Techniques to Examine Crystal Defects

Microscopic Techniques

It is defined as, "techniques used to examine structural elements and defects in crystalline materials".

• Microstructure  ↠  combination of crystal structure + grain size + defects in crystal structure + phase + impurities.
• Microscopy  ↠  used for the investigation of microstructural features (instruments).

Applications of microscopy and micro-structure are:

• Ensure association between the properties, structures and defects.
• To predict property of material (after relationship created).
• To design alloys with new property combination.
• Check material has been properly heat treated.
• To study various mode of mechanical fracture.

Types of Microscopy 

1. Optical Microscopy

It is defined as, "microscopy in which visible light and system of lenses to magnify images of small samples". Upper limit magnification is 2000x.

It consists of two major systems, which are

1. Magnification System  ↠  have lenses to magnify.
2. Illuminating System  ↠  have light and camera for capturing images.

Metallographic investigation (optical microscopy of metals) consist of following steps: 

1. Sectioning  ↠  cutting of material to observe a structure of large crystal structure.
2. Grinding  ↠  to remove buds over the surfaces.
3. Polishing  ↠  to further smoothen the surface.
4. Etching  ↠  workpiece is dipped in Nitric Acid to highlight boundaries.  

2. Electron Microscopy

It is defined as, "microscopy in which image of investigated structure is formed using beams of electrons".

• Magnification power is smaller than that of optical microscopy.
• Image is formed with magnetic lenses.

Techniques of Microscopy

a. Scanning Electron Microscopy (SEM)

It uses fine beams of focused electrons to scan sample's surface and find elemental composition based on reflection of electron beams.

• Uses scattered electrons.
• Magnification power is 2 million times.
• Used for surfaces, powders, polished and etched mirostructures, IC Chips, chemical segregation.

b. Transmission Electron Microscopy (TEM) 

It uses a broad beam of electrons to create an image of a sample's internal structure, composition and crystal structure.

• Uses transmitted electrons.
• Magnification power is 10-50 million times.
• Create 2D and gives characteristics of sample like morphology, crystallization, stress, magnetic domains.

Grain Size Determination

It is determined when properties of a polycrystalline material are under considerations. Methods of grains size determination are

1. Intercept Method

• Take micrograph (image obtained from microscope). 
• Draw 7 random lines (have 5 cm length), see how much grains are cut by these line. 
• Find sum of cutted grains.

2. ASTM Method

• Comparison charts (1-10) having different average grain size called grain size number ASTM-112. 
• Simply compare these charts with your your micro-chart. 
• Draw 1x1 sq.inch on your graph.
• Calculate number of grains (N)

References:

• Material from Class Lectures + Book named Materials Science and Engineering: An Introduction by Callister and Rethwick + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube.