Surface Imperfection/Interfacial Defects
Imperfections on crystal surface are surface imperfections. These imperfections have a thickness of a few atomic diameter. These are boundaries which have two dimensions and normally separate regions of materials possessing different crystal structure and/or crystallographic orientations.
These generally include
- Grain Boundaries
- Twin Boundaries/Twinning
- Stacking Faults
Grain Boundaries: Grain boundary is formed during solidification. A polycrystalline material is made up of many crystals called as crystallites or grains. In the boundary region between two adjacent grains, the atoms are irregularly arranged and this leads to lattice distortion. Grain boundaries are efined as the junction between two randomly growing dendrite. When this distortion is slight, of the order of a few degrees, then it is termed as small or low angle grain boundary and for large angle distortions it is termed as high angle grain boundary
Characteristics of Grain Boundary
• Grain boundaries are region of orientation mismatch.
• Grain boundaries are region of high potential energy.
• Grain boundaries are region of low melting point.
• Grain boundaries are region of heavy impurity concentration.
Transgranular fracture : When the fracture in material occurs through grains in small multi plane manner like in brittle material, known as transgranular fracture. This fracture occurs when temperature of material is less than its equi-cohesive temperature (Te).
Inter-granular fracture : When the fracture in material occurs through grain boundaries like in ductile material, the fracture is known as inter-granular fracture.
This fracture occurs when temperature of material is greater than its equi-cohesive temperature (Te).
Equi-cohesive temperature (Te) : The temperature at which both grains and grain boundaries have same strength is called equi-cohesive temperature. It is reference temperature for failure.
(ii) Twin Boundaries: When a poly-crystalline material is subjected to shear force, the atomic arrangement in the slipped and unslipped portion of the crystal can become mirror image of each other about two boundary known as twin boundaries.
Twins are produced by atomic displacement by mechanical shear force (mechanical twins found in BCC and HCP) and also during annealing heat treatment following deformation (annealing twin found in FCC).
Twin boundaries formation results in increase in net potential and hence non-uniform properties will be present in the lattice.
(iii) Stacking Faults: Stacking faults are the faults in stacking sequence. Stacking faults formation results in non-uniform properties and unstability in lattice due to increase in potential energy.
Stacking faults are found in FCC metals when there is an interruption in the ABCABCABC… stacking sequence of close-packed planes.
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