Chapter 4 - Imperfections in Solids Flashcards
interfacial defects definition
boundaries that have two dimensions and normally separate regions of the materials that have different crystal structures or crystallographic orientations
interfacial defects (5)
- external surfaces
- grain boundaries
- phase boundaries
- twin boundaries
- stacking faults
external surface
the boundary along which the crystal structure terminates, higher energy state which gives rise to surface energy
grain boundaries
the boundary separating two small grains or crystals having different crystallographic orientations in polycrystalline materials, magnitude of energy is a function of the degree of misalignment (large angle being greater energy), more chemically reactive than the grains, density remains the same
tilt boundary
small angle grain boundary, when a plane of atoms is inserted
twist boundary
small angle grain boundary, when the angle of disorientation is parallel to the boundary
phase boundaries
exist in multiphase materials where a different phase exists on each side of the boundary
twin boundaries
type of grain boundary where the lattice structure is mirrored across the boundary, region of materials between the boundaries is called a twin, result from applied mechanical shear forces (BCC, HCP) or during annealing heat treatments following deformation (FCC)
catalyst
a substance that speeds up the rate of a chemical reaction without participating in the reaction itself (it is not consumed)
adsorption
the adhesion of molecules of a gas or liquid to a solid surface
absorption
the assimilation of molecules into a solid or liquid
stacking faults
found in FCC metals where the ABCABC… stacking sequence is interrupted
atomic vibrations
the fact that every atom is vibrating very rapidly around its lattice position, can be thought of as an imperfection or defect
microscopy
the use of optical, electron and scanning probe microscopes to investigate the microstructure features of all material types, can take pictures on a photomicrograph
microscopic techniques (3)
- optical microscopy
- electron microscopy
- scanning probe microscopy
optical microscopy
light microscope is used to study the microstructure, basic elements are optical and illumination systems, upper limit approximately 2000x magnification
metallographic
investigations using optical microscopy where a light microscope is used in a reflecting mode and contrasts are visible due to the differences in reflectivity
etching
procedure where the microstructure is revealed by using a surface treatment of an appropriate chemical reagent
electron microscopy
capable of much higher magnifications, formed using beams of electrons, transmission and scanning
transmission electron microscope (TEM)
formed by an electron beam that passes through the specimen, details of internal microstructure features are observable, differences in beam scattering or diffraction create contrasts, material must be very thin, 10^5x magnification
scanning electron microscope (SEM)
surface of a structure is scanned with an electron beam and reflected, image represents the surface features of the specimen, surface must be electrically conductive, 10-50,000x magnification
scanning probe microscope (SPM)
neither light nor electrons are used to form an image, microscope generates a 3D topographical map on an atomic scale representing the surface features and characteristics of the specimen, 10^9x magnification
grain size determination
determined when the properties of a polycrystalline material is under consideration, average grain volume, diameter or area, photographed at a magnification of 100x,
crystalline defect
a lattice irregularity having one or more of its dimensions on the order of an atomic diameter
point defect
defects associated with one or two atomic positions
- vacancy
- self-interstitial
vacancy
a lattice site that is normally occupied from which an atom is missing, all crystalline solids have vacancies
self-interstitial
an atom from the crystal that is crowded into a space that is not normally occupied
alloy
metals into which immunity atoms have been added intentionally to impart specific characteristics to the material
solid solution
formed when as impurity atoms are added to a metal the crystal structure is maintained, compositionally homogeneous
solvent
the element or compound that is present in the greatest amount, also called host atoms
solute
an element or compound present in a minor concentration
substitutional point defect
solute or impurity atoms replace or substitute for the host atoms, depends on Atomic Size Factor, Crystal Structure, Electronegativity, and Valences
interstitial point defect
impurity atoms fill the voids among host atoms
composition or concentration
weight percent or atom percent of constituent elements
weight percent
the weight of a particular element relative to the total alloy weight
atom perecent
the number of moles of an element in relation to the total moles of elements in the alloy
dislocation
linear or one-dimensional defect
edge dislocation
when an extra half plane of atoms is inserted in, linear defect that centers on the line that is defined along the end of the extra half-plane of atoms, edge dislocation is perpendicular to the plane of the page, burgers vector perpendicular to the dislocation line
screw dislocation
shear stress is applied to produce a shift in atomic distances, can make a spiral or helical path, burgers vector is parallel to the dislocation line
burgers vector
the magnitude and direction of the lattice distortion associated with a dislocation
