structure of metals

Question 1

what is a crystal

Answer 1

an orderly array of atoms

Question 2

what is the unit cell of a crystal structure

Answer 2

the smallest group of atoms having the symmetry of the crystal which, when repeated in all directions, can develop the crystal lattice

Question 3

how many types of crystal lattice exists

Answer 3

14

Question 4

how many crystal systems exists

Answer 4

7

Question 5

anisotropy is due to…

Answer 5

the fact that spacign amog atoms is not always the same in all crystallographic directions

Question 6

which are the types of defects in crystals

Answer 6

-point defects (vacancy, interstitial atom, substitutional atom)
-line defects
-surface defects

Question 7

what is atomic diffusion in metals and which types exists

Answer 7

Diffusion is the stepwise movement of atoms from site to site in a crystal lattice.
-interdiffusion (atoms of 2 different materials)
-self-diffusion (atoms exchanging positions are of the same type)

Question 8

2 mechanism of diffusion

Answer 8

-vacancy diffusion
-interstitial diffusion

Question 9

requirements for diffusion to take place

Answer 9

• there must be an empty adjacent site
• the atom must have enough energy to break the bonds with neighbouring atoms and then cause some lattice distortion

Question 10

how do we measure diffusion

Answer 10

with the diffusion flux J

Question 11

how does plastic deformation of metals happens

Answer 11

Slip occurs along specific and most favourable crystallographic planes at a value much lower than it would be predicted theoretically in a perfect crystal.

Plastic deformation occurs much easier because real crystals contain a relatively large number of line defects called dislocations
✓ Edge dislocations
✓ Screw dislocations
✓ Mixed-type dislocations

Question 12

describe edge dislocations

Answer 12

An extra half-plane (ab) is present above the slip plane (SP)
By an applied stress tau, the dislocation line can move leftwards step by step along the slip line,
eventually producing the shearing of the crystal by one atomic distance

Question 13

describe screw dislocations

Answer 13

The upper front portion of the crystal is supposed to be sheared by one atomic distance to the left relative to the lower
portion

Question 14

what is a solid solution

Answer 14

Homogeneous mixtures of two or more kinds of atoms in the solid state

Question 15

define solvent and solute

Answer 15

✓ solvent refers to the more abundant species
✓ solute refers to the less abundant atoms

Question 16

which are the 2 types of solid solution

Answer 16

-substitutional solutions
-interstitial solutions

Question 17

general features of grain boundaries (behviour at low and high temperature and another one)

Answer 17

✓ At low temperatures GBs are normally quite strong and metals usually fail by cracks at grain interiors (transgranular
fracture)
✓ At high temperatures and slow strain rates, GBs lose their strength more rapidly than the crystal and fracture can
occur along the grain boundaries (intergranular failure)
✓ Precipitation of secondary phases and embrittling phenomena preferably occur at GBs.
✓GBs are in condition of higher free energy so they are more reactive

Question 18

what happens when dislocations reach the GBs

Answer 18

they have to stop, if we increase stress the dislocation cannot jump the GB anyway but a new dislocation will form in the other grain

Question 19

how can we increase material strenght acting on GBs

Answer 19

we have to increase them reducing the grain size

Question 20

parameters defining the stregnht of a metal

Answer 20

✓ The elastic modulus represents a physical property for
each metal, related to the atomic binding and to crystal
structure
✓ The yield strength and the post yielding behaviour
strongly depend on the ease by which dislocations can
glide

Question 21

how can we modify dislocation mobility

Answer 21

✓ Grain refining
✓ Strain hardening
✓ Solid solution strengthening
✓ Dispersion strengthening
✓ Martensite strengthening

Question 22

effects of increasing/decreasing dislocation mobility

Answer 22

• If we reduce dislocation mobility: same elastic modulus, higher yield point, higher uts, lower fracture elongation.
• If we increase dislocation mobility: same elastic behaviour (same young modulus), lower yield strength, lower it’s, higher fracture elongation

Question 23

how can we do hardening (3 types)

Answer 23

-strain hardening
-solid solution strengthening
-dispersion strentgthening

Question 24

strain/work hardening

Answer 24

processes based on plastic deformation are performed at room temperature that produce metals with a workhardened structure (higher density of dislocations because of multiplication mechanisms) The increase in dislocation density (ρ) results in improved strength and reduced ductility of the material

Question 25

describe dispersion strenghtening

Answer 25

Strengthening is achieved by the formation of ultra-fine dispersions of second-phase particles homogeneously
distributed in the primary-phase structure (also called “precipitation hardening”)

Question 26

Depending on precipitate condition (coherency, size, crystal structure), dislocations can…

Answer 26

✓ shear the particles and slide through them, but only with increased applied stress
✓ leave a loop after passing around them, but also this requires increased applied stress, and makes further cross slip even more difficult