4-Crystal Defects & Strengthening

Transcription

1 4-Crystal Defects & Strengthening A perfect crystal, with every atom of the same type in the correct position, does not exist. The crystalline defects are not always bad! Adding alloying elements to a metal is one way of introducing a crystal defect. There are basic classes of crystal defects: Point Linear defects 4-1-Point Defects Area or Planar defects Volume or Bulk defects Point defects are lattice defects of zero dimensionality, Le., they do not possess lattice structure in any dimension. They are where an atom is missing or is in an irregular place in the lattice structure. Point defects include a) Vacancies b) Interstitials c) Substitutions Vacancies are common, especially at high temperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites. N v =N.exp(-Q/KT) Dr.Ali Salemi Golezani Page 1

2 4-2-Line Defects Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are out of position in the crystal structures. Dislocations are generated and move when a stress is applied. The motion of dislocations allows slip plastic deformation to occur. There are two basic types of dislocations, the edge dislocation and the screw dislocation. a) Edge The edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The dislocation is called a line defect because the locus of defective points produced in the lattice by the dislocation lie along a line. b) Screw There is a second basic type of dislocation, called screw dislocation. The screw dislocation is slightly more difficult to visualize. c) Mixed Dislocations Movement of Dislocation: The dislocations move along the densest planes of atoms in a material, because the stress needed to move the dislocation increases with the spacing between the planes. FCC and BCC metals have many dense planes, so dislocations move relatively easy and these materials have high ductility. 4-3-Area or Planar Defects Grain or Phase Boundaries in Polycrystals Solids generally consist of a number of crystallites or grains. Grains can range in size from nanometers to millimeters across Dr.Ali Salemi Golezani Page 2

3 and their orientations are usually rotated with respect to neighboring grains. Where one grain stops and another begins is known as a grain boundary. Grain boundaries limit the lengths and motions of dislocations. Stacking Faults A stacking fault is a one or two layer interruption in the stacking sequence of atom planes. Stacking faults occur in a number of crystal structures, but it is easiest to see how they occur in close packed structures. For example, it is know from a previous discussion that face centered cubic (fcc) structures differ from hexagonal close packed (hcp) structures only in their stacking order. In the fcc arrangement the pattern is ABCABCABC. A stacking fault in an fcc structure would appear as one of the C planes missing. In other words the pattern would become ABCABCAB_ABCABC. Twine boundaries 4-5-Strengthening Mechanisms in Metals 1) Solid Solution Strengthening (Alloying) Dr.Ali Salemi Golezani Page 3

4 2) Work Hardening (Strain- cold work) Commercially pure aluminium (1000 series) and the non-heat-treatable aluminium alloys (3000 and 5000 series) are usually work hardened. The work hardening superimpose on any solution hardening, to give considerable extra strength.work hardening is achieved by cold rolling. The yield strength increases with strain (reduction in thickness) according to σ y = A ε n, where A and n are constants. For aluminium alloys, n lies between 1/6 and 1/3. Dr.Ali Salemi Golezani Page 4

5 Note: Recovery and recrystallisation Effect Cold Work on Microstructure Cold work elongates grains in the direction of working Dr.Ali Salemi Golezani Page 5

6 Heat treatment can be used to remove the effects of strain hardening. Three things can occur during heat treatment: Recovery Recrystallization Grain growth 3) Grain Size Control (Grain refinement) 4) Dispersion Strengthening (Precipitati on hardening) Dr.Ali Salemi Golezani Page 6