Effects of Imperfections on the Mechanical Properties

Mechanical properties (such as the strength, ductility, toughness, and ductile-brittle transition) depend upon the crystal structure and imperfections. Point defects influence electrical conductivity, mechanical strength, and diffusivity. Lattice, as well as bulk defects, are relevant in material processing involving solidification, deformation, and powder metallurgy.

For example, rimmed steel sheets are particularly susceptible to strain (within 30 days). This is due to the presence of dissolved carbon and nitrogen in steel. This is eliminated by vacuum degassing and in aluminium killed steels. Similarly, the strength of copper is increased by the addition of nickel owing to the formation of a solid solution.

The additional imperfection tends to hinder dislocation motion and consequently the strength increases. Also, hard ceramic particles of alumina (Al2O3) are dispersed in a soft, ductile matrix of aluminium. These particles resist the movement of dislocations in the aluminium matrix, and hence increase the strength of aluminium metal. This is called dispersion hardening. Sintered Aluminium Powder (SAP) is based on this advantage.

Hall-Petch equation is based on the dislocation theory that relates yield point stress ‘σyp‘ to the strain size ‘d’ as

Hall-Petch equation

Where ‘σo‘ is the strength of one crystal and ‘k’ is a constant. Thus, a solid with smaller grains is stronger due to the presence of more grain boundaries. Therefore, the Hall-Petch equation is also known as an expression of grain-boundary strengthening.

Large grains have lower free energy than small grains. Lowest energy state occurs in a single crystal (without grain boundaries). Whiskers are single crystals, which in some cases are made directly from vapor.

Parallel planes of high atomic density and corresponding large interplanar spacing exist in the crystal structure. Any movement in the crystal takes place either along these planes or parallel to them. Therefore, slip occurs in most closely packed directions since they require the least amount of energy. In FCC (e.g. aluminium, gold, copper), there are 12 possible slip directions, so easily deformed. In HCP (Hexagonal Close Packed), twinning occurs and in FCC(Face Centered Cubic), so slip occurs easily.

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