Dislocations – Linear Defects –Two-dimensional or line defect –Line around which atoms are misaligned – related to slip Edge dislocation: –extra half-plane of atoms inserted in a crystal structure –Or – think of it as a partially slipped crystal –b  to dislocation line Screw dislocation: –spiral planar ramp resulting from shear deformation –b  to dislocation line Burger’s vector, b: measure of lattice distortion or the amount of displacement. Burger’s vector is equal in magnitude to interatomic spacing.

Edge Dislocation Source: G. Dieter, Mechanical Metallurgy, McGraw Hill, This is a crystal that is slipping Slip has occurred in the direction of slip vector over the area ABCD Boundary between portion that has slipped and not slipped is AD AD is the edge dislocation The Burger’s vector b is  = magnitude to the amount of slip  Is acting in the direction of slip  Note that b is ┴ dislocation line

Dislocations – Linear Defects Fig. 4.3, Callister 7e. Edge Dislocation

Dislocation motion requires the successive bumping of a half plane of atoms (from left to right here). Bonds across the slipping planes are broken and remade in succession. Atomic view of edge dislocation motion from left to right as a crystal is sheared. (Courtesy P.M. Anderson) Motion of Edge Dislocation

Dislocations – Linear Defects Adapted from Fig. 4.4, Callister 7e. Burgers vector b Dislocation line b (a) (b) Screw Dislocation

Edge, Screw, and Mixed Dislocations Adapted from Fig. 4.5, Callister 7e. Edge Screw Mixed

Dislocations – Linear Defects Dislocations are visible in electron micrographs Adapted from Fig. 4.6, Callister 7e. Transmission Electron Micrograph of Titanium Alloy. Dark lines are dislocations X

Interfacial - Planar Defects Surfaces –Atoms do not have the same coordination number –Therefore are in higher energy state –Surface energy,  [=] J/m 2 –Materials always try to reduce surface energy – tendency towards spherical shapes

Solidification - result of casting of molten material –2 steps Nuclei form Nuclei grow to form crystals – grain structure Start with a molten material – all liquid Grain Boundaries – Interfacial Defects Crystals grow until they meet each other nuclei crystals growing grain structure liquid

Grain Boundaries regions between crystals transition from lattice of one region to that of the other slightly disordered low density in grain boundaries –high mobility –high diffusivity –high chemical reactivity High energy locations where impurities tend to segregate to

Planar Defects in Solids One case is a twin boundary (plane) –Special kind of grain boundary –Mirror lattice symmetry –Essentially a reflection of atom positions across the twin plane. Stacking faults –For FCC metals an error in ABCABC packing sequence –Ex: ABCABABC Adapted from Fig. 4.9, Callister 7e. Brass at 60X Figure4.13c

Diffusion Diffusion - Mass transport by atomic motion Mechanisms Gases & Liquids – random (Brownian) motion Solids – vacancy diffusion or interstitial diffusion

Interdiffusion: In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. Initially Adapted from Figs. 5.1 and 5.2, Callister 7e. Diffusion After some time

Self-diffusion: In an elemental solid, atoms also migrate. Label some atoms After some time Diffusion A B C D

Diffusion Mechanisms Vacancy Diffusion: atoms exchange with vacancies applies to substitutional impurity atoms rate depends on: --number of vacancies --activation energy to exchange. increasing elapsed time

Simulation of interdiffusion across an interface: Rate of substitutional diffusion depends on: --vacancy concentration --frequency of jumping. (Courtesy P.M. Anderson) Diffusion Simulation

Diffusion Mechanisms Interstitial diffusion – smaller atoms can diffuse between atoms in lattice positions. Which will be faster – vacancy diffusion or interstitial diffusion? Adapted from Fig. 5.3 (b), Callister 7e.

Case Hardening: Diffuse carbon atoms into the host iron atoms at the surface. Use a controlled atmosphere with a specific carbon potential (effective concentration) Elevated Temperature Example of interstitial diffusion is a case hardened gear. Result: The higher concentration of C atoms near the surface increases the local hardness of steel. Processing Using Diffusion