Q:
Why is Phase/Microstructure Analysis Important?





A:
  • These components of your metals/alloys/coatings, etc.
    control their mechanical/chemical properties and behavior.
    When you do surface modification and/or coating, you may
    affect the starting microstructure (in some cases this may
    be done intentionally).

  • Additionally, the interface between a coating and substrate
    may determine the success of the coating, or be the source
    of a failure, if not properly engineered and managed.

    Etchants for Aluminum Alloys.

  • Al alloys containing Cu, Mn, Si, Mg, Ti. Cast Al alloys
    with high Si content.
    Hydrochloric Acid (1.19 N) - 75 mil
    Nitric Acid (1.40 N) - 25 mil
    Hydrofluoric Acid (40%) - 25 mil
    (seconds to minutes exposure)

  • Al Alloys - most types, especially containing Cu
    Distilled water - 75 mil
    Nitric Acid (1.4 N) - 25 mil
    Heat to 70 degrees C (160 degrees F)
    40 seconds exposure

    Etchants for Steels and Cast Iron

  • Nital - most common etchant for steels and
    gray cast irons:


  • Ethanol or Methanol (95%) - 100 mil
    Nitric Acid (1.40 N) - 1 to 10 mil
    (but do not exceed 10 mil)
    Seconds to minutes exposure.

  • Murakami's Reagent - used to highlight
    carbides
    (in some alloys may be stained
    or colored
    ).

  • Distilled water - 100 mil
  • Potassium or Sodium Hydroxide - 10 g
  • Potassium Ferricyanide - 10 g
    (concentration is variable)
    2 to 20 minutes exposure 20-50 degrees C
    (70-120 degrees F) temperature solution
    (Use only fresh solution).

  • Examples of polish/etch interactions:
    Available in our Gallery, the micrographs show the
    importance of sample preparation/etchant effects,
    etched colored structures, and a combination of
    SEM/surface chemical analysis to identify phases.