the beam into two parts with a small phase difference between the two.

    Both beams then pass through the condenser and then the specimen.

    http://www.microscopyu.com/articles/dic/images/desenarmontintrofigure1.jpg

  • 119

    http://upload.wikimedia.org/wikipedia/commons/4/46/DIC_Light_Path.png

    Please take a look at the Wikipedia page on DIC microscopy.

    http://en.wikipedia.org/wiki/Differential_interference_contrast_microscopy

    Light path through a DIC Microscope

    Transmission mode

  • 120

    Figure 1.41 (a) Optical arrangement of Nomarski microscopy in a reflected light microscope, (b) bright field image of a carbon steel, and (c) DIC image of the same region in (b). In (a), 1 = polarizer; 2 = /4-plate; 3 = DIC prism; 4 = objective lens; 5 = specimen; 6 = light reflector; 7 = analyzer.

    (a) (b)

    (c)

    Reflected light mode

    Images from Leng

  • 121

    Light path through a DIC Microscope

    http://www.microscopyu.com/articles/dic/reflecteddic.html

  • 122

    Comparison of Examination Modes

    ASM Handbook ,Vol. 9, Metallography and Microstructures, ASM International, Materials Park, OH (2004), pg. 340

    BF DF DIC

    BF DF DIC