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Different energy requirement at different temperature

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Different energy requirement at different temperature ± Iron: • BCC structure below 912 o C and above 3194 o C • FCC structure in between ¾ Allotropism or polymorphism (many shapes) = appearance of more than one crystal structure Page 1-8 Deformation and strength of a single crystal ¾ Elastic deformation : ± When a crystal goes back to its original shape after an external force is removed ¾ Plastic (permanent) deformation : ± Does not return to its original shape ¾ Mechanisms of Plastic Deformation ± Slipping of one plane of atoms over another ± Twinning Page 1-9 Slipping of atom planes ¾ First cause of plastic deformation ± Shear stress causes slipping ± Critical shear stress for plastic deformation • Proportional to b/a (atomic spacing in two planes) • Slipping takes place in tightly packed planes and in tightly packed directions. •A single crystal is anisotropic because b/a is different in different directions Page 1-10 Slip systems ¾ Related to the number of directions in which slipping can occur. ¾ If slip system is 5 or more Æ ductile ¾ Body-Centred-Cubic: ± 48 possible slip systems ± Good strength and moderate ductility ¾ Face-Centerd-Cubic: ± 12 slip systems ± Moderate strength and good ductility ¾ Hexagonal closed-packed ± 3 slip systems ± Brittile at room temperature Page 1-11 Twinning ¾ Second cause of plastic deformation: Twinning ¾ Crystal forms a mirror
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Different energy requirement at different temperature Iron...

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