Chapter 4 Flashcards
solidification is ?
resulting of casting of molten material
what are the two steps of solidification?
nuclei form
nuclei grow to form crystals-grain structure
grains can be ______ or __________
equiaxed (roughly the same size in all directions)
- can result from rapid cooling
columnar (elongated grains)
- can result from less cooling or slow cooling
may of the important properties of the materials are due to the presence of ___________
imperfections
what are the 3 general general categories for types of imperfections and what are the sub categories?
point defects=vacancy atoms, interstitial atoms, substitutional atoms
line defects-dislocations
area defects-grain boundaries
describe more about point defects?
vacancies
-vacant atomic sites in a structure
self interstitials
-extra atoms positioned between atomic sites
equlibrium concentration varies with _______
temperature
each lattice site is a potential _______- site
vacancy site
why can the surface island of atoms grow as T grows?
this is because the equil. vacancy conc. increases via atom motion from the crystal to the surface, where they join the island
what are the two outcomes if impurities B are added to host A:
substitution or interstitial
what are the conditinos for substitutional solid solution (S.S). aka W. Hume- Rothery rule
- atomic radius <15%
- proximity in periodic table, similar electronegativities
- same crystal structure for pure metals
- valency
-all else being equal a metal will have a greater tendency to dissolve a metal of higher valency than one of lower valency
dislocaitons:
- are line defects,
- slip between crystal plane result when dislocations move
- produce permanent (plastic)deformation
linear defects (dislocations) are defined as
are one dimensional defects around which atoms are misalign
define edge dislocation:
-extra half-plane of atoms inserted in a crystal structure. burgers vector is perpendicular to dislocation line.. ads a plane of atoms.
define screw dislocation:
-spiral planar ramp resulting from shear deformation. burger vector is parralle to dislocation line. basically it moves or twists the plane and it does not add anything.
what does the burgers vector do?
burger’s vector is a measure of lattice distortion
motion of edge dislocation information?
-dislocation motion require the successive bumping of a half plane of atoms (from left to right here)
bond across the slipping planes are broken and remade in succession
dislocation visible in _________________
electron micrographs
grain boundaries: describe?
- regions between crystals
- transition from lattice of one region to that of other
- slightly disordered
- causes LOW DENSITY IN GRAIN BOUNDARIES
low density in grain boundaries assists in ?
- high mobility
- high diffusivity
- high chemical reactivity
what is 2 example of a planar defect in solids?
one case is the twin boundary (plane)
- this is essentially a reflection of atom positions across the twin plane
stacking faults:
-for FCC metals an error in ABCABC packing sequence
_______,_________,_____ defects exist in solids
point, line, planar defects
the number and type of defects can be varied and controls (T controls ________)
T controls vacancy conc.
defects affect material properties (grain boundaries control ___________)
control crystal slip
are defects desirable or undesirable
they can be either
