Metallography: Structure & Properties

• Metallography is a tool for understanding the structure and properties of metals and alloys
• It involves microscopic examination of materials
• It reveals internal organization and provides insights into performance

Metallographic Microscopy

• A crucial technique used to analyze the microstructure of materials
• Provides insights into properties and performance
• Involves preparing a sample, observing it under a microscope, and interpreting observed features

Macrostructure

• Refers to features visible to the naked eye
• Examples include grain boundaries, inclusions, and pores

Mesostructure

• Refers to features at an intermediate scale, between macro and micro
• Examples include the arrangement of grains in a polycrystalline material

Microstructure

• Refers to features only visible under a microscope
• Examples include grain size, phase distribution, and precipitates

Solidification

• The transformation of a liquid metal into a solid

Diffusion

• The movement of atoms within a material
• Driven by concentration gradients

Phase Transformations

• Changes in the microstructure of a material
• Caused by heat treatment or other processes

Macroscopic Observations: Segregations

• Uneven distribution of elements in a metal

Macroscopic Observations: Inclusions

• Foreign particles trapped in a metal during solidification

Macroscopic Observations: Thermochemical Treatments

• Heat treatments that alter the microstructure and properties

Macroscopic Observations: Manufacturing Defects

• Flaws that arise during processing
• Examples include pores and shrinkage cavities

Microscopic Observations: Grain Size

• The average size of the grains in a metal

Microscopic Observations: Phase Distribution

• The arrangement of different phases in a metal

Microscopic Observations: Precipitates

• Small particles that form within a metal during heat treatment

Microscopic Observations: Microcracks

• Tiny fractures that can initiate failure

Metallographic Analysis Procedure: Sample Preparation

• Involves cutting, mounting, grinding, polishing, and etching

Metallographic Analysis Procedure: Microscopic Examination

• Use an optical or electron microscope to observe the microstructure

Metallographic Analysis Procedure: Image Analysis

• Analyze images to measure grain size, phase distribution, and other features

Metallographic Analysis Procedure: Interpretation

• Relate the observed microstructure to the properties and behavior of the material

Metallographic Microscope Usage: Sample Preparation

• Prepare a representative sample for observation
• Involves cutting, mounting, grinding, polishing, and etching the material to reveal its microstructure

Metallographic Microscope Usage: Microscopic Observation

• Place the prepared sample under the microscope and illuminate it with various lighting techniques
• Including bright field, dark field, polarized light, or Nomarski interference contrast

Metallographic Microscope Usage: Image Analysis

• Analyze the observed image to identify the different phases present in the material, their distribution, size, shape, and orientation
• This provides information about the material's properties, processing history, and potential for failure

Sample Selection Considerations: Material Type

• Select a sample representative of the material you want to analyze
• Properties, composition, and processing history significantly impact its microstructure

Sample Selection Considerations: Location

• Choose the sample's location carefully, especially for components or structures
• Different parts may exhibit different microstructures due to varying heat treatments or processing

Sample Selection Considerations: Processing History

• Consider the material's processing history
• Including any heat treatments, forming operations, or welding
• Significantly affects the final microstructure and properties

Metallographic Sample Preparation: Cutting

• Remove a small piece of the material for examination

Metallographic Sample Preparation: Mounting

• Secure the sample in a holder for easier handling and grinding

Metallographic Sample Preparation: Grinding

• Remove surface irregularities with progressively finer abrasive papers

Metallographic Sample Preparation: Polishing

• Achieve a smooth, mirror-like surface with polishing cloths and abrasives

Metallographic Sample Preparation: Etching

• Reveal the microstructure by selectively dissolving certain phases

Metallographic Specimen Preparation: Sample Selection

• Choosing a representative sample is critical for accurate analysis
• Consider material type, location, and processing history

Metallographic Specimen Preparation: Metallurgical Cutting

• Cut the sample to a desired size and shape using a cutting machine or saw This ensures the sample is suitable for mounting and subsequent preparation steps

Metallographic Specimen Preparation: Mounting

• Embed the sample in a mounting medium like epoxy resin or acrylic
• This provides a stable grinding and polishing platform

Cutting and Mounting Methods: Cutting

• Use a cutting machine or saw to cut the sample to a suitable size and shape
• Perform cutting carefully to avoid introducing defects

