Metallography and Sample Preparation

Metallography and Sample Preparation

• Metallography is the study of the microstructure of metals and their alloys
• It's a branch of materials science, relating the constitution and structure of metals and alloys to their properties
• It involves observing and determining the chemical and atomic structure, and spatial distribution of the constituents, inclusions or phases in metal alloys

What is Microstructure?

• Microstructure is the structure of a prepared specimen revealed by a microscope
• Dimensions of structural features range from subatomic particles to macrostructural features like porosity

Sample Preparation

• Preparing metallographic specimens requires five major operations:
  • Sectioning
  • Mounting
  • Grinding
  • Polishing
  • Etching
• A well-prepared specimen:
  • Represents the sample accurately
  • Has no surface damage (disturbed or flowed metal) caused by mechanical deformation
  • Is free from scratches and pits from polishing
  • Is flat enough to examine at high magnification
• Sectioning:
  • Important for process control and investigating defects in finished/partially finished products or parts that have failed in service
  • Can require more than one specimen or sectioning operation
  • Failed parts may require specimens at the failure origin and surrounding areas
• Sectioning methods include: crushing, cutting, sawing, and abrasive cutting
• When cutting samples, preserve the axes orientation (rolling, transverse, normal) to avoid erroneous results

Mounting of Specimens

• Mounting is for handling convenience and protecting/preserving specimen edges and defects
• Specimens may require mounting for use with automatic devices or microscope stages
• Mounting also facilitates specimen identification by marking the mount
• Mounting often uses a cold curing resin or hot mounting compound for small specimens

Mounting Methods

• Mounting should not injure the specimen's microstructure
• Mechanical deformation and heat are potential sources of damage
• The mounting medium should be compatible with the specimen in terms of hardness and abrasion resistance and chemically resistant to the polishing and etching solutions

Clamp Mounting

• Clamps are commonly used for mounting thin metal sheets
• Several specimens can be clamped in a sandwich form
• The clamp hardness should be similar to or greater than the specimen material

Compression (Hot) Mounting

• Involves molding around the specimen using heat and pressure
• Materials include bakelite, diallyl phthalate resins, and acrylic resins
• Thermosetting resins require heat during the molding process, while thermoplastic resins also require pressure

Cold Mounting

• Uses materials including: polyesters, epoxides, and acrylics
• Compared to hot mounting, cold mounting is faster for large numbers of specimens

Grinding

• Grinding is an essential stage in specimen preparation
• It uses abrasive grit to progressively reduce surface deformation
• Grit size ranges from 40 to 600 mesh
• Grinding direction should be changed to ensure complete removal of deformed metal

Grinding (continued)

• Grind to lessen the depth of deformation to a point that polishing steps can remove remaining damage.
• Grind with different abrasive grit sizes
• Microscopic examination can aid in evaluating the grinding sequence and ensure the scratch size uniformity

Most Grinding

• The specimen is manually held against the abrasive
• Equal pressure is applied on both sides to prevent a convex surface
• Clean the specimen after grinding to remove any abrasive particles

Grinding (continued)

• Abrasive types include silicon carbide, aluminum oxide, emery and diamond particles.
• These abrasives are typically bonded onto paper or cloth backing material

Polishing

• Polishing is the final specimen preparation step, aiming for a flat, scratch-free, and mirror-like surface
• The surface should be of at least the same quality as obtained by grinding using a 1200-grit abrasive
• Polishing also prepares the surface for subsequent metallographic (microscopic) examination

Etching

• Etching enhances microstructure (grain size, phase features) by selectively altering features based on composition, stress, or crystal structures
• Etching is necessary to visualize structures that have little or no difference in reflectivity (especially microstructural features with weak color or similar hardness differences)
• Common etching is through selective chemical etching using different formulas

Specimen Storage

• Polished and etched specimens need protection from atmospheric corrosion
• Desiccators and vacuum desiccators provide this protection
• Stored specimens maintain their condition

Materials Characterization

• Used to examine material structure and properties (Methods include: SEM, AFM, Microscopy, mechanical measurements, XRD, XPS, etc.)

Optical Microscopy

• Microstructure is directly related to the material's physical, chemical, and mechanical properties
• Diverse variations of optical microscopy are used in material characterization for assessing the structural state of a material

Scanning Electron Microscopy (SEM)

• Images the sample surface by scanning it with a high-energy beam of electrons
• Information about sample surface topography, composition, and properties (like electrical conductivity) are obtained through interacting with atoms on the sample

SEM - X-ray Analysis (EDS)

• Qualitative and quantitative elemental analysis using x-rays
• Analyzes the surface and near-surface region

Scanning Tunneling Microscopy (STM)

• Atomic-level imaging using a tunneling current between a tip and a sample
• Produces high-resolution images of the surface
• Used in various environments (vacuum, air, liquid)

Atomic Force Microscopy (AFM)

• Topographical imaging of surfaces, semiconductors, conductors and insulators
• Creates images using a nano-scale tip to probe the surface

X-ray Photoelectron Spectroscopy (XPS)

• Quantitative spectroscopic analysis of elemental composition and chemical and electronic states
• Uses XPS spectra, obtained by irradiating the sample with x-rays while measuring the electrons that escape

X-ray Diffraction (XRD)

• Uses x-rays to determine the crystalline structure of materials
• Often used for powdered or bulk samples
• Bragg's Law is a key equation to understand how it works.