Topics Up to Midterm

Question 1

metals

Answer 1

malleable (can be deformed permanently), metallic bonds

Question 2

ceramics

Answer 2

brittle - no permanent deformation and they crack instead, has poor strength in tension

Question 3

polymer

Answer 3

has permanent deformation

Question 4

stress

Answer 4

strength, force/area

Question 5

strain

Answer 5

ductility, change in length/initial length

Question 6

young’s modulus

Answer 6

measure of elasticity, structure independent. the higher, the stiffer. stress = youngs modulus x strain

Question 7

elastic strain

Answer 7

reversible/recoverable – atoms return to their original positions after unloading

Question 8

tensile test

Answer 8

used to determine material properties and mechanical behavior for metals and polymers. stress is highest in reduced sections because cross-sectional area is small

Question 9

why are tensile tests not used for ceramics

Answer 9

difficult w low strain (it will fracture), the shape of tensile test is difficult to make with ceramic, difficult to grip

Question 10

plastic deformation

Answer 10

permanent change in the shape or size of a material when subjected to a stress that exceeds its yield strength. occurs from the step-by-step movement of dislocations

Question 11

elastic deformation

Answer 11

where the material returns to its original shape upon the removal of the applied stress

Question 12

3 point bending

Answer 12

to test material properties and mechanical behavior for ceramics

Question 13

Crystal

Answer 13

solid materials whose atoms, ions, or molecules are arranged in a highly ordered, repeating three-dimensional pattern. This regular arrangement extends in all directions, creating a structure known as a crystal lattice

Question 14

tempered glass

Answer 14

toughened glass. it is heated and cooled to solidify the surface; the surface cools much faster than the core. residual stresses – tensile at core, compression at surface

Question 15

Face Centered Cubic (FCC)

Answer 15

most metals are cubic FCC
- diagonal 4r, r = radius
- 4 total atoms
- a = 2 x sqrt2 R
- CN = 12
- APF = 0.74
- stacking sequence = ABCABC
- 4 unique close packed planes

Question 16

Atomic Packing Factor (APF)

Answer 16

max fraction of a volume that can be filled with spheres, the volume of spheres/volume of the unit cell

Question 17

Rock salt structure

Answer 17

• number of anion (-ve) = 4
• number of cations (+ve) = 4
• cation coordination number = 6
• volume of cube = (2Ra+2Rc)^3

Question 18

Interstitial Sites

Answer 18

space between other atoms. in rock salt structure, it has an octahedron interstitial site. Rc/Ra = 0.414

Question 19

Body-Centered Cubic (BCC) Crystal Structure

Answer 19

metallic crystal structure, body-centered position, contact along the body diagonal
- CN = 8
- a = 4/sqrt3 x R
- APF = 0.68, thus not closed packed as that is less than FCC

Question 20

Hexagonal Closed Packed (HCP)

Answer 20

• ABAB stacking sequence of closed packed planes
• APF = 0.74
• tetrahedra
• CN = 12
• has 6 total number of atoms in unit cell

Question 21

proportional limit

Answer 21

end of the linear elastic region

Question 22

ultimate tensile strength

Answer 22

the highest value of stress/strain

Question 23

yield strength

Answer 23

where plastic deformation begins. yield strength and proportional limit are not the same. 0.2% offset yield

Question 24

necking

Answer 24

when the cross-sectional area reduces, nonuniform plastic is where necking starts. After ultimate tensile strength

Question 25

dislocation

Answer 25

crystalline linear imperfection responsible for step-wise breaking and reforming of bonds during plastic deformation, 1 dimensional defect, plastic deformation of a metal increase number of dislocations which inhibits dislocation movement

Question 26

dislocation density

Answer 26

to lower dislocation density, heat metal. to increase dislocation density, plastically deform. to strengthen a metal, inhibit dislocation movement

Question 27

zero dimensional imperfections

Answer 27

point defects, interstitial impurities. more difficult for a dislocation to move through the lattice close to an impurity thus an increase of impurities make it harder to deform

Question 28

one dimensional imperfections

Answer 28

linear imperfections/dislocations

Question 29

two dimensional imperfections

Answer 29

interfacial imperfections

Question 30

three dimensional imperfections/second phase particles

Answer 30

volume defects, second phase - region of a solid that has a different crystal structure. dislocations moving in a crystal that encounter second phase particles must deal with obstacles discussed in grain boundaries

Question 31

interstitial defect

Answer 31

zero dimensional imperfection. point defect produced when an atom is placed into the crystal at a site that is normally not a lattice point

Question 32

substitutional impurity

Answer 32

atom of a different element replaces a host atom in the crystal

Question 33

vacancies

Answer 33

zero dimensional. atoms vibrate in their lattices trying to jump out of their sites and some atoms are successful leaving a vacancy. when atoms don’t jump out, its because binding energy is stronger than thermal energy. number of vacancies increase with temp

Question 34

boltzman distribution

Answer 34

interpret this when more clear headed

Question 35

free surfaces ?

Answer 35

2 dimensional imperfection that doesn’t increase the strength of a metal

Question 36

internal interfaces/grain boundaries

Answer 36

gorm of internal surface that occurs when one crystal contacts another. if dislocation moves during plastic deformation, it has to go through grain boundary, changing its direction. More difficult for dislocation travel through grain boundary as it requires more energy. can be used to inhibit dislocation movement, strengthening a metal. decreasing grain size would create more grain boundaries, strengthening a metal