Cutting and Mounting Methods: Mounting

• Embed the sample in a mounting medium
• This provides a stable platform for grinding and polishing
• Choose mounting material compatible with the sample and the intended etching procedure

Cutting and Mounting Methods: Cold Mounting

• Embed the sample in a cold mounting medium, like epoxy resin, using a press to ensure uniform embedding

Cutting and Mounting Methods: Hot Mounting

• Utilize a hot mounting medium, like acrylic melted around the sample to provide a secure bond
• Hot mounting is often preferred for larger or more complex samples

Metallographic Specimen Preparation: Grinding and Polishing

• Grind and polish the mounted sample using a series of abrasive papers and polishing cloths
• Achieve a smooth, flat surface suitable for observation
• This removes surface imperfections and reveals the true microstructure

Metallographic Specimen Preparation: Etching

• Final step selectively attacks the surface of the material
• This process reveals the microstructure by highlighting different phases and grain boundaries
• Helps distinguish between phases and provides information about the material's composition and properties

Grinding

• Use a series of progressively finer abrasive papers to remove surface imperfections
• Prepares the sample for polishing

Polishing

• Use a polishing cloth with a polishing compound
• Achieve a smooth, mirror-like surface
• This removes any scratches or imperfections left by the grinding process

Etching Techniques: Chemical Etching

• Immerse the polished sample in an etching reagent for a specific time period
• The reagent selectively attacks the surface
• This reveals different phases and grain boundaries

Etching Techniques: Electrolytic Etching

• Use electrical current to etch the sample's surface
• The etching process is controlled by the applied current and the electrolyte used
• Electrolytic etching is suitable for materials difficult to etch chemically

Chemical Etching Reagents: Nital

• Consists of nitric acid in ethanol
• Is used for etching steels and other ferrous alloys

Chemical Etching Reagents: Picric Acid:

• Consists of 2,4,6-trinitrophenol in water
• Used for etching aluminum and copper alloys

Chemical Etching Reagents: Kellers Reagent

• Consists of hydrochloric acid, nitric acid, and ferric chloride in water
• Used for etching stainless steels

Chemical Etching Reagents: Fry's Reagent

• Consists of nitric acid, hydrochloric acid, and ferric chloride in water
• Used for etching cast irons

Chemical Etching Reagents: Others

• Ammonium hydroxide and hydrogen peroxide is generally used for copper and many of its alloys
• Ammonium persulfate consists of ammonium persulfate and water and is a general reagent for copper, brass, bronze, silver, nickel and aluminum
• Palmerton reagent consists of chromic oxide, sodium sulfate and water, and is a general reagent for zinc and its alloys
• Ammonium molybdate consists of molybdic acid, ammonium hydroxide and water used for fast attack for lead and its alloys
• Hydrofluoric acid consists of hydrofluoric acid and water and is used for microscopic examination for aluminum and its alloys

Microscopic Observations: Bright Field

• The most common illumination technique
• Light is directly transmitted through the sample
• Provides good overall contrast and detail

Microscopic Observations: Dark Field

• Light is directed at an angle to the sample, illuminating only scattered light
• Highlights features like boundaries and inclusions

Microscopic Observations: Polarized Light:

• Utilizes polarized light to enhance contrast and reveal anisotropic features
• Examples include crystalline structures and stress patterns

Microscopic Observations: Nomarski

• Utilizes interference contrast to create a three-dimensional effect
• Highlights surface features and depth variations

Microstructure Analysis in Steel: Ferrite

• A body-centered cubic phase, soft and ductile
• Responsible for steel's toughness

Microstructure Analysis in Steel: Pearlite

• A lamellar structure consisting of alternating layers of ferrite and cementite
• Provides strength and hardness

Microstructure Analysis in Steel: Cementite

• A hard and brittle phase, iron carbide
• Contributes to steel's wear resistance

Microstructural Analysis via Metallography

• Provides insight into the internal structure of materials
• Reveals microstructural properties and characteristics

Grain Structure: Equiaxed Polyhedra

• Grains have roughly equal dimensions in all directions
• Forms a compact, interlocking structure

Grain Structure: Dendrites

• Exhibit a branched, tree-like morphology
• Often forming during rapid solidification processes