Chapter 4

Question 1

solidification is ?

Answer 1

resulting of casting of molten material

Question 2

what are the two steps of solidification?

Answer 2

nuclei form

nuclei grow to form crystals-grain structure

Question 3

grains can be ______ or __________

Answer 3

equiaxed (roughly the same size in all directions)
- can result from rapid cooling

columnar (elongated grains)
- can result from less cooling or slow cooling

Question 4

may of the important properties of the materials are due to the presence of ___________

Answer 4

imperfections

Question 5

what are the 3 general general categories for types of imperfections and what are the sub categories?

Answer 5

point defects=vacancy atoms, interstitial atoms, substitutional atoms

line defects-dislocations

area defects-grain boundaries

Question 6

describe more about point defects?

Answer 6

vacancies
-vacant atomic sites in a structure

self interstitials
-extra atoms positioned between atomic sites

Question 7

equlibrium concentration varies with _______

Answer 7

temperature

Question 8

each lattice site is a potential _______- site

Answer 8

vacancy site

Question 9

why can the surface island of atoms grow as T grows?

Answer 9

this is because the equil. vacancy conc. increases via atom motion from the crystal to the surface, where they join the island

Question 10

what are the two outcomes if impurities B are added to host A:

Answer 10

substitution or interstitial

Question 11

what are the conditinos for substitutional solid solution (S.S). aka W. Hume- Rothery rule

Answer 11

1. atomic radius <15%
2. proximity in periodic table, similar electronegativities
3. same crystal structure for pure metals
4. valency

-all else being equal a metal will have a greater tendency to dissolve a metal of higher valency than one of lower valency

Question 12

dislocaitons:

Answer 12

• are line defects,
• slip between crystal plane result when dislocations move
• produce permanent (plastic)deformation

Question 13

linear defects (dislocations) are defined as

Answer 13

are one dimensional defects around which atoms are misalign

Question 14

define edge dislocation:

Answer 14

-extra half-plane of atoms inserted in a crystal structure. burgers vector is perpendicular to dislocation line.. ads a plane of atoms.

Question 15

define screw dislocation:

Answer 15

-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.

Question 16

what does the burgers vector do?

Answer 16

burger’s vector is a measure of lattice distortion

Question 17

motion of edge dislocation information?

Answer 17

-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

Question 18

dislocation visible in _________________

Answer 18

electron micrographs

Question 19

grain boundaries: describe?

Answer 19

• regions between crystals
• transition from lattice of one region to that of other
• slightly disordered
• causes LOW DENSITY IN GRAIN BOUNDARIES

Question 20

low density in grain boundaries assists in ?

Answer 20

• high mobility
• high diffusivity
• high chemical reactivity

Question 21

what is 2 example of a planar defect in solids?

Answer 21

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

Question 22

_______,_________,_____ defects exist in solids

Answer 22

point, line, planar defects

Question 23

the number and type of defects can be varied and controls (T controls ________)

Answer 23

T controls vacancy conc.

Question 24

defects affect material properties (grain boundaries control ___________)

Answer 24

control crystal slip

Question 25

are defects desirable or undesirable

Answer 25

they can be either