MaterialDeformations.PDFVIP

Material Deformations Academic Resource Center Agenda • Origin of deformations • Deformations dislocations • Dislocation motion • Slip systems • Stresses involved with deformation • Deformation by twinning Origin of Deformations • Theoretical strengths of perfect crystal were much higher than those actually measured. • It was determined that this discrepancy in mechanical strength could be explained by dislocations. • On a macroscopic scale, plastic deformation corresponds to the net movement of large numbers of atoms in response to an applied stress. Edge and Screw Dislocations • In an edge dislocation, localized lattice distortion exists along the end of an extra half-plane of atoms. • A screw dislocation results from shear distortion. • Many dislocations in crystalline materials have both edge and screws components; these are mixed dislocations. Direction of Dislocation Motion Edge dislocation line moves parallel to applied stress Screw dislocation line moves perpendicular to applied stress 5 Dislocation Motion •Dislocation motion leads to plastic deformation. Dislocation Motion •An edge dislocation moves in response to a shear stress applied in a direction perpendicular to its line. •Extra half-plane at A is forced to the right; this pushes the top halves of planes B, C, D in the same direction. •By discrete steps, the extra 1/2-plane moves from L to R by successive breaking of bonds and shifting of upper 1/2-planes. •A step forms on the surface of the crystal as the extra 1/2-plane exits. Formation of a step on the surface of a crystal by the motion of (a) edge dislocation and (b) screw dislocation. Slip • The process by which plastic deformation is produced by dislocation motion is called slip (movement of dislocations). • The extra 1/2-plane moves along the sli